Program Director/Principal Investigator (Last, First, Middle): Napoli, Joseph. L. This research has long-term goals to determine 9-cis-retinoic acid (9cRA) physiological function and mechanisms of action. 9cRA has diverse pharmacological actions, which have prompted evaluation of analogs (rexinoids) for treating metabolic disease. Yet, 9cRA has not been detected in vivo in liver, serum, brain, adipose et al. with a sensitive LC/MS/MS assay, leaving its status unsettled (autacoid or drug?). The data here identify 9cRA in vivo (only in pancreas), show that 2-cells biosynthesize 9cRA, and that glucose induces mRNA of the cis-retinoid reductase Dhrs3 (4-fold) and represses mRNA of the cis-retinoid dehydrogenase Rdh5 (10-fold) in 832/13 cells. We also show that 9cRA decreases insulin secretion from isolated mouse islets by 70% within 15 min, and in the 2-cell line 832/13 by reducing Glut2 and glucokinase activities, ATP and calcium. Dosing 9cRA impairs glucose tolerance in vivo and reduces serum insulin during a glucose tolerance test. 9cRA decreases in the transition from the fasted to the fed state, and glucose dosing reduces 9cRA in vivo >80% within 15 min. These data support a model in which 9cRA and glucose work in opposition to regulate glucose-stimulated insulin secretion, and indicate non-genomic mechanisms of action for 9cRA (in addition to genomic mechanisms of action). In the longer term, 9cRA decreases expression of at least two genes crucial for pancreas development and insulin secretion: Pdx-1 and HNF41. Notably, this shows that at least two of the six monogenic diseases known as maturity onset diabetes of the young (MODY) are caused by defects in genes regulated by 9cRA, i.e. defects in Pdx-1 and HNF41 underlie MODY4 and MODY1, respectively. A defect in the gene that encodes glucokinase underlies a third MODY, i.e. MODY 2: 9cRA modulates glucokinase activity. These data validate 9cRA as a naturally occurring retinoid with physiological function(s) unique among retinoids, and identify a novel component of 2-cells that contributes to insulin secretion. This project will test 9cRA function as an autacoid that attenuates insulin secretion during low glucose to prevent hypoglycemia.
The specific aims are to determine: 1) physiological function(s) of 9cRA in short-term regulation of glucose stimulated insulin secretion;2) mechanisms of rapid 9cRA actions that diminish insulin secretion;3) whether 9cRA requires RAR for non-genomic and/or genomic actions in the 2- cell;4) functions of Dhrs3 and Rdh5 in 2-cell 9cRA biosynthesis.
Aim 1 will test the hypothesis that modest increases in 9cRA desensitize the pancreas to glucose, and that increasing blood glucose overcomes ability of 9cRA to impair insulin secretion.
Aim 2 will test the hypothesis that 9cRA functions through post-translational modification of Glut2 and GK.
Aim 3 will test the hypothesis that non-genomic and genomic actions of RAR mediate the actions of 9cRA in the 2-cell.
Aim 4 will test the hypothesis that glucose controls 9cRA in the 2-cell through Dhrs3 and Rdh5. This project should contribute novel insight into mechanisms of retinoid action, nuclear hormone receptor function, regulation of Glut2 and GK, and insulin secretion. PHS 398/2590 (Rev. 06/09) Page Continuation Format Page
Program Director/Principal Investigator (Last, First, Middle): Napoli, Joseph. L. This research would determine physiological function and mechanisms of 9-cis-retinoic acid (9cRA) action in the pancreas. Supporting data indicate that 9cRA functions in opposition to glucose to modulate insulin secretion. Insight into this heretofore overlooked aspect of regulating insulin secretion promises to provide new understanding of the process of regulating insulin secretion, a major hormone that modulates energy metabolism. PHS 398/2590 (Rev. 06/09) Page Continuation Format Page
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