INDUCTION OF MDR1B MESSENGER-RNA AND P-GLYCOPROTEIN EXPRESSION BY TUMOR-NECROSIS-FACTOR-ALPHA IN PRIMARY RAT HEPATOCYTE CULTURES

Mammalian liver exhibits expression of members of the family of multid rug resistance (mdr) transporters (P-glycoproteins). P-glycoprotein is oforms encoded by mdr1 genes participate in extrusion of an array of x enobiotics into the bile, Induction of mdr1b mRNA expression has been shown to occur in rat hepatocytes in response to hepatotrophic growth factors. As the cytokine tumor necrosis factor alpha (TNF-alpha) is kn own to exert a direct mitogenic effect on hepatocytes, its influence o n mdr1b expression was investigated. In primary rat hepatocytes cultur ed in the absence of TNF-alpha, a time-dependent increase in basal exp ression of mdr1b mRNA and in immunodetectable P-glycoprotein was obser ved. In cells created with TNF-alpha (4,000 U/ml) for 3 days, expressi on of mdr1b mRNA and of immunodetectable P-glycoprotein was induced ap proximately twofold. Moreover, intracellular steady-stale levels of th e mdr1 substrate rhodamine 123 were decreased in cells pretreated with TNF-alpha in comparison to controls, indicating an increase in functi onal transporter(s) mediating dye extrusion. Treatment of hepatocytes with antioxidants (1 mM ascorbic acid and 2% dimethyl sulfoxide) for 3 days markedly suppressed mdr1b mRNA and P-glycoprotein expression bot h in cells cultured in the presence of TNF-alpha and in the absence of the cytokine, but did not fully abolish mdr1b mRNA induction by TNF-a lpha, supporting the notion that reactive oxygen species participate i n regulation of basal mdr1b gene expression during hepatocyte culture. In conclusion, the present data indicate that by inducing mdr1b expre ssion in hepatocytes, TNF-alpha may affect the capacity of the liver f or extrusion or detoxification of endogenous or xenobiotic mdr1 substr ates. Cell. Physiol. 176:506-515, 1998. (C) 1998 Wiley-Liss, Inc.