Growth hormone acts on the synthesis and secretion of α-casein in bovine mammary epithelial cells

To study the effect of growth hormone (GH) on the functions of mammary epithelia, we examined the effect of GH on the synthesis and secretion of α-casein in a bovine mammary epithelial cell (BMEC) clonal line, which was established from a 26-d-pregnant Holstein heifer. GH receptors (GHR) were observed in the BMEC on the membrane as well as in the cytoplasm. After BMEC were plated onto cell culture inserts, GH stimulated α-casein release in both the presence and absence of the lactogenic hormone complex, which included dexamethasone, insulin and prolactin (DIP). DIP enhanced the effect of GH on α-casein release. Although α_s1-casein mRNA expression was not detected in untreated control cells, its expression was observed in BMEC in response to the GH, DIP and GH + DIP treatments. Expression was greater for GH and GH + DIP than for just DIP. Expression of GHR mRNA was increased by DIP treatment, while the mRNA expression was little changed by GH treatment. We conclude that GH acts on BMEC and induces the expression of both the α-casein gene and protein. GHR gene expression was shown to be regulated by DIP and GHR. GHR may be involved in a synergic effect between GH and DIP on casein secretion. These results suggest that GH, in addition to its widely accepted homeorhetic role in vivo, also can act on the mammary parenchyma, and that the effects of GH on mammary epithelial cells could partly account for the clear galactopoietic effect of recombinant bovine GH seen in lactating dairy cows.