Differential effect of insulin treatment on islet amyloid polypeptide (amylin) and insulin gene expression in streptozotocin-induced diabetes in rats

CGRP in the trigeminovascular system: a role for CGRP, adrenomedullin and amylin receptors?

The neuropeptide calcitonin gene-related peptide (CGRP) is reported to play an important role in migraine. It is expressed throughout the trigeminovascular system. Antagonists targeting the CGRP receptor have been developed and have shown efficacy in clinical trials for migraine. However, no CGRP antagonist is yet approved for treating this condition. The molecular composition of the CGRP receptor is unusual because it comprises two subunits; one is a GPCR, the calcitonin receptor-like receptor (CLR). This associates with receptor activity-modifying protein (RAMP) 1 to yield a functional receptor for CGRP. However, RAMP1 also associates with the calcitonin receptor, creating a receptor for the related peptide amylin but this also has high affinity for CGRP. Other combinations of CLR or the calcitonin receptor with RAMPs can also generate receptors that are responsive to CGRP. CGRP potentially modulates an array of signal transduction pathways downstream of activation of these receptors, in a cell type-dependent manner. The physiological significance of these signalling processes remains unclear but may be a potential avenue for refining drug design. This complexity has prompted us to review the signalling and expression of CGRP and related receptors in the trigeminovascular system. This reveals that more than one CGRP responsive receptor may be expressed in key parts of this system and that further work is required to determine their contribution to CGRP physiology and pathophysiology.

LINKED ARTICLES

This article is part of a themed section on Neuropeptides. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.170.issue-7.

DPP-4 inhibition improves glucose tolerance and increases insulin and GLP-1 responses to gastric glucose in association with normalized islet topography in mice with beta-cell-specific overexpression of human islet amyloid polypeptide

Inhibition of dipeptidyl peptidase-4 (DPP-4) is currently explored as a novel therapy of type 2 diabetes. The strategy has been shown to improve glycemia in most, but not all, rodent forms of glucose intolerance. In this study, we explored the effects of DPP-4 inhibition in mice with beta-cell overexpression of human islet amyloid polypeptide (IAPP). We therefore administered the orally active and highly selective DPP-4 inhibitor, vildagliptin (3 micromol/mouse daily) to female mice with beta-cell overexpression of human IAPP. Controls were given plain water, and a series of untreated wildtype mice was also included. After five weeks, an intravenous glucose tolerance test showed improved glucose disposal and a markedly enhanced insulin response in mice treated with vildagliptin. After eight weeks, a gastric tolerance test showed that vildagliptin improved glucose tolerance and markedly (approximately ten-fold) augmented the insulin response in association with augmented (approximately five-fold) levels of intact glucagon-like peptide-1 (GLP-1). Furthermore, after nine weeks, islets were isolated. Islets from vildagliptin-treated mice showed augmented glucose-stimulated insulin response and a normalization of the islet insulin content, which was reduced by approximately 50% in transgenic controls versus wildtype animals. Double immunostaining of pancreatic islets for insulin and glucagon revealed that transgenic islets displayed severely disturbed intra-islet topography with frequently observed centrally located alpha-cells. Treatment with vildagliptin restored the islet topography. We therefore conclude that DPP-4 inhibition improves islet function and islet topography in mice with beta-cell specific transgenic overexpression of human IAPP.

Amylin gene expression mediated by cAMP/PKA and transcription factors HNF-1 and NFY

The regulation of amylin gene expression is not clearly understood. In this study, we used the pancreatic beta cell line, INS-1, to evaluate the regulation of amylin gene expression. When INS-1 cells were cultured in media containing IBMX, the ratio of amylin secretion to insulin secretion increased by 200%. This coincided with an increase in amylin mRNA content. Moreover, there was a three to four-fold increase in amylin promoter activity which was inhibited by the specific protein kinase A (PKA) inhibitor, H89. Electrophoretic mobility shift assays (EMSA) demonstrated that IBMX induced protein-DNA binding to the FLAT and CAAT elements of the amylin promoter. Competitive EMSA experiments revealed that these proteins are likely to be HNF-1 and NFY, respectively. IBMX-induced amylin promoter activity was inhibited by mutations in the FLAT and CAAT elements. These results indicate that amylin is positively regulated by cAMP and PKA through the transcription factors HNF-1 and NFY.