Valproate and copper accelerate TRH-like peptide synthesis in male rat pancreas and reproductive tissues

Abstract

Treatment with valproate (Valp) facilitates the synthesis of TRH-like peptides (pGlu-X-Pro-NH 2) in rat brain where “X” can be any amino acid residue. Because high levels of TRH-like peptides occur in the pancreas and pGlu-Glu-Pro-NH 2 (Glu-TRH) has been shown to be a fertilization promoting peptide, we hypothesized that these peptides mediate some of the metabolic and reproductive side effects of Valp. Male WKY rats were treated with Valp acutely (AC), chronically (CHR) or chronically followed by a 2 day withdrawal (WD). AC, CHR and WD treatments significantly altered TRH and/or TRH-like peptide levels in pancreas and reproductive tissues. Glu-TRH was the predominant TRH-like peptide in epididymis, consistent with its fertilization promoting activity. Glu-TRH levels in the epididymis increased 3-fold with AC Valp. Phe-TRH, the most abundant TRH-like peptide in the pancreas, increased 4-fold with AC Valp. Phe-TRH inhibits both basal and TRH-stimulated insulin release. Large dense core vesicles (LDCV's) contain a copper-dependent enzyme responsible for the post-translational processing of precursors of TRH and TRH-like peptides. Copper (500 μM) increased the in vitro C-terminal amidation of TRH-like peptides by 8- and 4-fold during 24 °C incubation of homogenates of pancreas and testis, respectively. Valp (7 μM) accelerated 3-fold the processing of TRH and TRH-like peptide precursors in pancreatic LDCV's incubated at 24 °C. We conclude that copper, an essential cofactor for TRH and TRH-like peptide biosynthesis that is chelated by Valp, mediates some of the metabolic and reproductive effects of Valp treatment via acceleration of intravesicular synthesis and altered release of these peptides.