Interactive Effects of c-myc and Transforming Growth Factor α Transgenes on Liver Tumor Development in Simian Virus 40 T Antigen Transgenic Mice

Abstract

To analyze the effects of c-myc and transforming growth factor alpha (TGFalpha) on hepatocarcinogenesis induced by simian virus 40 T antigen (TAg), livers from single and bitransgenic mice, 3 to 11 mice per line, were examined morphologically 1 to 8 weeks after birth. Mice carrying c-myc or TGFalpha alone exhibited centrilobular hypertrophy and increased apoptosis (c-myc mice only) of hepatocytes after 3 or 4 weeks of age, but no detectable changes in cell proliferation or proliferative lesions were observed in either line during the 8 weeks. Mice carrying TAg alone exhibited increased cell proliferation, apoptosis, and dysplasia of hepatocytes with notably high mitotic and apoptotic indices as major changes before development of putative preneoplastic lesions after 4 weeks of age and neoplastic lesions after 6 weeks. In bitransgenic mice coexpressing c-myc or TGFalpha with TAg, nonproliferative lesions and mitotic and apoptotic indices were similar to those in mice carrying TAg alone. In TAg x c-myc bitransgenic mice, however, both preneoplastic and neoplastic lesions developed sooner and grew more rapidly than those in TAg mice, whereas in TAg x TGFalpha bitransgenic mice, rapid tumor growth was the principle observation. Because of the effects of transgene coexpression, livers from TAg x c-myc and TAg x TGFalpha mice had multiple tumors as early as 3 and 6 weeks of age, respectively. The results indicate cooperative functions of c-myc and TGFalpha with TAg during development and/or growth of liver tumors in vivo.

Quantifying growth and transformation frequency of oncogene‐expressing mouse hepatocytes in vivo†

“…This liver phenotype is consistent with the maintenance of normal liver mass, long tumor latency (>12 months), and low tumor incidence and multiplicity observed in AL‐c‐myc transgenic mice 3, 4. In contrast, although the viral TAg stably increases hepatocyte turnover (increased BrdU labeling and apoptosis) both in AL‐TAg transgenic mice12 and in transplant foci, it does not directly increase net hepatocyte growth under permissive conditions. Rather, as demonstrated by an increase in EOs, it acts by measurably increasing the risk that a TAg‐expressing hepatocyte will accumulate changes that allow it to escape normal growth controls.…”

“…These oncogene combinations decrease hepatocyte size in transplant foci, raising the possibility that partial cell dedifferentiation accompanies their expression. With respect to neoplasia, these pairings are remarkably potent, both in transgenic animals6, 12 and after hepatocyte transplantation. Both pairings are associated with synergistic growth increases in permissive liver, caused principally by changes in hepatocyte replication rather than apoptosis, and with elevated EO frequency.…”

“…In transgenic mice, TAg can induce benign and malignant liver neoplasms by 3 to 4 months of age with an incidence of 100% 3, 10. Transgenic mice coexpressing two oncogenic transgenes in hepatocytes displayed increased tumor multiplicity and decreased latency compared with single transgenic littermates 3, 4, 6, 11‐13. However, the types of analyses performed using these models, which include gross and microscopic observation of lesion development and molecular examination of tumors, remain similar to earlier experimental designs.…”

“…In all cases, mice were excluded as donors if they displayed focal lesions visible on gross examination. Transgenic mouse livers lacking these alterations contain few or no areas of parenchyma that would be microscopically diagnosed as neoplastic, although dysplastic cells may be present 3, 5, 6, 12. The concentration of viable large cells (hepatocytes) was determined by trypan blue exclusion using a hemacytometer.…”

Biological Basis of Differential Susceptibility to Hepatocarcinogenesis among Mouse Strains

“…These include ras and myc oncogenes under the influence of the albumin gene regulatory elements 29,30 as well as SV40-TAg driven by a variety of regulatory elements 27,31 . Growth factors such as TGFalpha and IGF2 under the influence of the metallothionein-1 or major urinary protein regulatory elements, respectively, yield dysplastic foci in as little as 6 months and hepatocellular neoplasms in 8 to 18 months 27,32 with evidence of severe hepatic dysplasia in newborn mice 33 .…”