Mouse Strain Susceptibility to Diethylnitrosamine Induced Hepatocarcinogenesis Is Cell Autonomous Whereas Sex‐susceptibility Is Due to the Micro‐environment: Analysis with C3H ↔ BALB/c Sexually Chimeric Mice

In man, liver cancer is on the increase, especially in males. Sex differences also exist in rodent models. To elucidate the mechanisms, chimeric mice were produced by amalgamation of early embryos from high and low hepatocarcinogen‐susceptible strains, C3H and BALB/c. Tumor formation was initiated with 10 mg/kg of diethylnitrosamine at the ages of 7 and 14 days and mice were sacrificed at 30 and 45 weeks. The chimeras were classified into XY↔XY, XY↔XX, XX↔XY, and XX↔XX in terms of sex chromosomes by means of polymerase chain reaction‐simple sequence length polymorphism analysis (SSLP) using Y chromosome‐specific Sry primers in combination with the D3Mit21 marker. Liver lesions were analyzed histopathologically, by immunostaining using a C3H strain‐specific antibody and by DNA in situ hybridization with the Y chromosomespecific digoxigenin‐labeled Y353/B probe. Sex and strain genotyping by SSLP analysis matched histological observations, confirming the reliability of our system. The strain differences in liver tumor numbers of each strain type in XY↔XY and XX↔XX subtypes of C3H↔BALB/c chimeras were retained well (P< 0.0001 and P< 0.001, respectively), indicating a minimum influence of the C3H or BALB/c surrounding milieu on development of individual lesions. On the other hand, significant promotion of XX cell tumors was evident in phenotypically male sexually chimeric XY↔XX and XX↔XY chimeras for both C3H (P< 0.02) and BALB/c (P< 0.01) lesions compared to the XX↔XX case. The results suggest the presence of hormonal or micro‐environmental factors specific for males, which are not caused cell‐autonomously. Basic strain differences, however, are determined by intrinsic genetic factors rather than the strain‐dependent micro‐environment