DOWN-REGULATION OF CYTOKERATIN 14 GENE-EXPRESSION BY THE POLYOMA-VIRUS MIDDLE T-ANTIGEN IS DEPENDENT ON C-SRC ASSOCIATION BUT INDEPENDENT OF FULL TRANSFORMATION IN RAT-LIVER NONPARENCHYMAL EPITHELIAL-CELLS

Polyoma virus middle T antigen (mT) transforms the T51B cell line and induces the loss of the cytokeratin 8 and 14 pair (CK8/CK14) present i n these rat nonparenchymal liver epithelial cells (LECs), because of t he selective down-regulation of CK14 gene expression. To identify the initial steps of the mT-induced signaling pathway(s) leading to this i nhibition, T51B cells were transfected with vectors encoding the NG59, dl23, and 248M mT mutants, which are known to interact in a different ial manner with c-Src, PI3-kinase, and Shc. Immunofluorescence microsc opy and Northern blot analysis showed a loss of cytokeratins in dl23 o r 248M but not in NG59 mT mutant-containing cells. An in vitro kinase assay demonstrated that only the dl23 and 248M mT mutants could associ ate with c-Src. This c-Src-mediated action of mT on CK14 gene expressi on was further confirmed by adding the v-src gene product in T51B cell s. The assessment of the transforming capacity of the mT mutants demon strated that the NG59 and dl23 mT mutants were nontransformant, wherea s the 248M mT mutant expressed an appreciable transforming activity, T hese results show that the down-regulation of CK14 gene expression by mT in the LEC line T51B is dependent on the association with the c-Src tyrosine kinase, but interestingly, this c-Src-mediated action of mT can occur in the absence of transformation, Furthermore, when coupled with recent data on the plasticity of LECs, the present findings provi de the first essential element in our definition of the signaling path way(s) that link growth/differentiation events with CK gene regulation in typical simple epithelial cells.