c-Jun N-terminal kinase activation of activator protein-1 underlies homologous regulation of the gonadotropin-releasing hormone receptor gene in alpha T3-1 cells.

Reproductive function is dependent on the interaction between GnRH and its cognate receptor found on gonadotrope cells of the anterior pituitary gland. GnRH activation of the GnRH receptor (GnRHR) is a potent stimulus for increased expression of multiple genes including the gene encoding the GnRHR itself. Thus, homologous regulation of the GnRHR is an important mechanism underlying gonadotrope sensitivity to GnRH. Previously, we have found that GnRH induction of GnRHR gene expression in alpha T3-1 cells is partially mediated by protein kinase C activation of a canonical activator protein-1 (AP-1) element. In contrast, protein kinase A and a cAMP response element-like element have been implicated in mediating the GnRH response of the GnRHR gene using a heterologous cell model (GGH(3)). Herein we find that selective removal of the canonical AP-1 site leads to a loss of GnRH regulation of the GnRHR promoter in transgenic mice. Thus, an intact AP-1 element is necessary for GnRH responsiveness of the GnRHR gene both in vitro and in vivo. Based on in vitro analyses, GnRH appeared to enhance the interaction of JunD, FosB, and c-Fos at the GnRHR AP-1 element. Although enhanced binding of cFos reflected an increase in gene expression, GnRH appeared to regulate both FosB and JunD at a posttranslational level. Neither overexpression of a constitutively active Raf-kinase nor pharmacological blockade of GnRH-induced ERK activation eliminated the GnRH response of the GnRHR promoter. GnRH responsiveness was, however, lost in alpha T3-1 cells that stably express a dominant-negative c-Jun N-terminal kinase (JNK) kinase, suggesting a critical role for JNK in mediating GnRH regulation of the GnRHR gene. Consistent with this possibility, we find that the ability of forskolin and membrane-permeable forms of cAMP to inhibit the GnRH response of the GnRHR promoter is associated with a loss of both JNK activation and GnRH-mediated recruitment of the primary AP-1-binding components.

Effects of post-exercise supplements on glycogen repletion in skeletal muscle.

Post-exercise carbohydrate supplementation has been routinely used to enhance glycogen concentrations in skeletal muscle, particularly during multiple-day athletic events. Consumption of protein hydrolysates mixed with carbohydrate supplements during the post-exercise period may increase insulin response and cause glycogen repletion in skeletal muscle. A group of Alaskan sled dogs were selected to examine post-exercise supplementation in a paired crossover study design. The dogs were subjected to the same exercise regimen and provided one of three treatments-water, glucose polymers, or glucose polymers with protein hydrolysates-over a 2-month period. Parameters tested at various post-exercise time points included plasma insulin, glucagon and glucose concentrations, and skeletal muscle glycogen content to gain a better understanding of glucose metabolism and glycogen repletion. The results showed an enhanced insulin, glucose, and glucagon response immediately after supplementation and significantly increased glycogen concentrations in skeletal muscle within 24 hours when dogs received either of the glucose-containing supplements compared with water alone. There were no differences in the plasma parameters or skeletal muscle glycogen stores in dogs provided the glucose polymers alone or the glucose polymers plus protein hydrolysates. Thus, post-exercise carbohydrate supplementation increased muscle glycogen repletion, but inclusion of protein hydrolysates in the supplements provided no additional benefits.