Progesterone-mediated inhibition of casein mRNA and polysomal casein synthesis in the rat mammary gland during pregnancy

Abstract

The levels of casein mRNA, polysomal casein synthesis, and intracellular casein have been determined during mid-pregnancy in the rat mammary gland. Ovariectomy of the mid-pregnant rat and hormone replacement were used as a model for studying the regulation of casein synthesis. Following ovariectomy a twofold increase in both casein mRNA activity, determined in a wheat germ translation assay, and casein mRNA sequence concentration, measured using a selective cDNA hybridization probe, was observed. This effect on mRNA accumulation was first observed within 16 h following ovariectomy and was maximal between 24 and 48 h. Progesterone, but not estradiol or hydrocortisone, administered at the time of ovariectomy prevented the increase in casein m R N A . These changes in casein m R N A were compared with the mRNA levels normally observed during development of the rat mammary gland from virginity until lactation. A comparison of polysomes isolated from mid-pregnant and lactating mammary glands revealed a shift from small polysomes, containing predominantly monosomes, disomes, and trisomes E e v i o u s results from our laboratory have demonstrated that appreciable amounts of casein m R N A are present in the rat mammary gland during early and mid-pregnancy . Although secretion of casein does not occur at these times we found that at mid-pregnancy the level of casein mRNA per alveolar cell was 25 to 50% of the maximal levels reached during lactation. This increase in casein m R N A sequences, measured with a selective cDNA-hybridization probe, was coincident with an increase in casein m R N A activity, as measured by cell-free translation (Rosen and Barker, 1976). Thus the relationship between the levels of casein m R N A and the synthesis and secretion of casein are not understood, and the precise hormonal mechanisms regulating these processes remain unclear.The levels of casein mRNA observed during mid-pregnancy may rcflect both the positive and negative effects of several peptide and steroid hormones. Thus, the increase in casein mRNA activity seen during early and mid-pregnancy may be related to changes in serum prolactin and, especially in the rat, placental lactogen. These two polypeptide hormones have been shown to induce casein synthesis (Lockwood et al., 1966; Turkington, 1968) and casein mRNA (Houdebine and Gaye, 1975 both in vivo and in mammary gland organ culture. Furthermore, high levels of progesterone during pregnancy are thought to antagonize the lactogenic effects of these hormones (Davis et ai., 1972;Assairi et al., 1974). The present studies were therefore designed to investigate the relationship among casein m R N A levels, polysomal casein synthesis. and the intracellular concentrations of casein during mid-pregnancy in the rat.to larger polysomal aggregates, containing 7 to 1 1 ribosomes per mRNA. A similar shift was observed within 48 h following ovariectomy. Using specific immunoprecipitation, casein synthesis on isolated polysomes was measured in a cell-free translation assay. Casein comprised ...

Absolute requirement of glucocorticoid for expression of the casein gene in the presence of prolactin.

“…The pregnancy mammary parenchyma, rich in glucocorticoid receptor (26), is exposed to an increased level of circulating glucocorticoid in the pregnant animal (27). The glands also contain abundant casein and its mRNA (28,29). Consequently, preincubation in a medium with insulin and cortisol is required to reduce the endogenous mRNA.. in the explants (12,13).…”

Lactogenic Hormonal Induction of Long Distance Interactions between β-Casein Gene Regulatory Elements

“…1C). The maximal dissociation of YY-1 in cells treated with both hormones was observed by [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] (2). The association of p-pol II at both regulatory regions suggested either the possibility of a long range interaction between the ␤-casein promoter and enhancer or, alternatively, independent recruitment of RNA pol II and perhaps even the transcription of short RNAs at the distal enhancer.…”

Cloning and characterization of ovine αS1-casein gene promoter: A transfection study in rat mammary gland cell line

“…It had been shown that glucocorticoids, prolactin and 17b-estradiol were responsible for the induction of casein gene expression (Groenen and Poel 1994). Glucocorticoids were involved in the induction of casein gene expression (Hobbs et al 1982) while progesterone inhibited casein synthesis during pregnancy (Rosen et al 1978). The expression of milk protein genes is mediated through activation of hormone-responsive elements (HRE) in the promoter region of a gene.…”