Liver cell-specific transcriptional regulation of connexin32

Gap junctional intercellular communication facilitates liver homeostasis an d growth control in the liver. The major gap junction protein expressed by hepatocytes is connexin32 (Cx32) and non-parenchymal hepatic cells do not e xpress this gene. We investigated the regulation of Cx32 transcription by t rans-activating factors in liver cells. Transient transfection assays using deletions of the rat Cx32 promoter (nt -753 to -33) linked to the lucifera se gene were performed in MH1C1 rat hepatoma cells that express endogenous Cx32 compared with WB-F344 rat liver epithelial cells that do not. The basa l promoter element was located within nt -134 to -33 and was 1.4-fold more active in MH1C1 cells than WB-F344 cells whereas the entire promoter fragme nt (nt -754 to -33) was four-fold more active in MH1C1 cells. Specific nucl ear protein-DNA complexes that bound to Sp1 consensus sites within the basa l promoter were formed using nuclear extracts from both types of cells. Add itional promoter sequences increased promoter activity more strongly in MH1 C1 cells than WB-F344 cells and this was correlated with the binding of hep atocyte nuclear factor-1 (HNF-1) to two HNF-1 consensus sites centered at - 187 and -736. Expression of HNF-1 and binding to these elements was only ob served with MH1C1 cells. Other specific protein-DNA complexes were formed, however, that included YY-1- and NF-1-containing complexes, but these were not related to promoter activity. Dexamethasone increased Cx32 promoter act ivity and expression in MH1C1 cells, but had little effect in WB-F344 cells and did not alter protein-DNA complex formation. These data suggest that S p1 is responsible for Cx32 promoter basal activity, that HNF-1 determines t he cell-specific expression of Cx32, and that dexamethasone increases Cx32 expression through other mechanisms. (C) 2000 Elsevier Science B.V. All rig hts reserved.