Post-Transcriptional Regulation of Casein Gene Expression by Prolactin and Growth Hormone in Lactating Rat Mammary Gland

Abstract

It is well established that prolactin (PRL) is a major lactogenic hormone. Lowering its serum concentration with bromocryptine (Br) results in a marked decrease in milk yield as reflected by a decrease in pup weight gain and by direct measurement. In addition growth hormone (GH) plays an increasingly important role in maintaining milk synthesis as lactation proceeds since combined ablation of PRL and GH (with an antiserum to rat GH (anti-rGH) reduces milk yield further (Barber et al., 1992) to around 20% of control animals. The mechanism by which these hormones regulate milk synthesis and secretion remains unclear. However, a number of experiments conducted in vitro with mammary tissue explants and isolated cells has shown that expression of the genes for the major milk proteins, the caseins, are regulated by PRL at the level of transcription and mRNA stability (Guyette et al., 1979). The role of PRL in controlling the expression of casein mRNAs in vivo has not been addressed. Indeed casein mRNA abundances are relatively high per mammary cell in the rat during pregnancy at a time when serum prolactin is relatively low (Rosen and Barker, 1976).