HORMONAL-CONTROL OF INSULIN-LIKE GROWTH-FACTOR-I AND GROWTH-HORMONE RECEPTOR MESSENGER-RNA EXPRESSION BY PORCINE HEPATOCYTES IN CULTURE

The effects of various hormones commonly added to hepatocyte culture media upon the expression of the GH receptor (GHR) and insulin-like growth factor-I (IGF-I) genes in cultured porcine hepatocytes were investigated. Preliminary investigations indicated that there was an absolute requirement only for insulin, with high losses of cell viability upon long term exclusion of insulin from the culture medium. The decline in GHR expression with time in culture was found to be less when high levels of glucose were included in the medium. Therefore the basal culture medium used in these studies was Williams' medium E supplemented with 0.2% (w/v) BSA, 5000 mg glucose/l and 100 nmol porcine insulin/l. The addition of dexamethasone (100 nmol/l) increased the expression of both GHR and IGF-I (class 1 transcripts only) mRNA (P<0.001 and P<0.05 respectively), and resulted in an increased responsiveness of IGF-I mRNA expression to GH (1 mu g/ml), when the two were added in combination (although only class 1 transcripts were shown to be statistically significant, P<0.01). The addition of either thyroid hormone (1 nmol/l T-3 or T-4) alone also increased the expression of GHR mRNA (P<0.01) in addition to the dexamethasone stimulated expression, with T-4 appearing to decrease IGF-I expression slightly (P<0.05) (either on its own or with T-3). As with dexamethasone, the thyroid hormones increased the response of IGF-I mRNA expression to GH (1 mu g/ml) when added in combination with GH (P< 0.001). These observations demonstrate one possible mechanism for the interactions of glucocorticoids and thyroid hormones with the GH-IGF axis.