Increased expression of actin-like contractile protein in preneoplastic and neoplastic lesions in rat liver.

Cryostat sections of 16 preneoplastic and 14 neoplastic hepatic lesions induced in rats by 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB) were examined by indirect immunofluorescence with human serum containing smooth-muscle antibody (SMA). Preneoplastic lesions showed strong cytoplasmic staining of proliferating oval cells and of the cell outlines of hepatocytes, in areas of nodular hyperplasia. In carcinomas, poorly differentiated hepatocytes showed staining of cell outlines, while well differentiated tumour cells forming glandular structures showed only staining of the luminar surfaces. The stromal cells also showed cytoplasmic staining. Morphologically normal areas of 3'-Me-DAB-treated livers showed weak staining of cell outlines, similar to normal liver. Specificity of the staining reactions was established by failure of staining in parallel control sections treated with normal human serum, or SMA serum neutralized by absorptions with homogenates of smooth muscle or extracts of actin. The results suggest that there is an increased expression of actin-like contractile protein in preneoplastic and poorly differentiated neoplastic liver cells.