Lactogenic hormones regulate xanthine oxidoreductase and beta-casein levels in mammary epithelial cells by distinct mechanisms

Xanthine oxidoreductase (XOR) is a prominent component of the milk lipid gl obule, whose concentration is selectively increased in mammary epithelial c ells during the transition from pregnancy to lactation, To understand how X OR expression is controlled in the mammary gland, we investigated its prope rties and regulation by lactogenic hormones in cultured HC11 mammary epithe lial cells. XOR was purified as the NAD(+)-dependent dehydrogenase by benza midine-Sepharose chromatography and was shown to be intact and to have bioc hemical properties similar to those of enzyme from other sources, Treating confluent HC11 cells with prolactin and cortisol produced a progressive, fo ur- to fivefold, increase in XOR activity, while XOR activity in control ce lls remained constant, Elevated cellular XOR activity was correlated with i ncreased XOR protein and was due to both increased synthesis and decreased degradation of XOR, Prolactin and cortisol increased XOR protein and mRNA i n the presence of epidermal growth factor, which blocked the stimulation of p-casein synthesis by these hormones. Further, hormonal stimulation of XOR was inhibited by genistein (a protein tyrosine kinase inhibitor) and by PD 98059 (a specific inhibitor of the MAP kinase cascade). These findings ind icate that lactogenic hormones stimulate XOR and p-casein expression via di stinct pathways and suggest that a MAP kinase pathway mediates their effect s on XOR, Our results provide evidence that lactogenic hormones regulate mi lk protein synthesis by multiple signaling pathways. (C) 2000 Academic Pres s.