ABSTRACT
In the present study, the effects of 17beta-estradiol (E(2)) treatment on the expression of preprosomatostatin (PPSS) I, PPSS II', and PPSS II" mRNA in the hypothalamus and endocrine pancreas (Brockmann body), as well as the effects of E(2) treatment on plasma somatostatin (SS)-14 and -25 concentrations in sexually immature rainbow trout (Oncorhynchus mykiss), were investigated. E(2) treatment significantly (P < 0.001) depressed both plasma SS-14 and SS-25. In the hypothalamus, E(2) treatment significantly (P < 0.001) decreased the levels of PPSS I and PPSS II" mRNA. However, there was no effect of E(2) treatment on PPSS II' mRNA levels. In the pancreas, E(2) treatment had no significant effect on the levels of either PPSS II' mRNA or PPSS II" mRNA. However, E(2) treatment significantly (P < 0.005) decreased levels of PPSS I mRNA. These data suggest that E(2) acts, in part, to increase plasma growth hormone levels in rainbow trout by decreasing the endogenous inhibitory somatostatinergic tone by inhibiting plasma levels of both SS-14 and SS-25 and hypothalamic levels of mRNA encoding these proteins.
Subject(s)
Estradiol/pharmacology , Gene Expression/drug effects , Oncorhynchus mykiss/genetics , Somatostatin/genetics , Animals , Estradiol/physiology , Gene Expression Regulation/physiology , RNA, Messenger/metabolism , Somatostatin/bloodABSTRACT
We previously characterized two cDNAs that encode for distinct preprosomatostatin molecules containing [Tyr(7), Gly(10)]-somatostatin-14 at their C-termini (PPSS II' and PPSS II") and found that these cDNAs were differentially expressed in the endocrine pancreas (Brockmann body) of rainbow trout, Oncorhynchus mykiss. In this study, we examined the control of PPSSII' mRNA and PPSS II" mRNA expression by glucose. Fish injected with glucose displayed elevated plasma levels of glucose in association with nearly three-fold higher levels of PPSS II mRNAs compared to saline-injected control animals. Glucose directly stimulated the expression of both PPSS II mRNAs in vitro in a dose-dependent manner; however, glucose was a more potent stimulator of PPSS II" expression than of PPSS II' expression. The hexoses, mannose, galactose, and fructose, as well as glucose, all induced the expression of PPSS II mRNAs, whereas, sucrose and the glucose analogs, 3-o-methylglucose and 2-deoxyglucose, were without effect. In addition, the expression of PPSS II mRNAs was stimulated by dihydroxyacetone, pyruvate, lactate, acetate, and citrate. Furthermore, the expression of PPSS II mRNAs was inhibited by iodoacetate, an inhibitor of glycolysis, but was stimulated by dichloroacetate, a stimulator of Krebs cycle flux via pyruvate dehydrogenase activation. Finally, glucose-stimulated PPSS II expression was inhibited by actinomycin. These results indicate that the expression of PPSS II mRNAs in the Brockmann body of trout is regulated by nutrients such as glucose and suggest that glucose-stimulated expression of PPSS II mRNAs requires the uptake and subsequent metabolism of the sugar and is transcription sensitive.
Subject(s)
Glucose/pharmacology , Islets of Langerhans/metabolism , Oncorhynchus mykiss/metabolism , Protein Precursors/biosynthesis , RNA, Messenger/biosynthesis , Somatostatin/biosynthesis , Animals , Blood Glucose/metabolism , Citric Acid Cycle/drug effects , Dactinomycin/pharmacology , Female , Glycolysis/drug effects , Hexoses/pharmacology , Islets of Langerhans/drug effects , Male , Nucleic Acid Synthesis Inhibitors/pharmacology , Sucrose/pharmacologyABSTRACT
Rainbow trout were used as a model system to evaluate the role of glucose in regulating the expression of preprosomatostatin I. Glucose increased pancreatic levels of preprosomatostatin I mRNA in vivo, in concert with elevated plasma somatostatin levels, and in vitro. Glucose-stimulated expression of preprosomatostatin I mRNA required the uptake, phosphorylation, and subsequent metabolism of the sugar in pancreatic islets. These results suggest that glucose modulates both the production and release of somatostatin.
Subject(s)
Gene Expression Regulation/drug effects , Glucose/pharmacology , Pancreas/drug effects , Protein Precursors/genetics , Somatostatin/genetics , Animals , Oncorhynchus mykiss , Pancreas/metabolism , RNA, Messenger/geneticsABSTRACT
Previously, we isolated a 624-bp cDNA encoding for a 115-amino acid preprosomatostatin containing [Tyr7,Gly10]-somatostatin (SS)-14 (now designated PPSS-II') obtained from the endocrine pancreas (Brockmann bodies) of rainbow trout. In this study we have characterized a second cDNA obtained from trout pancreas that is 600-bp in length and encodes for a 111-amino acid precursor containing [Tyr7,Gly10]-SS-14 (PPSS-II''). The nucleotide and amino acid identity between the two cDNAs is 82.3 and 80.5%, respectively. Both PPSS-II' and PPSS-II'' mRNA were present in esophagus, pyloric ceca, stomach, upper and lower intestine, and pancreas, whereas only SS-II" mRNA was present in brain. PPSS-II'' mRNA was more abundant than PPSS-II' mRNA in pancreas, whereas PPSS-II' mRNA was more abundant than PPSS-II" mRNA in stomach. Fasting increased pancreatic PPSS-II'' mRNA levels but had no effect on the levels of PPSS-II' mRNA. These results indicate the existence of two nonallelic pancreatic SS-II genes that are differentially expressed, both in terms of distribution among tissues and in terms of relative abundance within the tissues.
