Proliferative Hepatocellular
Lesions of the Rat:
Review and Future Use in
Risk Assessment
Prepared for the
Risk Assessment Forum
U.S. Environmental Protection Agency
Washington, DC
February 1986
Esther Rinde, Ph.D.
Richard Hill, M.D., Ph.D.
)ffice of Pesticides and
Toxic Substances
Authors
Arthur Chiu, M.D., Ph.D.
Bernard Haberman, D.V.M., M.S.
Office of Research and
Development
|arry Anderson, Ph.D. (ODW)*
lary Argus, Ph.D. (OPTS)
\i\n Baumel, Ph.D. (OPTS)*
Judith Bellin, Ph.D. (OSWER)
Iteven Bayard, Ph.D. (ORD)
Jhao Chen, Ph.D. (ORD)
Irthur Chiu, M.D., Ph.D. (ORD)
Technical Panel
David Cleverly, M.S. (OAR)
Bernard Haberman, D.V.M., M.S. (ORD)*
Richard Hill, M.D., Ph.D. (OPTS)
Louis Kasza, Ph.D., D.V.M. (OPTS)
Paul Milvy, Ph.D. (OPPE)
Esther Rinde, Ph.D. (OPTS)
Jackson Schad, M.S. (OSWER)
*Technical Panel Co-Chairman
Risk Assessment Forum Staff
Dorothy Patton, Ph.D., J.D., Executive Director (Acting)
Alan Ehrlich, Ph.D., Executive Secretary
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, DC 20460
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IN MEMORY OF
DR. BERNARD HABERMAN
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Disclaimer
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Contents
Page
List of Figures
List of Tables
I. Introduction .................................... 1
A. Background .................................... ........... ^
B. Purpose and Scope .......................... ............. 1
II. Chemical Hepatocarcinogenesis, A Model ..................... 3
III. Characteristics of Proliferative Hepatocellular Lesions ........ 5
A. Morphological Classification ....................... ........... .5
B. Background Incidence of Lesions in Untreated Animals. '.'.'.'.'.'.'.'.'. 7
C. Comparative Morphology .............................. ..... 7
IV. Relationship Between Certain Hepatocellular Proliferative
Lesions and Hepatocellular Carcinoma ........................... 9
A. Regression of Early Lesions .................... ............... B
1 . Foci ............................................ ......... g
2. Neoplastic Nodules ........................... ........... 10
B. Progression .................................. ............. 1 1
1 . Foci ........................................ ........... 12
2. Neoplastic Nodules ................................. ..... 12
V. Current Governmental Use of Data on Rat Hepatocellular Lesions ... 1 4
VI. Future Use of Rat Hepatocellular Lesions by EPA ................ 15
A. Rationale for Positions ....................... " ' ] ] .......... 15
B. EPA Positions ....................... ............... ........ 16
References ...................................... 1 o
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List of Figures
Page
... 4
Number
1. Hypothetical scheme of neoplastic development in rat liver..
2. Terminology of rat liver lesions 5
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List of Tables
Number
Page
1. Proliferative Liver Lesions: Background Incidence in
2-Year-Old Rats 8
2. Evidence for Progressions of Hepatocellular Lesions 11
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I. Introduction
A. Background
When liver carcinogens are administered to rats, there is an increase in
the frequency of certain morphological lesions and often a chronology in
their formation. Generally foci of cellular alteration are first to appear, then
nepplastic nodules, and finally carcinomas are noted in the livers of the rats
This time sequence suggests an interpretation of the carcinogenic process
in which foci are a precursor stage in the development of neoplastic nod-
ules, which in turn are precursors of carcinomas. However, under certain
circumstances, if carcinogen administration ceases, many foci and neoplas-
tic nodules disappear, and the formation of carcinomas occurs at a low
frequency. Thus, an alternative interpretation is that foci and neoplastic
nodules may be independent of carcinoma development.
Because of these general findings, it is difficult to evaluate the signifi-
cance of increases in lesions in long-term tests in rats. Since there is no
certain scientific knowledge or consensus about the biological significance
of liver nodules, the EPA needs a policy for the interpretation of rat carcino-
gen bioassays in which liver nodules are an end point.
Differences as to the way rat hepatocellular lesions might be used in risk
assessment arose within the Agency. In July 1984, the Office of Toxic Sub-
stances submitted to the EPA Risk Assessment Forum a proposal to inves-
tigate the nature and use of neoplastic nodules of the rat liver. Issues were
formulated that dealt with the histological classification of hepatocellular
proliferative lesions and the way these lesions would be used for hazard
assessment and dose-response assessment. It was agreed that the EPA
should have a clear position on these issues.
B. Purpose and Scope
The classification of certain experimentally induced proliferative hepato-
cellular lesions in the rat liver, other than carcinoma, has been the subject
of great controversy. This paper addresses three questions that are crucial
in the evaluation of such rat tumor data and in the assessment of carcino-
genic risk for humans. One question is whether to consider these lesions
hyperplastic" or "neoplastic" in nature, the latter classification being in-
dicative of tumor formation.1 A second question is to interpret the finding
that in some experimental systems, certain nonmalignant proliferative le-
sions may regress following removal of an inciting, carcinogenic stimulus
with only a small proportion of the lesions persisting. Finally, it is important
to know whether any of these lesions may progress to carcinoma.
This paper begins with a composite view of chemically induced hepato-
carcmogenesis. Proliferative hepatocellular lesions are described histo-
iDorland's Illustrated Medical Dictionary (1974) defines the term neoplasm as "... a mass of new
nh^? 7 ' Per»'tf an, 9r°WS indePendent|y °f "s surrounding structures and which has no
physologic use." Neoplasm has also been defined as ". . . an abnormal tissue that grows by
cellular proliferation more rapidly than normal and continues to grow after the stimuli that
initiated the new growth cease" (Stedman's Medical Dictionary, 1982). Dorland's defines hyper-
plasia as ... abnormal multiplication or increase in the number of normal cells in normal
arrangement m tissue." Stedman's definition of hyperplasia is ". . . increase in the nCmS™or
cells in a tissue or organ, excluding tumor formatioq, where-by the bulk of the organ is in-
crsssGu.
1
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pathologically, and comparisons among species are made. Discussions are
developed on the regression of proliferative lesions following removal of a
carcinogenic stimulus, as well as their potential for progression to car-
cinoma. Current uses of rat liver proliferative lesions in risk assessments by
governmental agencies are reviewed, and recommendations are made for
the future use of these lesions by EPA in cancer risk assessment.
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II. Chemical Hepatocarcinogenesis, A Model
This section presents concepts generally accepted to represent the natu-
ral history of chemical hepatocarcinogenesis, and provides a background
for the larger discussion of neoplastic nodules and foci of cellular alteration
in evaluating chemical carcinogenesis. Only key elements are discussed.2
Hepatocellular carcinoma, like other tumors, is generally recognized as
the last stage in a multistage process, in which several stages can be de-
scribed in terms of the atypical phenotypic expression(s) of cells. The con-
cept of a stepwise development in the carcinogenic process is mainly based
on morphological observations, correlated with cellular behavior. However,
the precise developmental stages in carcinogenesis have not been unequiv-
ocally demonstrated, but are mainly inferred by post-hoc reconstruction.
The first step in hepatocarcinogenesis (usually termed initiation) is
thought to be an irreversible alteration of one or more hepatocytes, either
as a "spontaneous" event or induced by contact with an active chemical
substance. The initial lesion becomes fixed following one or more cycles of
cell proliferation, such that there appears to be lasting memory of its occur-
rence in all following cell generations. In some systems initiation appears to
be manifested by a resistance of the initiated cell to the growth-inhibiting
effect of carcinogens on normal cells; thus the cell can proliferate into a
focus, the first observable morphologic hepatocellular lesion (Scherer and
Emmelot, 1976; Farber, 1980; Pitot and Sirica, 1980; Williams, 1982). Fol-
lowing initiation, the progression of the initiated cell to cancer can be greatly
accelerated by the administration of promoting agents (Berenblum, 1975;
Pitot and Sirica, 1980).
Foci are microscopic lesions that can be distinguished from surrounding
normal liver tissue by a number of phenotypic markers. For example, rou-
tine staining with hematoxylin and eosin differentiates foci into several
types (e.g., clear cells, eosinophilic foci, and basophilic foci). Foci can also
differ in their histochemical properties, such as alterations in enzyme activ-
ities (e.g., deficiencies in glucose-6-phosphatase and beta-glucuronidase;
increases in gamma-glutamyl transpeptidase), and reduced iron accumula-
tion. While these are general characteristics, individual lesions may differ in
their phenotypic marker expression. Foci do not show any disruption of
normal hepatic lobular architecture, although some types of foci show ele-
vated rates of cell division as compared with the surrounding "normal"
parenchyma (Squire and Levitt, 1975; Farber, 1980; Williams, 1980;
Bannasch et al., 1980; Pitot and Sirica, 1980; Emmelot and Scherer, 1980;
Stewart, 1980; Bannasch et al., 1982).
Neoplastic nodules usually appear later in time than foci, but before the
advent of carcinoma; there may be overlap, however. Nodules are com-
posed of cells that are similar to those of foci (e.g., show similar phenotypic
2Various authors have used a plethora of terms over the years to describe the various hepatocel-
lular proliferative lesions and associated phenomena. Sometimes they have meant to indicate
specific types of lesions or cellular processes. However, in order to keep this review simple in
concept, we use the term "focus" (foci) for any of the various literature references that describe
small, focal proliferative areas or areas of cellular alteration. We use the term "neoplastic nod-
ules" to refer to any "nodular" proliferative lesions, whether the authors called them hyperplas-
tic, neoplastic, or any other comparable terms. Lastly, processes leading to morphological rever-
sal of proliferative lesions back to normal-appearing morphology (termed variably such things
as phenotypic maturation, remodeling, reversion, re-differentiation) are called regression.
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alterations following routine and special histochemical staining) and show
elevated rates of cell division. Nodules, however, are larger in size and,
unlike foci, compress the surrounding parenchyma, from which they are
sharply demarcated (Squire and Levitt, 1975; Stewart, 1980; Williams,
1980; Farber, 1980).
Still later following initiation and the development of foci and nodules,
hepatocellular carcinomas are seen to develop. Carcinogens usually induce
multiple foci or nodules and generally fewer carcinomas.
Although the morphology and timing of the lesions suggests a progres-
sion from focus to nodule to carcinoma, the case has certainly not been
proven. Farber (1984) states that nodules are one identified source, while
Williams (1981) suggests that foci do not necessarily give rise to neoplastic
nodules, but may be direct precursors of carcinoma. Yet, unidentified le-
sions that are antecedents of carcinoma may also exist.
A hypothetical projection consistent with these observations is shown in
Figure 1.
Progression of lesions to carcinoma is only part of the scheme. As will be
discussed later in this paper, under certain experimental conditions it ap-
pears that some foci and neoplastic nodules may regress following removal
of a chemical stimulus.
Figure 1. Hypothetical scheme of neoplastic development in rat liver.
Normal Hepatocyte
I
Initiated Cell
. Focus
Neoplastic Nodule
Carcinoma
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III. Characteristics of Proliferative
Hepatocellular Lesions
A. Morphological Classification
In recognition of the diversity of terms used to describe proliferative
lesions of the rat liver, a conference was held in 1974 to develop a consen-
sus among researchers in the field (Squire and Levitt, 1975). Many of the
previous terms that had existed in the literature up to that time, such as
nodular hyperplasia, hyperplastic nodule, hepatoma, and hepatic cell ade-
noma, were replaced by other terms. Three hepatocellular proliferative di-
agnoses were identified: foci (or areas) of cellular alteration, neoplastic
nodule, and hepatocellular carcinoma (see Figure 2).
Figure 2, Terminology of rat liver lesions.
Pre-1975
Since 1975
1985(NTP)
Preneoplastic
Hyperplastic Foci \
Basophilic Hyperplasia
Enzyme-Deficient
Islands
Hyperplastic Nodule
Nodular Hyperplasia
Neoplastic
Benign
Malignant
Hepatoma
Hepatic Cell Adenoma
Hepatoma
Hepatic Cell
Carcinoma
Trabecular
Carcinoma
Neoplastic
Nodule
\
V
Hepatocellular
Carcinoma
Hepatocellular
Carcinoma
SOURCES: Squire and Levitt, 1975; Stewart et al., 1980; Maronpot et al., 1985.
Foci were defined as small lesions of less than one liver lobule in size,
whereas areas of alteration were the size of a lobule or greater. The primary
distinguishing features involve the staining and texture of the cytoplasm,
leading to the use of descriptive terms such as, clear cell, eosinophilic (or
ground glass), and basophilic to describe the differences. Foci did not show
any disruption of the normal hepatic architecture. Because of the structural
similarities with neoplastic nodules, most participants agreed that foci and
areas of alteration may be part of a spectrum of lesions capable of progress-
ing to neoplastic nodules (Squire and Levitt, 1975).
-------
Neoplastic nodules were defined as generally spherical lesions, several
hepatic lobules in size, which show architectural distortion and sharp de-
marcation from the surrounding liver over at least a portion of its periphery.
Neoplastic nodules also display compression of surrounding liver par-
enchyma. It was recognized that nodules occur at a low incidence in control
animals, could be induced by administration of carcinogenic substances,
and "at the least, indicate an increased probability for the development of
hepatocellular carcinoma" (Squire and Levitt, 1975). A majority of persons
thought the previously used term, hyperplastic nodule, was a misnomer,
and preferred the designation neoplastic nodule. Some question still ex-
isted about the nature of neoplastic nodules, however, since the conferees
concluded at that time that benign hepatocellular tumors (that is, those
without malignant behavior potential) were not recognized in rats and,
therefore, the term adenoma was not recommended.
The hepatocellular carcinoma designation was applied to lesions with
characteristic histological and cytological features. They are larger and
more irregular than neoplastic nodules and may involve major portions of
the liver lobes. At the periphery they compress or extend into the surround-
ing parenchyma (Squire and Levitt, 1975).
A second group of experts was assembled by the National Academy of
Sciences (NAS) (Stewart et al., 1980) to consider the nomenclature of rat
liver proliferative lesions. This group affirmed and extended the classifica-
tion suggested by the previous group (Squire and Levitt, 1975). Hepatocel-
lular lesions were divided into focus and primary neoplasms (i.e., neoplastic
nodule and various subtypes of carcinoma).
The NAS group recognized that although foci occur in aging control rats,
an increase in their incidence following chemical administration raises the
suspicion that the substance may be carcinogenic. The neoplastic nodule
was interpreted as a manifestation of the carcinogenic process, with the
focus representing an earlier stage.
The nature and significance of neoplastic nodules was captured in the
report by the following:
Before and to some extent after this workshop, some pathologists
termed this lesion a "hyperplastic nodule." Some who use this term
regarded, and continue to regard, the lesion so designated as a stage of the
neoplastic process; others consider it to be unrelated to neoplasia. Still
others have called the neoplastic nodule a hepatoma or adenoma, thus
implying that the lesion is a benign neoplasm. The consensus of those
attending the workshop, however, was that neoplastic nodules, as
described, do progress. End-stage benign neoplasms of hepatic cell origin
have not been identified with certainty in the rat liver. From these
considerations, the use of the term "hyperplastic nodule" as a synonym for
neoplastic nodule is to be discouraged. (Stewart et al., 1980).
The Squire and Levitt and NAS papers developed a new classification
scheme for hepatocellular proliferative lesions, but the exact position and
significance of the neoplastic nodule was not totally clarified. The nodule
was seen not as a hyperplastic lesion but as somewhat neoplastic in nature,
yet still not an end-stage benign tumor.
Bannasch (1976) proposed that neoplastic nodules seem to include a
mixture of forms, some precancerous, some cancerous, and various inter-
mediate cells.
Dissatisfaction with the term neoplastic nodule is reflected in the discus-
sion of a recent international conference organized by the Society of Toxico-
logic Pathologists (Symposium Panel Discussion, 1982). An informal con-
-------
sensus of the conference participants (determined by a show of hands)
recommended that the term neoplastic nodule be abandoned.
Most recently, after discussion with an outside group of experts in the
field, the National Toxicology Program (NTP) has instituted a change in the
classification of rat liver proliferative lesions in the chronic toxicity program
(Maronpot et al., 1986; Maronpot, personal communication, 1985). Basi-
cally, the NTP has retained the terms focus and hepatocellular carcinoma as
used before, but has replaced neoplastic nodule with two terms: hyper-
plasia and hepatocellular adenoma (see Figure 2). Hyperplasia is character-
ized by mild compression of the surrounding parenchyma that is not noted
in the focus of cellular alteration. Hyperplasia is viewed as the result of cell
injury with accompanying regeneration. In contrast, hepatocellular ade-
noma shows clear-cut demarcation from surrounding normal tissue with
compression and loss of the normal lobular architecture.
The new NTP classification scheme is similar to that for some other ep-
ithelial organs: cellular reactions include non-neoplastic hyperplasia as well
as benign and malignant neoplastic lesions. The choice of terms should
help to alleviate some of the problems associated with the term neoplastic
nodule. The effect that the change in nomenclature will have on the propor-
tion of neoplastic nodules that will now be called hyperplasia must await its
use in a number of chronic studies. Although the NTP is applying the new
terminology to the Fischer 344 rat, the new nomenclature could be extended
to other strains. Finally, it is important to see how the proposed classifica-
tion will hold up in experimental studies concerned with hepatocarcinogen-
esis in which a host of biological indicators are used to assess the nature of
the various proliferative lesions.
B. Background Incidence of Lesions in Untreated Animals
A few authors have reported the incidence of hepatocellular proliferative
lesions in control animals in various strains of rats. A representative set of
values is tabulated in this report (Table 1). Over a lifetime, it appears that the
bulk of untreated animals develop foci, whereas many fewer develop neo-
plastic nodules or carcinomas. For instance, for Fischer 344 rats, it seems
that over 75% of control animals develop foci, whereas usually less than 5%
and less than 1% of animals develop neoplastic nodules and liver-cell can-
cers, respectively (Goodman et al., 1979; Haseman et al., 1984; Popp et al.,
1985).
C. Comparative Morphology
Proliferative liver lesions have been identified in a number of different
species. Foci showing alteration in routine or special (e.g., enzymatic) histo-
logical staining have been identified in the mouse, rat, golden hamster,
monkey, and human (Ward, 1984; Stenback et al., 1986). The same is true
for "benign" nodules and carcinomas. Both Ward (1984) and Popper et al.
(1977) point out the similarity in proliferative lesions (i.e., foci, nodules
[adenomas], and carcinomas) among species, although Ward claims that
"dysplastic" foci are more common in the monkey and human than in the
mouse,, and Popper et al. indicate that human lesions show a much more
pronounced fibrotic reaction than those in rodents. This latter difference is
probably a reflection of the fact that normal rat liver contains only one-third
the hydroxyprolihe (a measure of collagen, a necessary component in fibro-
sis) of normal human liver.
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Table 1. Proliferative Liver Lesions: Background Incidence in 2-Year-Old
Rats
Animals with (%)
Strain
Wistar
Sherman
COBS
CD (Charles
River)
Fischer 344a
Fischer
344b
Fischer 344
(early NCI
experi-
ence)
Fischer 344
(recent
NTP ex-
perience)
Sprague-
Dawley0
(Dow
Chemical)
Wistar (Col-
worth
strain)
Number Neoplastic
Sex examined Foci nodules
M
F
M
F
M
M
F
M
F
M
F
M
F
M
F
7 100
173 16
20 15
20 50
9 100
85 78
79 87
1794
1754
2306
2356
171
172
45
45
—
—
—
—
11
4.6
1.3
2.7
3.4
3.0
3.5
7.0
13
11
Carcinomas Reference
—
—
—
—
1.2
0
0.39
0.39
0.8
0.2
2.9
1.7
16
29
Ogawa et al..
1981
Kimbrough
et al., 7975
Ulland et al.,
1977
Ogawa et al.,
1981
Popp et al.,
1985
Goodman et
al., 1979
Haseman et
al., 1984
Adapted
from
Thorpe et
al., 1982
Adapted
from
Thorpe et
al., 1982
"72 weeks old.
b111 weeks old; total from three laboratories.
Tote/ from two studies.
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IV. Relationship Between Certain Hepatocellular
Proliferative Lesions and Hepatocellular Carcinoma
Administration of hepatocarcinogenic substances to rats is associated
with two different phenomena. One is the induction of proliferative hepato-
cellular lesions, foci, and neoplastic nodules, and usually later, hepatocellu-
lar carcinomas. The other is reversion of early lesions back to "normal"
morphology, which occurs under some experimental conditions, if the car-
cinogenic stimulus is removed early in the process. This section summa-
rizes some of the evidence dealing with the potential of various early prolif-
erative lesions either to progress to carcinoma or regress to normal
appearance.
A. Regression of Early Lesions
Early proliferative lesions induced by hepatocarcinogens often change
following discontinuation of administration of the carcinogen. The mor-
phology of foci and nodules becomes more like that of normal adult liver.
Many of the biochemical changes noted in these lesions reverse (e.g.,
glucose-6-phosphatase activity is regained, gamma-glutamyl transpepti-
dase activity declines). Due to this regression, the area becomes grossly
indistinguishable from the surrounding liver (Kitagawa and Pitot, 1975-
Farber, 1980).
1. Foci
Goldfarb and Zak (1961) reported regression of foci 4 weeks after with-
drawal of 3'-methyl butter yellow, fed for 26 days. (When the same carcino-
gen was fed for 40 days, neoplastic nodules and hepatocellular carcinomas
were found.)
Kitagawa (1976) demonstrated after feeding N-2-fluorenylacetamide
(2-AAF) that a majority of "hyperplastic" areas (it is assumed that the author
was referring in part to foci) underwent regression. On the other hand, in a
small group of foci, enzyme deficiency persisted during the observation
period. The author speculated that these areas appeared to be more impor-
tant than the ones that had regressed, in relation to the later development
of carcinoma.
Williams and Watanabe (1978) reported that over 95% of altered foci,
induced by 2-AAF, regressed within 6 months after withdrawal of 2-AAF (but
that a small fraction progressed to neoplastic nodules and carcinoma). Also
after promotion with phenobarbital (following 2-AAF withdrawal) "over
90% of livers developed carcinoma by 6 months."
Regression of foci has also been reported with N-nitrosomorpholine
(NNM) (Moore et al., 1983). There have also been reports in which foci did
not regress. For instance, daily oral doses of NNM for 7 weeks at doses of
80-200 mg/L in drinking water resulted in increases in the number of foci
which persisted for periods up to 50 weeks after cessation of dosing (Moore
et al., 1982).
Irreversible foci were also induced with a single dose administration of
diethylnitrosamine (DEN) followed by partial hepatectomy (Scherer and
Emmelot, 1975) or followed by feeding of phenobarbital (Pitot et al., 1978).
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2. Neoplastic Nodules
Sprague-Dawley rats fed 0.064% methyl butter yellow for 26 days or
40 days and then put on a standard lab diet showed different carcinogenic
responses (Goldfarb and Zak, 1961). Neoplastic nodules formed in animals
treated for 40 days, but most underwent regression upon cessation of treat-
ment; many animals had developed hepatocellular carcinomas by the end
of the study. However, few nodules or carcinomas were noted after 26 days
of treatment. This compound induced hepatotoxic effects in addition to
parenchymal cell changes (e.g., lesions of bile ducts and cirrhosis). Confirm-
ing studies showing regression of nodules initially produced by methyl
butter yellow were reported by Goldfarb (1973).
Teebor and Becker (1971) reported heterogeneity among neoplastic nod-
ules. They found that when Sprague-Dawley rats were fed 0.06% 2-AAF for
3 monthly cycles (3 weeks on treatment; 1 week on basal diet) and then
returned to control diet, many nodules were produced which then re-
gressed. Only a low incidence of hepatocellular carcinomas developed over
time. However, in another group of rats that had been fed for only 1 addi-
tional monthly cycle, nodules persisted, and there was a high incidence of
animals with hepatocellular carcinomas.
Kitagawa (1976) found that many neoplastic nodules induced by feeding
0.03% 2-AAF to Sprague-Dawley or Donryu rats for 12 weeks returned to a
more normal phenotype upon withdrawal of the agent.
In contrast to the above experiments, Hirota and Williams (1979) failed to
find regression of neoplastic nodules in F344 rats fed 0.021% 2-AAF for 3,4,
or 5 cycles (4 weeks on treatment; 1 week on basal diet) and then returned
to control diet, although some nodules did show changes in enzyme stain-
ing that are often seen in nodules that regress.
Moore et al. (1982) also failed to find regression of neoplastic nodules and
foci in male Sprague-Dawley rats that had been given NNM in drinking
water (80-200 mg/L) for 1 to 20 weeks and then sacrificed 8 weeks after
removal of the chemical. Some animals given NNM for 7 weeks were not
sacrificed until 50 weeks post-treatment; again, there was no indication of
regression. Unlike these findings, when the carcinogen is given for a limited
period of time and then stopped, when the same laboratory (Moore et al.,
1983) evaluated NNM in a short-term exposure system,3 regression of nod-
ules and foci was observed.
In a series of experiments using a related model,4 Farber and co-workers
have clearly indicated that nodules synchronously appear in great numbers
following treatment, only to regress following termination of 2-AAF (Solt et
al., 1977; Ogawa et al., 1979b; Enomoto and Farber, 1982; Farber, 1982;
Tatematsu et al., 1983). These results have been confirmed by Van der
Heijden and Dormans (1981). During regression, staining characteristics in
most nodules return to normal and the characteristic morphology of the
lesions gradually disappears, with the cells in the area again appearing
"normal." Tritiated thymidine labeling has demonstrated that regresssion
to normalcy in this system is not attended with profound cellular loss;
instead, existing cells in nodules return to normal phenotypes (Kitagawa,
1976; Enomoto and Farber, 1982).
SPartial hepatectomy, 3 days NNM (400 mg/L in drinking water), 2 weeks of 2-AAF (0.2 mg/kg in
feed) with a single dose of carbon tetrachloride (1 mL/kg) midway during 2-AAF exposure.
4A single dose of N-nitrosodiethylamine followed 2 weeks later by feeding 2-AAF for 1 week and
partial hepatectomy.
10
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Thus, it appears that both foci and neoplastic nodules can regress under
at least two circumstances following removal of a carcinogenic stimulus. In
"stop" experiments, where a carcinogen is given for a short period and then
stopped, both foci and nodules have reverted to normal morphology. In
addition, foci and nodules have also regressed in short-term experiments,
when a carcinogen is given for a period of time following administration of
another carcinogen to initiate the process and a stimulus is given to cause
liver regeneration. Regression has not, however, been noted by every inves-
tigator, nor have all experimental systems been investigated.
Even in those cases in which regression has been demonstrated, some
proliferative lesions persist. Farber has stated that despite a high rate of
regression of nodules in his experimental model, 70% of animals develop
hepatocellular carcinomas by 9 months following cessation of treatment
(Ogawa et al., 1979a; Farber, 1982).
For those proliferative lesions that regress, the question remains whether
they retain "memory" of the initial lesion. Farber (1982) hypothesized that
these hepatocytes revert back to an initiated cell; however, evidence for this
position is still lacking.
B. Progression
The general acceptance of the developmental sequence: focus to neo-
plastic nodule (NN) to hepatocellular carcinoma (HCC) (i.e., progression) is
based mainly on indirect evidence. Table 2 presents a summary of some of
this evidence in simplified form.
Although there has been much discussion of candidates for a phenotypic
marker of cells committed to develop carcinoma, none as yet has proven to
be a reliable indicator of the origin of hepatocellular carcinoma cells. Thus,
Table 2. Evidence for Progressions of Hepatocellular Lesions
Foci
NN
HCC
Incidence increased by carcino-
gen administration +
Sequence in the development of
lesions ^t
Proliferative lesion +
Number of lesions per liver + + +
Morphology: compression of
adjacent tissue —
Markers,
Enzyme-staining abnormalities +
Decreased iron accumulation +
Site of hepatocellular carcinoma
formation ?
Transplantability -/?
+
2nd
+
+ + +/++?
+
3rd
+/_
+/_
+ -f
n/a
11
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most of the conclusions in the literature on this aspect of hepatocarcinogen-
esis are based on inference, rather than direct evidence. Some of these are
presented.
1. Foci
It has been claimed that some carcinomas arise within foci (Schulte-
Hermann et al., 1982). However, the authors cautioned that there was no
proof that all foci progressed to malignancy.
Cells of certain carcinogen-initiated foci cells are able to proliferate (in-
crease in number) in the absence of the carcinogen for considerable periods
of time (Scherer and Emmelot, 1975).
Bannasch et al. (1982) observed that "foci became larger and acquired
phenotypic markers closerto neoplasia, independent of further action of the
carcinogen."
Although foci undergo regression, it has been suggested that these re-
verted cells may retain some permanent carcinogen-induced alteration
which could make them susceptible to progression if exposed to additional
carcinogens (Williams and Watanabe, 1978). These authors describe an
experiment where PB was given as a promotor after withdrawal of AAF,
which resulted in increased persistence of foci, and up to 90% of livers so
treated developed carcinomas by 6 months.
Scherer and Emmelot (1975) suggested there is a precursor-product rela-
tionship between foci and hepatocellular carcinoma, which they claim to
have demonstrated by a quantitative technique. (See Emmelot and Scherer,
1980 for a detailed discussion.)
It has also been claimed that only a small fraction of foci may develop into
nodules or carcinomas: about 1 in 1000 following administration of AAF
(Watanabe and Williams, 1978) and about 1 in 11,000 after partial hepatec-
tomy and dosing with methyl(acetoxy-methyl)nitrosamine (Kaufman et al.,
1985).
2. Neoplastic Nodules
Cancer has been found to arise in nodules (Goldfarb, 1973; Williams, 1976
[observed cancer arising both inside and outside nodules]; Solt et al., 1977;
Popper et al., 1977; Farber, 1982). Some nodules have been found to be a
site for cancer development with five different carcinogens in different mod-
els (Farber et al., 1984); however, cancer cannot be said to arise exclusively
inside nodules.
A minority of nodules do not undergo regression following carcinogen
removal, but persist (Ogawa et al., 1979a; Van der Heijden and Dormans,
1981; Enomoto and Farber, 1982; Moore et al., 1983; Tatematsu et al., 1983;
Farber, 1984). These may play a role in the development of carcinoma.
Autonomy
Neoplastic nodules sometimes develop long after removal of a carcino-
gen and are progressively growing entities: a carcinogen can be withdrawn
for 8 to 12 weeks, and liver cancer will still appear weeks or months later
(Epstein etal., 1967). Several researchers have published experimental re-
sults that clearly demonstrate this concept of autonomous growth for neo-
plastic nodules. For many weeks after chemical withdrawal, neoplastic nod-
ules continue to enlarge or invariably increase in number (Goldfarb, 1973;
Hirota and Williams, 1979). In fact, Hirota and Williams (1979) presented
data from studies in F344 rats fed N-2-fluorenylacetamide for a limited
number of cycles, which clearly showed that nodules were persistent after
carcinogen withdrawal and had progressive growth ability (in situ), a char-
acteristic of neoplasms. Similar conclusions regarding persistence and au-
12
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tonomy were also reported for various experimental protocols (Scherer and
Emmelot, 1975; Emmelot and Scherer, 1980).
Transplantation
Since on transplantation, hepatocellular carcinomas display continued
growth, invasiveness, and metastasis, several investigators have studied
the behavior of "putative preneoplastic lesions" following transplantation.
The results are not consistent.
Lee et al. (1983) described the transplantation of carcinogen-altered hep-
atocytes and their subsequent development into metastasizing cancer. Dis-
sociated cells from livers bearing chemically induced nodules were im-
planted into spleens of syngenic rats. Cells from "early nodules,"5 grew
slowly in the spleen, but their evolution into cancer was not observed in
studies lasting up to 2 years. Cells from "persistent nodules,"6 on the other
hand, generated hepatocellular carcinoma in the spleen within 18 months
(Finklestein et al., 1983; Farber, 1984).
In another transplantation model, hepatocytes from nodules showed pro-
gressive invasion into chick embryo heart fragments in the same way as did
those from hepatocellular carcinoma. Control hepatocytes showed no inva-
sion (Wanson et al., 1981).
In still another series of studies summarized by Williams (1980), earlier
attempts at transplantation of neoplastic nodules were unsuccessful. Nod-
ule transplants placed in the mammary fat pads of syngenic rats were found
to persist for months, but did not display "continued growth, invasiveness,
or metastasis," although cell division was noted.
Ward et al. (1975) claimed that two well-delineated nodules, induced by
dietary aflatoxin B-i and transplanted into syngenic recipients, grew slowly.
In a later study, Ward and Lynch (1984) were unsuccessful in transplanting
a hepatocellular "adenoma" and a hepatocellular carcinoma.
In summary, there is some information that indicates that foci and nod-
ules may continue to grow and possibly progress toward carcinoma. The
case is stronger for the progression of some nodules to carcinoma than it
is for foci, but in neither case is the evidence conclusive. It is also recognized
that under certain experimental conditions many foci and nodules regress
after the administration of a hepatocarcinogen has been discontinued.
Thus, even if some foci or nodules do progress, all such lesions are not
necessarily obligate precursors of carcinoma.
One can conclude that foci and nodules are part of the carcinogenic
process. It is generally recognized that foci are hyperplastic lesions, but it is
still difficult to discern whether rat liver nodules are hyperplastic or neoplas-
tic lesions. A position consistent with the data is that at least some nodules
may be neoplastic (e.g., those that persist after carcinogen removal) and
have the potential to progress to carcinoma.
5Those nodules induced in the short-term test system employed by Farber and co-workers; see
footnote 4.
6The small percentage of nodules that do not regress following removal of a carcinogenic stim-
ulus.
13
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V. Current Governmental Use of Data on Rat
Hepatocellular Lesions
Several governmental agencies have adopted procedures for the use of
proliferative lesions in experimental animals in the evaluation of chemicals
for carcinogenicity.
The National Toxicology Program (NTP) commonly pools the incidence of
certain benign and their corresponding malignant neoplasms in evaluating
the carcinogenic hazard of chemicals in long-term animal tests (McConnell
et al., 1986); this practice has been endorsed by an advisory group to the
NTP (1984). For rat hepatocellular lesions, separate incidence figures are
generated for animals bearing neoplastic nodules, hepatocellular car-
cinomas, and for the combination of the two. Carcinogenicity determina-
tions are based on treatment versus control comparisons and dose-trend
relationships for all these groupings. Neoplastic nodules have been inter-
preted by NTP as if they were benign neoplasms (Maronpot and Boorman,
1982); chemically related increases in any of the three groupings constitute
"clear" evidence of carcinogenicity. The NTP also collects information on
the frequency of different types of foci of cellular alteration among animals
in their long-term studies.
Recently, NTP has proposed to change the classification of rat hepatocel-
lular proliferative lesions in their chronic toxicity testing program (Maronpot
et al., 1986). The major difference in their new classification scheme is
replacement of the term neoplastic nodule with two terms: hyperplasia and
hepatocellular adenoma. At the end of a 2-year trial period, NTP will evalu-
ate the adequacy of this new nomenclature.
The Office of Science and Technology Policy (1985) report and the EPA
Proposed Guidelines for Carcinogen Risk Assessment (U.S. EPA, 1984) sup-
port the pooling of benign and malignant tumors of the same histological
type when scientifically defensible. The EPA Guidelines go on to state that
the contribution of benign tumors to the total extrapolated risk should be
expressed when pooling has been used. In qualitative hazard assessment,
the Guidelines accord less weight to cases where the only neoplastic re-
sponse attributed to chemical exposure is the production of benign tumors.
Such data constitute limited evidence of animal carcinogenicity, and such
chemicals are generally put into Group C (Possible Human Carcinogen). The
EPA Proposed Guidelines also state that decisions as to whether to proceed
with quantitative risk assessment on Group C substances would be made on
a case-by-case basis.
The Food and Drug Administration, Occupational Safety and Health Ad-
ministration, and Consumer Product Safety Commission have used rat neo-
plastic nodules in the assessment of carcinogenicity of chemical sub-
stances, but no specific policies have been established. Hepatocellular
carcinomas and nodules are used in a weight-of-evidence determination as
to whether a substance may pose a carcinogenic hazard. All agencies use
foci in their hazard assessments to help in the interpretation of hepatocellu-
lar neoplastic responses.
14
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VI. Future Use of Rat Proliferative Hepatocellular
Lesions by EPA
A. Rationale for Positions
Although the disagreements seem to have narrowed in recent years,
there are still differences of opinion within the scientific community on the
biological significance of certain parenchyma! cell proliferative lesions in rat
liver. Several factors have influenced this situation, including questions
about nomenclature and possible progression and regression of these le-
sions under certain circumstances. As the National Academy of Sciences
(NRC, 1983) pointed out, even in the absence of scientific consensus, Fed-
eral agencies have the responsibility for developing inference guidelines
(i.e., science policiesj which lay out the positions to be taken on specific
issues together with their rationale.
From the accompanying review it seems apparent that scientists have
described a number of proliferative lesions of rat hepatocytes which are in
some way part of the carcinogenic process. Morphological entities similar
to those found in the rat have been described in the livers of other rodents,
monkeys, and humans. Foci appear to be some type of preneoplastic lesion,
but controversy still surrounds the term neoplastic nodule. Some consider
the nodule to be a hyperplastic lesion while others interpret it to be a
neoplastic entity (a benign or possibly a malignant lesion), and the term
may include entities from all such morphological designations.
Foci are commonly found in the livers of control rats at the end of long-
term studies; neoplastic nodules and hepatocellular carcinomas are less
common. Administration of carcinogens, but not of noncarcinogens, in-
creases the incidence of these three lesions. These lesions are also in-
creased in animals initially given a carcinogen and then a promoter. There
seems to be agreement that an increase in foci and nodules in treated
animals is some indication that the liver is at increased risk of cancer forma-
tion.
The appearance of morphological entities generally follows a time-
dependent pattern following administration of a hepatocarcinogen: foci
form first, followed by nodules, and still later, by carcinomas.
Although there is some evidence supporting the notion of progression
from a latent-initiated cell to focus to nodule and then to carcinoma, that
relationship cannot be said to be established. Carcinoma sometimes seems
to arise from a nodule; a focus or some yet unidentified precursor or a
combination of the ab'ove may be other sources.
Regression experiments serve two roles: they help to elucidate steps in
the carcinogenic process, and they may give insights into phenomena that
may occur under conditions of limited or intermittent exposure to carcino-
genic substances. Human exposures to environmental toxicants may be
limited or intermittent and, therefore, regression experiments may be rele-
vant to risk evaluation.
It also appears that under certain experimental conditions, early prolifer-
ative lesions (i.e., some foci and nodules) that have been induced by hepa-
tocarcinogens will largely regress following removal of the carcinogenic
stimulus. Whether these lesions return to a latent-initiated state or back to
a totally normal cell, or whether the cells may be lost or destroyed under
some circumstances, is not known. To the extent that proliferative lesions
15
-------
regress, however, there may be some modulation of the potential hazard for
cancer development.
Even in those cases in which the bulk of the foci and nodules have been
shown to regress following carcinogen removal, some lesions persist. The
persistent lesions are often associated with carcinoma formation at a later
time. It also seems possible that some of the persistent lesions may actual ly
progress to carcinoma.
Federal guidance for carcinogen risk assessments, as developed in the
Office of Science and Technology Policy (1985) document, recommends
that the incidence of animals bearing malignant and corresponding benign
lesions should be combined when scientifically defensible. It also points out
the importance of reviewing the incidence of preneoplastic lesions in target
organs as being an adjunct to the assessment of a chemical's carcinogenic-
ity. EPA's Proposed Guidelines for Carcinogen Risk Assessment (1984) af-
firm these positions, but state that an increase in benign tumors alone is
accorded less weight in assessing carcinogenic hazards than an increase in
malignant tumors or a combination of malignant and corresponding benign
tumors. Increases in benign tumors only are given a weight-of-evidence
classification of Group C (Possible Human Carcinogen). Decisions as to
whether to proceed with a quantitative risk assessment are made on a
case-by-case basis.
Thus, in regard to rat liver proliferative lesions, it appears that Federal
agencies are using foci and neoplastic nodules along with hepatocellular
carcinomas in risk assessments on chemicals, and it seems appropriate to
continue such practices. Accordingly, the Agency provides the following
guidance for interpreting liver toxicity data in rats.
B. EPA Positions
In formulating a policy position on the interpretation of neoplastic nod-
ules, one needs to be cognizant of the existing scientific evidence as well as
the inconsistencies in the data, the absence of information in certain areas,
and the lack of a consensus in the scientific community.
Morphologically described proliferative lesions of rat parenchymal cells
will be used in carcinogen risk assessments in conjunction with the EPA
Proposed Guidelines for Carcinogen Risk Assessment (1984) as follows:
1. Foci of cellular alteration are to some extent indicative of an ongoing
carcinogenic process and should be used in risk assessments. Accordingly,
EPA adopts the following position:
An analysis of the incidence of foci of cellular alteration among treat-
ment groups can contribute to the interpretation of a given rat toxicity
study and, thus, becomes part of the hazard identification step of the
carcinogenic risk assessment. Dose-response and time-to-occurrence
parameters are useful in this qualitative analysis.
2. Neoplastic nodules are increased in animals receiving carcinogens,
and that occurrence is almost always associated with an increase in hepato-
cellular carcinoma. However, it appears that there is not a one-to-one rela-
tionship between nodules and carcinomas. Although some nodules may
have "malignant potential," others may only be "hyperplastic" lesions. Still
other nodules'may regress following cessation of carcinogenic administra-
tion.
Therefore, the exact contribution of neoplastic nodules to the overall
incidence of hepatocellular tumors in the rat is unclear at this time. EPA
adopts the following position until better means of articulating uncertainties
are developed.
Determination of carcinogenic hazard will be based upon consider-
ation of the incidence of hepatocellular carcinoma alone, neoplastic
16
-------
nodule alone, and a combination of carcinoma and nodule. The range
of responses could vary from cases where there are very significant
increases in the incidence of carcinomas backed up by increases in
nodules, through situations where only the combined frequency of
animals with carcinomas or nodules is significant, to cases where in-
creases are limited to nodules alone.
(i) At one end, responses showing an overwhelming increase in car-
cinomas alone will be interpreted as providing sufficient evidence of
animal carcinogen/city when other criteria for the sufficient category
are met. In the absence of human evidence and supporting information,
such evidence will generally be given a weight-of-evidence designation
of Probable Human Carcinogen (Group B2), and quantitative risk as-
sessment is appropriate.
(ii) At the other end, where increases in lesions and statistical signif-
icance are restricted to neoplastic nodules alone, such data will be
interpreted as only limited evidence of animal carcinogenioity. When
neither human evidence nor supporting information accompany the
increase in nodules, the weight-of-evidence designation is Possible Hu-
man Carcinogen (Group C), but quantitative assessment is not war-
ranted.
(Hi) Situations within the two extremes will be evaluated on a case-
by-case basis. In determining carcinogenic hazard, different factors
should be considered in deciding whether there is sufficient or limited
evidence of animal carcinogenicity.
Some of these factors include the proportion of proliferative lesions
that are malignant; the magnitude of increase in incidence of nodules
and carcinomas; considerations of dose response and time-to-tumor;
and the nature and the outcome of short-term tests and other support-
ing information. In deciding whether or not to proceed with quantita-
tion, consideration will be given to the overall weight of evidence. In
cases where quantitation is performed, methods of expressing the pos-
sible uncertainty in the estimated risks should be explored (e.g., ex-
pressing risk as a range between that determined from extrapolation of
carcinoma incidence alone and that from a combination of carcinomas
and nodules).
3. Hepatocellular adenoma will constitute a diagnostic classification in
some rat studies reviewed by the Agency in the future. The designation
adenoma includes a yet unknown proportion of lesions previously called
neoplastic nodules. The Agency concludes that:
The EPA current position on benign tumors will apply to the designa-
tion, hepatocellular adenoma. The pooled incidence of hepatocellular
adenomas and carcinomas will be used in making qualitative judg-
ments as to whether a rat study demonstrates evidence of a carcino-
genic hazard. Pooled incidence will also be used for dose-response
assessments. In cases where adenomas alone reach significance, deci-
sions as to whether to proceed with a dose-response assessment will
be made on a case-by-case basis.
17
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