REGULATION OF 11-BETA-HYDROXYSTEROID DEHYDROGENASE TYPE-1 IN PRIMARY CULTURES OF RAT AND HUMAN HEPATOCYTES

Two isozymes of the enzyme 11 beta-hydroxysteroid dehydrogenase (11 be ta-HSD) are responsible for the interconversion of the active glucocor ticoid, cortisol in man, (corticosterone in the rodent), to the inacti ve 11-keto metabolite, cortisone (11-dehydrocorticosterone). We have e xamined the regulation of type 1 11 beta-HSD (11 beta-HSD1) using prim ary cultures of rat and human hepatocytes, both of which express only 11 beta-HSD1. Only 11 oxo-reductase activity could be demonstrated in cultured hepatocytes (apparent K-m for cortisone 382 +/- 33 nM in huma n hepatocytes, apparent K-m for 11-dehydrocorticosterone 14.6 +/- 1.5 mu M in rat hepatocytes). There exists a marked discrepancy between 11 beta-HSD oxo-reductase activity and 11 beta-HSD1 mRNA levels in cultu red human hepatocytes and human liver. Thus oxo-reductase specific act ivity is much higher in the cultured hepatocytes (7.2 +/- 0.01 nmoles cortisol/mg/h vs 0.89 +/- 0.06 for whole liver homogenates) whilst the converse is true for steady state 11 beta-HSD1 mRNA levels (0.78 +/- 0.02 vs 1.94 +/- 0.07 in whole liver, 11 beta-HSD1/18S expressed as ar bitrary units). Carbenoxolone has a significant inhibitory effect on 1 1 oxo-reductase activity in both rat and human hepatocytes. However, t here is clear species-specific regulation of 11 oxo-reductase activity by thyroid hormone (tri-iodothyronine (T-3)), which increases 11 oxo- reductase activity in rat hepatocytes but has no effect on activity in human hepatocytes, and progesterone which inhibits activity in human hepatocytes, but has no effect on activity in rat hepatocytes. Neither T-3 nor progesterone altered 11 beta-HSD1 mRNA levels. A series of gr owth factors (hepatocyte growth factor, epidermal growth factor, basic fibroblast growth factor, transforming growth factor beta(1)) were wi thout effect on 11 oxo-reductase activity in cultured rat hepatocytes. In contrast to homogenates of human liver, cultured hepatocytes expre ss only 11 beta-HSD oxo-reductase activity. This is inhibited by carbe noxolone and shows species-specific regulation by Tg and progesterone. Growth factors do not appear to regulate activity or expression of 11 beta-HSD1. The discrepant enzyme activity data and 11 beta-HSD1 mRNA expression in hepatocytes and whole liver could reflect unstable 11 be ta-HSD1 oxo-reductase activity or, alternatively, an additional 11 bet a-HSD oxo-reductase isoform in cultured hepatocytes.