The regulation of casein expression during lactation is a

The regulation of casein expression during lactation is a complex process involving a coordinated response at several levels. A variety of hormones are known to play roles in lactogenesis, such as prolactin (PRL), insulin, and hydrocortisone, and there is now increasing evidence that growth factors, such as epidermal growth factor (EGF), transforming growth factor (TGF), fibroblast growth factor and insulin-like growth factor (IGF) are being implicated as local mediators of hormonal function in mammary growth and lactogenesis (Baumrucker and Stemberger, 1989, Oka et al., 1991, Robinson et al., 1993). Our results showed that ghrelin was located in the epithelial THZ1 australia of mammary glands, and ghrelin dose-dependently increased the β-casein expression in the cultured mammary tissues and primary MECs. However, it remains unclear whether ghrelin acts directly on mammary glands to regulate other hormones or growth factors secretion. Studies shown that regulatory factors (PRL, IGF, EGF and TGF) for locally produced in mammary glands have potential role in regulation of casein synthesis. PRL in plasma is mainly produced in the pituitary gland, it is well known that PRL is the main hormone that regulates the onset and maintenance of lactation and the synthesis of milk proteins, fat and immunoglobulins (Jonathan et al., 1996, Tucker, 2000). Extrapituitary PRL has been reported to be produced in the mammary glands of various species, including rats, sheep and goats (Steinmetz et al., 1992, Fields et al., 1993, Provost et al., 1994). Circulating PRL levels in human are increased by infusion ghrelin (Tassone et al., 2003). Our previous study found that ghrelin could significantly up-regulated the expression of PRL mRNA in the pituitary of dairy goat (Zhang et al., 2009). Studies in rodents have shown that ghrelin inhibits PRL secretion, whereas in vitro treatment of pituitary cells with ghrelin did not inhibit PRL secretion (Tena-Sempere et al., 2004). These results suggest that ghrelin regulate PRL secretion in human and animals. However, whether ghrelin directly regulate PRL secretion in mammary glands has not been reported. IGF, EGF and TGF for locally produced growth factors in mammary glands have important role in regulation of milk proteins synthesis. Future studies will be directed towards the effects of ghrelin on the expression of PRL, IGF, EGF and TGF in mammary glands. In conclusion, we have demonstrated that ghrelin and GHSR-1a immunostaining is located in the mammary epithelial cells, expression of ghrelin and GHSR-1a (mRNA and protein) in mammary glands are initiated in L30 to L180 in parallel with the physiological function of β-casein expression and milk production, continues throughout lactation. Ghrelin acts on cultured mammary explants and primary MECs to stimulate the β-casein expression, and the stimulative effect of ghrelin on β-casein expression in vitro was blocked by D-Lys3-GHRP6. Future studies will be directed towards the cellular and molecular mechanism by which ghrelin exerts its effects on β-casein expression and, perhaps, on milk production and mammary development in the mammary glands of dairy goat.
Acknowledgments This work was supported by the China Postdoctoral Science Foundation (20090451402) and Sci-Tech Innovation Project of Shaanxi Province (2010k01-8-1).
Introduction Ghrelin, the endogenous ligand for the growth hormone secretagogue receptor (GHSR) or ghrelin receptor, is mainly secreted by the stomach in response to fasting, but it is also synthesized locally in the hypothalamus and various peripheral tissues. Ghrelin stimulates growth hormone release via a dual action on hypothalamic growth hormone‐releasing hormone cells and on pituitary somatotrophs (Korbonits et al., 2004). Ghrelin was also shown to have an orexigenic effect and to regulate energy homeostasis, leading to increased body weight and adiposity, in rodents (Tschop et al., 2000). The ghrelin stimulation of appetite involves a complex hypothalamic action through stimulation of the orexigenic neuropeptide Y/agouti-related protein- and orexin-producing neurons in the hypothalamus (Cowley et al., 2003, Korbonits et al., 2004). Although knockout animals for ghrelin and GHSR showed relatively minor changes in growth, weight and food intake under basal conditions (Sun et al., 2003, Wortley et al., 2004), more recent studies have reported that either ghrelin or GHSR deficiency leads to a resistance to diet-induced obesity (Asakawa et al., 2003, De Smet et al., 2006, Shearman et al., 2006, Vizcarra et al., 2007, Wortley et al., 2005, Zigman et al., 2005). A few studies have reported a linkage between the ghrelin locus or GHSR locus and obesity-related parameters (Luke et al., 2003, Moslehi et al., 2003, Steinle et al., 2002, Wu et al., 2002). A GHSR haplotype was also associated with an increased risk of myocardial infarction (Baessler et al., 2007). Chronically, low circulating ghrelin levels are found in obese patients compared to normal subjects, and also in subjects with insulin resistance and thus high insulin levels: this is probably explained by a direct effect of insulin (Poykko et al., 2003, Purnell et al., 2003). Eating disorders include a broad spectrum of patterns and several have been associated with obesity, for example “snacking” or “grazing” (frequent small meals, producing an almost continuous eating pattern), binge eating (sudden periodic large amounts of food intake), “overeating” (unusually large amount of food consumed at meal times), and night eating (Adair and Popkin, 2005, Cooper and Fairburn, 2003). A reduction in ghrelin levels after food intake is blunted in patients with bulimia nervosa compared to BMI-matched healthy controls (Kojima et al., 2005, Monteleone et al., 2003). Ghrelin is a 28 amino-acid peptide with a fatty acid chain modification on the N-terminal third amino acid by the ghrelin O-acyltransferase enzyme (Yang et al., 2008). Ghrelin and obestatin are encoded by the same gene and propeptide, but different post-translational processes generate two peptides with opposing functions; obestatin was originally proposed to counteract the effects of ghrelin on food intake (Zhang et al., 2005), although ghrelin's original function was depicted in terms of a strong growth hormone releasing effect (Kojima et al., 1999, Seoane et al., 2000). Five novel single nucleotide polymorphisms (SNPs) were detected in the bovine GHSR gene, indicating that the GHSR genotype was significantly associated with birth weight and average daily gain up to 6months of age (Zhang et al., 2009). A general review of the available genomic information uncovers a candidate gene for genetic breeding encoding the GHSR. Polymorphism in the GHSR gene, however, has not been depicted previously in sheep. We therefore screened most exons of the sheep GHSR gene to find sequence variations that might have an effect on protein structure and function and might be related to growth and carcass traits. As well, it is worth mentioning that there has not been any publication regarding the effect of protein structure on production traits.