17β-estradiol reduces milk synthesis and increases stanniocalcin gene expression in the mammary gland of lactating cows

Increasing amounts of estrogens in the blood of pregnant lactating cows may be one factor inducing the progressive involution of the mammary gland after the peak of lactation. The present study was undertaken to assess the effects of 17β-estradiol on milk production and to analyze its mechanism of action at the molecular level. Non-pregnant, mid-late lactation cows received daily sc. injections of either 17β-estradiol (treated, n = 4) or ethanol (controls, n = 4). Treated cows showed a reduction in dry matter intake and presented signs of mammary gland involution, as indicated by their lowered milk production and modified milk composition. Real-time RT-PCR, performed on RNA extracted from mammary biopsies, showed that 17β-estradiol reduced the mRNA levels of β-casein, did not affect those of Bax and Bcl-2 but increased those of stanniocalcin, a hormone that regulates epithelial calcium and phosphate transport. Moreover, concentration of stanniocalcin was increased in the milk and serum of treated cows. Thus, our results indicate that the effect of 17β-estradiol on milk production is not mediated by Bax or Bcl-2 while they suggest a possible role for stanniocalcin in mammary involution.