Studies on the biosynthesis of alpha2u-globulin and albumin in rat liver

Abstract

a2u-globulin and albumin are secretory proteins synthesised in rat liver parenchymal cells and appear as major components in urine and serum, respectively. The syntheses of both proteins are under complex hormonal control. Sex-hormones appear to be particularly important in regulating a2u-globulin synthesis since a2u-globulin is normally only found in adult male rats. The intracellular site and mode of a2u-globulin synthesis were studied and a hormone-responsive hepatocyte in vitro system for studying the effects of hormones on protein synthesis was developed. Newly synthesised a2u-globulin and albumin were recognised by immunoprecipitation and SDS-polyacrylamide gel analysis of radiolabelled products. Specific antibodies were raised in rabbits to the purified proteins. Rat hepatic polysomes were isolated and used to direct translation in a polysome-free reticulocyte cell-free system. Identification of the in vitro products showed that a2u-globulin and albumin were synthesised exclusively on the membrane-bound polysomes. a2u-globulin-mRNA and albumin-mRNA were identified in the poly(A)- and non poly(A)-containing RNA fractions isolated from total hepatic RNA preparations, respectively. The molecular weights of [35S]-labelled a2u-globulin from RNA or polysome directed in vitro systems and [3H]-labelled a2u-globulin secreted from hepatocytes were compared by co-electrophoresis of the appropriate immuno-precipitates on 15 - 18% SDS polyacrylamide gels. The slight, although consistent, higher molecular weight found for the in vitro product suggests that a2u-globulin, like other secretory proteins, may involve a precursor. Rat hepatocyte cell suspensions prepared by collagenase treatment of perfused livers incorporated radiolabelled amino acids linearly into proteins for at least seven hours. The relative rates of albumin synthesis were similar in hepatocytes isolated from males, females, castrated males and ovariectomised females. a2u-globulin synthesis was found only in cells from males, and at reduced rates in those from castrated males and androgen-treated ovariectomised females. 5a-dihydrotestosterone failed to stimulate a2u-globulin, albumin and total protein synthesis in hepatocytes from castrated males and ovariectomised females but specifically doubled the rate of a2u-globulin synthesis in cells from androgen-pretreated, ovariectomised females. Although a2u-globulin synthesis rates could be stimulated or inhibited in hepatocytes from males by oestradiol-17B, depending on concentrations, these effects were non-specific. Inhibitor studies suggested the involvement of microtubules in the secretion of a2u-globulin and albumin from hepatocytes.