CORRELATION OF EXPRESSION OF CONNEXIN MESSENGER-RNA ISOFORMS WITH DEGREE OF CELLULAR-DIFFERENTIATION

Examination of rat hepatic cell lines has revealed a correlation betwe en the differentiated state of the cells and the gap junctional protei ns, or connexins, they express. The cell lines RLC (Gershenson et al, 1970) and FTO.2B (Killary et al, 1984) were examined and compared to p rimary adult hepatocytes for expression of fetal and adult hepatic ant igens under various tissue culture conditions. Maximal expression of f etal antigens was observed in cells grown in serum-supplemented medium , on either tissue culture plastic or type IV collagen. Maximal expres sion of adult specific antigens was seen in cells grown in a hormonall y defined medium containing heparin, on type I or type IV collagen. Th e cell line RLC strongly expressed fetal antigens, while FTO.2B expres sed both fetal and adult antigens. These cell lines and another poorly differentiated hepatic cell line, WB-F344 (Tsao et al., 1984) were us ed to assess the developmental profile of mRNAs encoding isoforms of g ap junctions: connexins 26, 32, and 43. The cell lines each transcribe d mRNAs of all three connexins, as determined by transcriptional elong ation analysis. By contrast, only certain of the connexin mRNAs could be detected in specific cell lines by Northern analysis: RLC expressed only connexin 43 mRNA; WB-F344 expressed connexin 26 and 43 mRNAs; an d FTO.2B, the most differentiated cell line, expressed connexin 32 and 43 mRNAs. Selection among the connexin mRNAs appears to occur post-tr anscriptionally. Culture of the cell lines in hormonally defined mediu m vs. serum supplemented medium did nor affect the patterns of connexi n mRNA abundance. When the cell lines were cultured in hormonally defi ned medium containing heparin, however, the level of connexin mRNAs di d vary: Connexin 26 mRNA increased in WB-F344 cells, and connexins 32 and 43 mRNAs increased in FTO.2B, but connexin 43 mRNA decreased in WB -F344 and RLC. The abundance of connexin mRNAs also varied when the ce ll lines were analyzed at different cell densities: connexin 43 mRNA i ncreased with cell density in RLC and WB-F344, and connexin 26 mRNA pe aked at an intermediate density and fell at higher cell densities in W B-F344. The differences in connexin mRNA expression among cell lines c haracteristic of different stages of hepatic a differentiation, and th e differences in regulation of connexin mRNAs in the hepatic cell line s, suggest distinct biological roles of the highly homologous proteins . Moreover, connexin gene expression may be a marker of hepatic develo pment: as hepatocytes differentiate the proportions of connexin 43 the n 26 mRNAs decrease while that of connexin 32 mRNA increases.