The aim of the present study was to determine the existence of melatonin membrane receptors and to examine the mRNA expression of nuclear orphan receptors in human pancreatic tissue, in an effort to explain differences between type 2 diabetic and metabolically healthy patients. Molecular and immunocytochemical investigations established the presence of the melatonin membrane receptors MT1 and MT2 in human pancreatic tissue and, notably, also in the islets of Langerhans. Results of a calculation model to determine mRNA expression ratios, as well as subjective analysis of immunoreactions, showed elevated MT1 receptor expression in comparison with MT2 expression. mRNA transcript levels of melatonin receptors appeared to be significantly higher in type 2 diabetic patients than in a control group. An upregulation of receptor expression in type 2 diabetic patients was also observed in immunocytochemical investigations. In addition, transcripts of the nuclear orphan receptors RORalpha, RZRbeta, RORgamma and RevErbalpha were detected in human pancreatic tissue and islets. In correlation with membrane melatonin receptors, data indicate increased mRNA expression levels of RORalpha, RZRbeta, and RORgamma in type 2 diabetic patients. Thus, our data demonstrate the existence of the melatonin membrane receptors MT1 and MT2 as well as mRNA expression of nuclear orphan receptors in human pancreatic tissue, with upregulated expression levels in type 2 diabetic patients.

Recent investigations have demonstrated an influence of melatonin on insulin secretion in pancreatic beta-cells. The effects are receptor-mediated via two parallel signaling pathways. The aim of this study was to examine the relevance of a second melatonin receptor (MT2) as well as the involvement of a third signaling cascade in mediating melatonin effects, i.e. the cyclic guanosine monophosphate (cGMP) pathway. Our results demonstrate that the insulin-inhibiting effect of melatonin could be partly reversed by preincubation with the unspecific melatonin receptor antagonist luzindole as well as by the MT2-receptor-specific antagonist 4P-PDOT (4-phenyl-2-propionamidotetraline). As melatonin is known to modulate cGMP concentration via the MT2 receptor, these data indicate transmission of the melatonin effects via the cGMP transduction cascade. Molecular investigations established the presence of different types of guanylate cyclases, cGMP-specific phosphodiesterases and cyclic nucleotide-gated channels in rat insulinoma beta-cells (INS1). Moreover, variations in mRNA expression were found when comparing day and night values as well as different states of glucose metabolism. Incubation experiments provided evidence that 3-isobutyl-1-methylxanthine (IBMX)-stimulated cGMP concentrations were significantly decreased in INS1 cells exposed to melatonin for 1 hr in a dose- and time-dependent manner. This effect could also be reversed by application of luzindole and 4P-PDOT. Stimulation with 8-Br-cGMP resulted in significantly increased insulin production. In conclusion, it could be demonstrated that the melatonin receptor subtype MT2 as well as the cGMP signaling pathway are involved in mediating the insulin-inhibiting effect of melatonin.

Identification of excessive IFN-γ production by blood and lymph node-derived T cells of patients with CVID with immune dysregulation will offer new therapeutic avenues for this subgroup. CD21 B cells might serve as a marker of this IFN-γ-associated dysregulation.

Melatonin influences the second messenger cyclic guanosine 3',5'-monophosphate (cGMP) signaling pathway in pancreatic beta-cells via a receptor-mediated mechanism. In the present study, it was determined how the regulation of cGMP concentrations by melatonin proceeds. The results provide evidence that melatonin acts via the soluble guanylate cyclase (sGC), as molecular investigations demonstrated that long-term incubation with melatonin significantly reduced the expression levels of the sGC mRNA in rat insulinoma beta-cells (INS1) cells, whereas mRNA expression of membrane guanylate cyclases was unaffected. Incubation with melatonin abolished the S-nitrosoacetyl penicillamine-induced increase of cGMP concentrations in INS1 cells. In addition, the cGMP-inhibitory effect of melatonin was reversed by preincubation with the sGC inhibitors 1H-(1,2,4)oxadiazolo(4,3-alpha)quinoxalin-1-one and 4H-8-bromo-1,2,4-oxadiazolo(3,4-d)benz(b)(1,4)oxazin-1-one. Nitric oxide (NO) production was not influenced after 1 hr of melatonin application, but was influenced after a 4 hr incubation period. Preincubation of INS1 cells with the NO synthase inhibitor N(G)-monomethyl-l-arginine did not abolish the cGMP-inhibitory effect of melatonin. Transcripts of cyclic nucleotide-gated (CNG) channels were significantly reduced after melatonin treatment in a dose-dependent manner, indicating the involvement of these channels in mediating the melatonin effect in INS1 cells. The results of this study demonstrate that melatonin mediates its inhibitory effect on cGMP concentrations in pancreatic beta-cells by inhibiting the sGC, but does not influence NO concentration or NO synthase activity in short-term incubation experiments. In addition, it was demonstrated that melatonin is involved in modulation of CNG channel mRNA.