HORMONAL-REGULATION OF ALBUMIN GENE-EXPRESSION IN PRIMARY CULTURES OFRAT HEPATOCYTES

When primary cultures of rat hepatocytes were placed in a chemically d efined serum-free medium containing a combination of insulin, glucagon , and dexamethasone, the synthesis of albumin and total protein and th e cellular content of RNA and DNA were maintained at constant values f or 8 days. Despite the constant rate of albumin synthesis, secretion o f the protein increased more than twofold during the initial 4 days in culture and was then maintained at a value similar to that observed i n vivo through day 8. This observation suggested an initial defect in albumin secretion that was corrected with time in culture. Deprivation of insulin between days 2 and 5 resulted in a decline in albumin secr etion to similar to 40% of the control value. The decline in albumin s ecretion was accompanied by proportional decreases in albumin synthesi s, albumin mRNA, and albumin gene transcription. Return of insulin-dep rived cells to complete medium on day 5 restored albumin synthesis and secretion as well as albumin mRNA to control values by day 8. Depriva tion of either glucagon or dexamethasone also resulted in reduced albu min synthesis and secretion accompanied by proportional decreases in a lbumin mRNA and gene transcription. However, the magnitude of the chan ges in these parameters was less with glucagon or dexamethasone depriv ation compared with insulin deprivation. Return of glucagon- or dexame thasone-deprived cells to complete medium on day 5 restored albumin sy nthesis and secretion as well as albumin mRNA to control values by day 8. The data demonstrate that insulin, glucagon, and dexamethasone eac h act to regulate albumin gene expression in primary cultures of rat h epatocytes by modulating the rate of transcription of the albumin gene .