The Role of Exogenous Somatotropin, Ovariectomy and Extracellular Matrix in Bovine Mammary Gland Development

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Virginia Tech

The highly regulated maturation of the mammary gland is poorly understood. Our studies were designed to further characterize the role of ovarian hormones, growth hormone (GH)/IGF axis proteins and extracellular matrix (ECM) in the growth and development of prepubertal mammary glands. Prepubertal heifers were injected with either exogenous GH or subjected to ovariectomy (OVX). Mammary parenchyma (PAR) and mammary fat pad (MFP) were harvested for DNA, protein, lipid, and western blot analysis. Remaining tissues were preserved for histological staining or snap frozen for quantitative real-time PCR. We examined 13 genes that work in conjunction with the extracellular matrix to regulate mammary proliferation and morphogenesis. Administration of GH, while impacting composition of MFP, had no effect on expression of the selected genes; there was a decrease in expression of fibronectin in PAR. Ovariectomy had no effect on gene expression in MFP but decreased expression of epimorphin, a potent regulator of morphogenesis, in PAR. In both experiments, the presence of a 55 kDa band corresponding to androgen converting enzyme aromatase was detected but its expression was unaffected. In another study, we used in vitro cell culture to evaluate the role of ECM in mammary gland maturation and employed quantitative real-time PCR to evaluate gene expression profiles of select genes involved in proliferation and differentiation. Expression of Rac1 was decreased in response to bovine insulin (BI) but increased on collagen I (Col). Expression of aldehyde dehydrogenase was decreased in BI and serum on plastic and on Col in the presence of BI. Expression of IGF binding proteins (BP) 3, -4, and -6 were decreased in the presence of serum on laminin (LM). Also, IGF-BP2 expression was decreased on Col while IGF-BP6 was increased on LM with BI. Clusterin, a ubiquitous non-adhesive ECM protein was not affected by ECM substrate but did increase over time. In conclusion, we propose that the mammary gland is not able to respond to GH at this age and that while OVX did effect the expression of some genes, the presence of aromatase maintained local estrogen concentrations. Furthermore, ECM alone is insufficient to regulate mammary gland development and growth.

growth hormone, MAC-T, heifer, mammary, ovariectomy