Effects of retinoids and thiazolidinediones on proliferation, insulin release, insulin mRNA, GLUT 2 transporter protein and mRNA of INS‐1 cells

Abstract

Both 9-cis-retinoic acid (9cRA) and all-trans-retinoic acid (ATRA) are active metabolites of vitamin A (retinol). There exists an interaction between retinoid receptors and peroxisome proliferator-activated receptors (PPARgamma). To define their functions in an insulin secreting system the effects of ATRA, 9cRA and the PPARgamma agonist rosiglitazone on cell proliferation, insulin release and glucose transporter (GLUT) 2 of INS-1 cells were tested. Retinoic acid receptor (RAR-alpha and -gamma) and retinoid X receptor (RXR-alpha and -beta) proteins are present (immunoblots). Both 9cRA and ATRA inhibit INS-1 cell proliferation ([3H]-thymidine assay) in a concentration dependent manner. Both 9cRA and ATRA increased insulin release, but only ATRA ralsed the GLUT 2 mRNA in a bell-shaped concentration response curve after 48 h. The insulinotropic effect of one compound is not significantly superimposed by the other indicating that the same binding sites are used by 9cRA and ATRA. The acute and chronic effects of the PPARgamma agonist rosiglitazone on insulin release were additionally determined since glitazones act as transcription factors together with RXR agonists. At high concentrations (100 microM) rosiglitazone inhibited glucose (8.3 mM) stimulated insulin secretion (acute experiment over 60 min). Insulin secretion, however, was increased during a 24 h treatment at a concentration of 10 microM and again inhibited at 100 microM. Changes in preproinsulin mRNA expression were not observed. Rosiglitazone (100 microM) increased GLUT 2 mRNA paralleled by an increase of GLUT 2 protein, but only after 24 h of treatment. This data indicate that RAR and RXR mediate insulin release. The changes in GLUT 2 have no direct impact on insulin release; the inhibition seen at high concentrations of either compound is possibly the result of the observed inhibition of cell proliferation. Effects of rosiglitazone on preproinsulin mRNA and GLUT 2 (mRNA and protein) do not play a role in modulating insulin secretion. With the presence of an RXR receptor agonist the effect of rosiglitazone on insulin release becomes stimulatory. Thus the effects of RAR-, RXR agonists and rosiglitazone depend on their concentrations, the duration of their presence and are due to specific interactions.

Effects of rosiglitazone and metformin on pancreatic beta cell gene expression

“…This section of the promoter has been shown in previous studies to contain key regulatory elements essential for the control of proinsulin gene transcription [10,13,23]. Although our data do not exclude the possibility that further elements which may respond to rosiglitazone lie outside this region, our findings are consistent with previous studies which show 100 μmol/l rosiglitazone has no effect on proinsulin mRNA levels in INS1 cells over a 24-h period [24]. In contrast to the marked effects of rosiglitazone, metformin had no effect on the activity of either the Ipf1 or the proinsulin promoter over the same 24-h incubation period.…”

“…The effects of rosiglitazone and metformin on Ipf1 promoter activity have not been reported previously. It is intriguing that metformin did not affect Ipf1 promoter activity, but did stimulate IPF1 protein levels over 24 h. This suggests that the observed effects of metformin on IPF1 production occur post-transcriptionally. Since metformin and rosiglitazone are often prescribed in combination for the treatment of insulin-resistance in type 2 diabetes [25,26], we tested the combined effect of these agents on our selected targets, both in Min6 cells and in freshly isolated rat islets.…”

13-Cis-Retinoic Acid-Induced Hyperglycemia in the Fresh Water Edible Crab, Oziothelphusa Senex Senex is mediated by Triggering Release of Hyperglycemic Hormone from Eyestalks

“…Retinoids have been implicated in both stimulation of insulin secretion and expression of the glucose transporter 2 gene [10]. Further, retinoids are believed to exert their effects through the retinoic acid receptor/PPARγ heterodimer [11, …”

“…cis-retinoic acids like 9-CRA and 13-CRA interacts with both retinoic acid receptors (RARα,β,γ) and retinoid X receptor (RXR), whereas ATRA mainly interacts with the RARs [8]. It was reported earlier that administration of cis-retinoic acid increases insulin release in cultured RINm5F cells [9,10]. The antidiabetic effects of retinoids in human skeletal muscle [11] and diabetic rodents [12] are accepted to be the mediated through the RXR/RAR heterodimer, and RXR homodimer.…”

Beta-Cell Function and Failure in Type 1 Diabetes

“…et al 1987) and retinoic acid increases insulin secretion in cultured islets , presumably by its stimulatory effect on pancreatic glucokinase expression and activity ). Retinoic acid is also capable of increasing insulin ) and GLUT2 mRNA (Blumentrath, J. et al 2001). …”