Ghrelin is expressed in the pregnant mammary glands of dairy goats and promotes the cell proliferation of mammary epithelial cells

Little is known about ghrelin's effects on cell proliferation in pregnant mammary epithelial cells (MECs) even though it is known to be a mitogen for a variety of other cell types. The objectives of this study were to evaluate the expression and localization of ghrelin and its functional receptor, GHSR-1a, in the mammary glands of dairy goats during pregnancy and to investigate the direct role of ghrelin in cell proliferation of primary cultured MECs. Compared to the early stage (days 30) of pregnancy, the abundance of transcripts and protein of ghrelin and GHSR-1a were significantly greater in mid- and late-phases (between days 90 and days 120) of pregnancy (p < .05). Immunohistochemistry analysis showed that ghrelin and GHSR-1a were predominantly localized in the alveolar and ductal mammary epithelial cells at various stages of pregnancy. In our in vitro experiments, ghrelin induced a dose- and time-dependent promotory effect on cell proliferation of MECs. At the dose of 103 pg/mL treatment 24 h, ghrelin augmented the expression of proliferation-related peptides (PCNA and cyclin B1). Furthermore, ghrelin promoted the expression of prolactin (PRL) and GHSR-1a in cultured MECs. Additionally, the stimulatory effects of ghrelin were blocked by d-Lys3-GHRP6, a selective antagonist of GHSR-1a. As the temporal changes in ghrelin and GHSR-1a expression in pregnant goat mammary glands coincided with the mammary growth and development during the pregnancy, activation of GHSR-1a signal transduction pathways by ghrelin may play a direct role in the regulation of mammary growth in dairy goats.