Apoptosis and autophagy in involuting bovine mammary gland is accompanied by up-regulation of TGF-beta1 and suppression of somatotropic pathway

Regulation of mammary gland remodeling during the lactation cycle in cattle still remains unclear. The present study focused on the role of TGF-beta1 and somatotropic pathways proteins in control of the switch between survival and death of bovine mammary epithelial cells (MEC). Expressions of TGF-beta1, TGF-betaRII, IGF-IRalpha, IGF-IRbeta, GH-R, IGFBP-3, -4, and -5 in mammary tissue explants in Holstein-Fresian heifers (n = 7) and cows (n = 23) in early lactation (1-100 day), late lactation (200-260 day) and drying off (280-340 day) were compared with biochemical indices of apoptosis (caspase 3, 89 kDa fragment of PARP) and autophagy (Beclin1). The results revealed that an increase in apoptosis during the dry period was accompanied by highly significant increases in TGF-beta1 and TGF-betaRII expression. Beside biochemical markers, typical morphological features of apoptosis, such as cell shrinkage, separation from the neighboring cells and condensation of chromatin were observed. TGF-beta1 expression and induction of apoptosis was facilitated by the suppression of somatotropic pathway during drying off, manifested with down-regulation of GH-R and IGF-IRalpha, and up-regulation of IGFBP-4 and -5. This is the first report describing autophagy in the bovine mammary gland. Similarly to apoptosis, the intensity of autophagy was the highest in the dry period, as shown by increased expression of Beclin1 and morphological features, e.g. autophagosomes, autophagic vacuoles. Autophagy observed in the involuting mammary tissue could be the natural cell defense against transient undernourishment and action of apoptogenic peptides (e.g. TGF-beta1, IGFBPs), thus maintaining cellular homeostasis in the dry period.