POSTTRANSCRIPTIONAL MECHANISMS ACCOUNT FOR DIFFERENCES IN STEADY-STATE LEVELS OF INSULIN-RECEPTOR MESSENGER-RNA IN DIFFERENT CELLS

Previous studies have indicated that, in general, the insulin receptor gene is expressed at a level in cells reflecting the level of insulin receptors on the cellular surface. For instance, insulin-responsive tissues, such as hepatocytes, express high levels of both insulin receptor protein and mRNA relative to less responsive cells, such as fibroblasts. Moreover, in the cells of a patient (Minn 1) with severe insulin resistance and very low levels of insulin receptors, it has been shown that insulin receptor gene transcripts are virtually undetectable. Our earlier studies of the insulin receptor gene promoter suggested that the differences in the steady state level of insulin receptor gene transcripts in different cells could be transcriptionally mediated. In this study we have attempted to assess the relative contribution of transcriptional and posttranscriptional mechanisms in determining steady state levels of insulin receptor mRNA in various cells, including Minn 1 fibroblasts. Using nuclear run-on assays, we have determined that the level of nascent insulin receptor gene transcripts is roughly equal in different cells, including Minn 1 fibroblasts. Therefore, transcriptional differences do not seem to account for the dramatic differences in steady state levels of insulin receptor mRNA in different cells, and there is no evidence in support of a transcriptional defect in Minn 1's insulin receptor gene alleles. However, the rate of insulin receptor mRNA turnover varies significantly in different cells, ranging from an mRNA half-life of as little as 2 h in fibroblast and IM9 (lymphocytic) cells up to 8 h in HepG2 (liver) cells, and accounts in part for the observed differences in steady state insulin receptor mRNA. In addition, other posttranscriptional control mechanisms are likely to be important in establishing the level of processed cytoplasmic insulin receptor mRNA.