JAM-A Knockdown in HepG2 Cells

Hepatocytes are polarized epithelial cells whose function depends upon their ability to distinguish between the apical and basolateral surfaces that are located at intercellular tight junctions. It has been proposed that the signaling cascades that originate at these junctions influence cellular activity by controlling gene expression in the cell nucleus. To assess the validity of this proposal with regard to hepatocytes, we depleted expression of the tight junction protein junctional adhesion molecule-A (JAM-A) in the HepG2 human hepatocellular carcinoma cell line. Reduction of JAM-A resulted in a striking change in cell morphology, with cells forming single-layered sheets instead of the normal multi-layered clusters. In the absence of JAM-A, other tight junction proteins were mislocalized, and canaliculi, which form the apical face of the hepatocyte, were consequently absent. While most changes in gene expression were modest, there was a strong transcriptional induction of the adherens junction protein E-cadherin in cells with reduced levels of JAM-A. This increase in E-cadherin was partially responsible for the observed alterations in cell morphology and mislocalization of tight junction proteins. We therefore propose that we have uncovered a novel mechanism for crosstalk between specific components of tight and adherens junctions that can be utilized to regulate adhesion between hepatic cells and to maintain hepatocyte cell polarity.

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Konopka G, Tekiela J, Iverson M, Wells C, Duncan SA