br Acknowledgments This work was supported by
Acknowledgments This work was supported by the NIH (R01DK103884 to JMZ, R01DK100659 to JKE, and F32DK104659 and K01DK111644 to CMC), the Diana and Richard C. Strauss Professorship in Biomedical Research, the Mr. and Mrs. Bruce G. Brookshire Professorship in Medicine, the Kent and Jodi Foster Distinguished Chair in Endocrinology, in Honor of Daniel Foster, M.D., and institutional funds from the University of Texas Southwestern Medical Center to J.M.Z. Catecholamine assays were performed by the VUMC Hormone Assay and Analytical Services Core (supported by NIH grants DK059637 and DK020593).
Introduction Ghrelin, the natural ligand of the GH (growth hormone, GH) secretagogue receptor type 1a (GHSR-1a), is a 28-amino LDN193189 peptide with a peculiar n-octanoyl esterification at the Ser3 residue (Kojima et al., 1999). Several lines of evidence suggested that ghrelin acts through GHSR-1a, at hypothalamic and hypophyseal levels, to regulate GH release, energy metabolism, food intake, and cell proliferation (Hosoda et al., 2006, Rak and Gregoraszczuk, 2008, Sugino et al., 2004). Ghrelin is produced primarily in the oxyntic glands of the gastric mucosa, which is also the main source of circulating ghrelin (Ariyasu et al., 2001). However, ghrelin is also expressed in other tissues, including the ovary, placenta, and mammary gland (Grönberg et al., 2008, Harrison et al., 2007, Miller et al., 2005, Zhang et al., 2008). The functional receptor of ghrelin, GHSR-1a, is also expressed in a variety of tissues (Gaytan et al., 2003, Gnanapavan et al., 2002, Zhang et al., 2013). This wide tissue distribution suggests that locally produced ghrelin might exert autocrine/paracrine effects on different tissues (Korbonits et al., 2004). Recent studies have shown that ghrelin plays an important role in the regulation of mammary function. In lactating rats and dairy cows, ghrelin influenced the yield and composition of milk, and mammary β-casein expression (Nakahara et al., 2003, Roche et al., 2008). It has been reported that the expression of GHSR-1a mRNA is higher in the hypothalamus and pituitary gland of lactating animals than in those of non-lactating animals (Abizaid et al., 2008). GH and prolactin (PRL) appear to play an important role in mammary function in humans and animals (Etherton and Bauman, 1998, Flint et al., 1992, Milsom et al., 1992). In monogastrics and ruminants, ghrelin stimulates the release of GH and PRL acting on the hypothalamus and pituitary gland (Date et al., 2000, Iqbal et al., 2006, Tassone et al., 2003, Wren et al., 2000). Whether or not ghrelin and its receptor are expressed in the mammary gland of pregnant dairy goats is not known. Accordingly, no information is available concerning the direct role of ghrelin in the mammary function of dairy goats during the pregnancy. The mammary gland attains its maximum development during pregnancy. The development which occurs during this period is dependent upon estrogen, progesterone, and pituitary hormones. Studies showed that mammary growth and development is also regulated by the actions of systemic hormones and local growth factors (Flint and Knight, 1997). Of note, ghrelin has potent mitogenic effects in several cells including cardiomyocytes, adipocytes, endothelial cells, and Leydig cells (Baldanzi et al., 2002, Barreiro et al., 2004, Kim et al., 2004, Nagaya and Kangawa, 2003). A previous study has demonstrated that ghrelin promoted cell proliferation in porcine ovarian granulosa cells (Sirotkin and Meszarosova, 2010). Ghrelin’s affects on other cell types raise the possibility that the peptide may have a similar mitogenic effect on MECs as well under physiological circumstances. PRL is best known for the multiple effects it exerts on the mammary gland. It is involved in the growth and differentiation of the mammary gland (Ben-Jonathan et al., 1996, Bern and Nicoll, 1968). Studies showed that PRL is expressed in mammary epithelial cells of rat, human, and goat mammary gland (Fields et al., 1993, Le Provost et al., 1994, Steinmetz et al., 1993). The effect of ghrelin on pituitary PRL secretion has been demonstrated in humans, rats, and dairy goats (Takaya et al., 2000, Tena-Sempere et al., 2004, Zhang et al., 2009). However, little is known regarding the role of ghrelin in the regulation of mammary PRL expression. Whether or not the ghrelin system is developmentally regulated in the mammary gland of dairy goats, and whether ghrelin interacts with other paracrine regulators of mammary growth such as PRL are unclear. In the present study we examined the potential role of ghrelin in regulating mammary growth and development of pregnant diary goats in vitro.