The overexpression of P-glycoprotein (Pgp) appears to be responsible for 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 regulation of Pgp gene expression (Lee et al., J. Cell. Physiol., 157: 392-402, 1993). We observed a dramatic and specific overexpression of class II Pgp as a function of the time in culture. This isoform of Pgp, which is expressed at a very low level in normal liver, has also been shown to be predominantly overexpressed in several models of rat liver carcinogenesis. In the present study, we have used nuclear run-on assays and mRNA decay studies to investigate the mechanism for the overexpression of class II Pgp in cultured hepatocytes. We conclude that an increased mRNA stability is the major factor involved in the increased expression of class II Pgp. Studies using various drugs indicate that the integrity of the cytoskeleton is important for the maintenance of high expression of class II Pgp. Disruption of the cytoskeleton in cultured hepatocytes with cytochalasin D did not affect the transcriptional 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 regulates class II Pgp mRNA stability. These findings have important implications for our understanding of the overexpression of class II Pgp during liver carcinogenesis.
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