OVEREXPRESSION OF THE CLASS-II P-GLYCOPROTEIN GENE IN PRIMARY RAT HEPATOCYTE CULTURE - EVIDENCE FOR INCREASED MESSENGER-RNA STABILITY

The overexpression of P-glycoprotein (Pgp) appears to be responsible f or multidrug resistance in some human cancers. The molecular basis of this overexpression is not understood. We have used primary monolayer cultures of adult rat hepatocytes as a model system to study the regul ation of Pgp gene expression (Lee ef al., J. Cell. Physiol., 157: 392- 402, 1993). We observed a dramatic and specific overexpression of clas s II Pgp as a function of the time in culture. This isoform of Pgp, wh ich is expressed at a very low level in normal liver, has also been sh own to be predominantly overexpressed in several models of rat liver c arcinogenesis. In the present study, we have used nuclear run-on assay s and mRNA decay Studies to investigate the mechanism for the overexpr ession of class II Pgp in cultured hepatocytes. We conclude that an in creased mRNA stability is the major factor involved in the increased e xpression of class II Pgp. Studies using various drugs indicate that t he integrity of the cytoskeleton is important for the maintenance of h igh expression of class II Pgp. Disruption of the cytoskeleton in cult ured hepatocytes with cytochalasin D did not affect the transcriptiona l activity of the class II Pgp gene but rapidly destabilized its mRNA. This raises the possibility that an association between class II Pgp mRNA and cytoskeletal elements may underlie the mechanism that regulat es class Il Pgp mRNA stability. These findings have important implicat ions for our understanding of the overexpression of class II Pgp durin g liver carcinogenesis.