This document discusses rodent liver tumors and their relevance to human health risk assessment. It provides histopathological images and descriptions of various types of pre-neoplastic and neoplastic liver lesions observed in rodents. The key points are: 1) Several types of foci of hepatocellular alteration are considered pre-neoplastic lesions that can progress to tumors in rodents. Their relevance to humans depends on the mechanism of tumor formation. 2) Rodent liver tumors include adenomas, which are benign tumors, and carcinomas, which are malignant. Their features differ between rodents and humans. 3) The International Life Sciences Institute/Health and Environmental Sciences Institute developed a framework for

1. Rodent Liver Tumours and Human Health
Risk Assessment
Rhian Cope

3. Progression to Neoplasia
(A)Low-power photomicrograph of an
AHF in a control mouse, which is
recognizable as dysplastic under
higher power (magnification, 63;
bar = 100 µm).
(B)Higher magnification of AHF in (A)
illustrating dysplasia including
nuclear enlargement, increased
nuclear/cytoplasmic ratio, nuclear
hyperchromasia, variation in nuclear
size and shape, irregular nuclear
borders, and nucleoli that are
increased in size and number with
irregular borders (magnification,
250; bar = 100 µm).

4. Progression to Neoplasia
(C) LFCA in a liver from a
mouse treated with 1 g/L
DCA; note irregular border
and lack of compression at
edge (magnification, 63; bar =
100 µm).
(D) Higher magnification of
LFCA in (C) illustrating a
focus of dysplastic cells
within the LFCA
(magnification, 400; bar = 100
µm).

5. Progression to Neoplasia
(E) Edge of a large AD from a
mouse treated with 3.5 g/L
DCA, demonstrating
compression of adjacent
parenchyma and "pushing"
border of lesion
(magnification, 63; bar = 100
µm).
(F) Higher magnification of AD
in (E) illustrating dysplastic
cells (magnification, 400; bar
= 100 µm).

6. Progression to Neoplasia
Carcinoma

9. Foci of Hepatocellular Alteration:
“Pre-neoplastic” change
 Society of Toxicologic Pathology Classifications:
• Foci of hepatocellular alteration:
• Basophilic cell foci, tigroid type and homogenous type –
increased RER and decreased cell glycogen;
• Eosinophilic (acidophilic) cell foci – deficient in glucose-
6-phosphatase; ground glass appearance;
• Clear cell foci – large unstained cytoplasm with no
vacuoles;
• Amphiphilic cell foci – intensely eosinophilic cytoplasm;
• Mixed cell foci.

10. Basophilic FHA

11. Eosinophilic FHA

12. Clear Cell FHA

13. Mixed FHA

14. Foci of Hepatocellular Alteration:
“Pre-neoplastic” change
 Occur spontaneously with age in rats; also occasionally in dogs
& non-human primates;
 Type and number of spontaneous foci vary with strain;
 Have the characteristics of initiated ± promoted cells;
 Number increase with exposure to genotoxic carcinogens;
 Represent an “adaptation” of the hepatocytes to a hostile
environment i.e. maladaptive response;

15. Foci of Hepatocellular Alteration:
“Pre-neoplastic” change
• Often express placental glutathione S-transferase (GST-P)
and are UDP-glucuronosyltransferase negative in rats.
Variable expression patterns found in mouse foci;
• Elevated replicative DNA synthesis;
• Altered expression of various growth factors;
• Over responsive to mitogens;
• Inherent defects in growth control (i.e. becoming
autonomous in terms of growth);
• Genomic instability;
• Aberrant methylation of p16 TSG;
• Mutations of ß-catenin;
• Decreased apoptosis;
• Clonal origin demonstrable in vitro

16. GST-P Positive FHA

17. Foci of Hepatocellular Alteration:
“Pre-neoplastic” change
 Relevance to humans:
• Similar pre-neoplastic foci occur in humans exposed to
hepatic carcinogens (both viral and chemical);
• Also occur with non-genotoxic hepatocarcinogens i.e.
anabolic steroids;
• Potentially relevant to humans depending on the
mechanism!

18. Foci of Hepatocellular Alteration:
“Pre-neoplastic” change
 Reversibility:
• In the case of chemically stimulated FHA’s, a high proportion
will partially or near-completely regress when the stimulus is
removed;
• Meet the criteria for “initiation + promotion”;
• Initiation is irreversible, but initiation is not phenotypically
detectable;

19. FHA Versus Focal Nodular Regenerative Hyperplasia
and Nodular Regenerative Hyperplasia
 Key differences:
• Cells phenotypically normal;
• Circumscribed i.e. not invading surrounding normal tissue;
• May be divided into pseudolobules by fibrous tissue (focal
nodular regenerative hyperplasia);
• Not pre-neoplastic.
BUT: Can be very difficult to distinguish from AHF!

20. Foci of Pancreatic Tissue
 Metaplasia NOT neoplasia;
 Islands of seemingly “normal” exocrine pancreatic tissue within
the liver;
 Induced by Arochlor1254;

21. Focal hepatocyte adenoma

22. Adenoma Acinar Type
(An adenoma is a benign tumor (-oma) of glandular origin)

24. Adenoma Trabecular Type
(An adenoma is a benign tumor (-oma) of glandular origin)

25. Adenoma – Human Vs Rodent
 Rodent
• Clearly distinguishable from regenerative hyperplasia;
• Usually larger than one lobule;
• Compress the surrounding tissue;
• Loss of normal lobular architecture but portal triads may be
present;
• Usually multifocal;
• Not encapsulated with fibrous tissue;
• Humans
• Difficult to differentiate from regenerative hyperplasia;
• Usually solitary;
• Usually encapsulated.

26. Carcinoma
Carcinoma: Carcinoma refers to an invasive malignant tumor consisting
of transformed epithelial cells. Alternatively, it refers to a malignant
tumor composed of transformed cells of unknown histogenesis, but
which possess specific molecular or histological characteristics that are
associated with epithelial cells, such as the production of cytokeratins or
intercellular bridges.

27. Carcinoma Trabecular Type (Malignant)

28. Carcinoma Acinar Type (Malignant)
What is this??

29. Carcinoma Clear Cell Type (Malignant)

30. Carcinoma Scirrhous Type (Malignant)

31. Carcinoma Poorly Differentiated (Malignant)

32. What is so important about this?

33. Carcinoma – Human Vs Rodent
• Humans
• Mixed cell tumors are relatively common;
• Concurrent cirrhosis is common;
• Usually associated with chronic hepatitis;
• Rarely spontaneous – usually a history of viral exposure
and/or aflatoxin exposure and/or alcohol exposure.

34. Carcinoma – Human Vs Rodent
 Rodent
• Classically metastasize to lung (why?);
• Derive from oval cells (pluripotent stem cells) in the periportal
area;
• Mixed cell tumors (i.e. hepatocyte plus bile duct cell
carcinomas) do not occur
• Usually do not involve concurrent cirrhosis or chronic hepatitis;
• “Spontaneous” in older animals (also in hamsters and beagle
dogs);
• “Spontaneous” tumors are common, particularly in some
strains.

35. So what sort of tumor is this?

36. ILSI/HESI MOA Framework
 Is the weight of evidence sufficient to establish the MOA in
animals?
• Genotoxic?
• Potentially relevant to humans, particularly if tumors at
multiple sites;
• Nongenotoxic?
• Relevance to humans is highly dependent on the
mechanism!

37. ILSI/HESI MOA Framework
 Are the key events in the animal MOA plausible in humans?
• Genotoxic
• Do the mutations occur in human cells in vitro and in vivo?
• Do the same spectrum of mutations occur?
• Is the genotoxic progression similar?
• Histopathology
• Is the same histopathological life history present in rodents
and humans?

38. ILSI/HESI MOA Framework
 Are the key events in the animal MOA plausible in humans?
• Nongenotoxic?
• Relevance is HIGHLY dependent on the mechanism;
• Do the hyperplastic effect + antiapoptotic effect occur in
humans?
• If a receptor-mediated pathway is involved, is this pathway
present in humans and of similar pathophysiological
relevance?
• Is there a clear dose threshold and what is its relationship
to human exposure?

39. ILSI/HESI MOA Framework
 Taking into account kinetic and dynamic factors, are the key
events in the animal MOA plausible in humans?
• TK is sufficiently similar to result in relevant concentrations at
the site of action?
• Promutagens activated to the same extent in humans (i.e. TD
issues)? (TD encompasses all mechanisms through which the
concentration/amount at the site of action elicits the toxic
effect);
• If redox damage is critical, does similar metabolism/events
occur in humans?
• Do the tumors occur in a non-rodent species?

40. • Observation of tumours under different circumstances lends support to the
significance of the findings for animal carcinogenicity. Significance is
generally increased by the observation of more of the following factors:
•Uncommon tumour types;
•Tumours at multiple sites;
•Tumours by more than one route of administration;
•Tumours in multiple species, strains, or both sexes;
•Progression of lesions from preneoplastic to benign to malignant;
•Reduced latency of neoplastic lesions;
•Metastases (malignancy, severity of histopath);
•Unusual magnitude of tumour response;
•Proportion of malignant tumours;
•Dose-related increases;
•Tumor promulgation following the cessation of exposure.

42. Relevance Depends on MOA