cAMP-mediated signaling normalizes glucose-stimulated insulin secretion in uncoupling protein-2 overexpressing β-cells

Abstract

We investigated whether an increase in cAMP could normalize glucose-stimulated insulin secretion (GSIS) in uncoupling protein-2 (UCP2) overexpressing (ucp2-OE) bcells. Indices of b-cell (b-TC-6f7 cells and rodent islets) function were measured after induction of ucp2, in the presence or absence of cAMP-stimulating agents, analogs, or inhibitors. Islets of ob/ob mice had improved glucoseresponsiveness in the presence of forskolin. Rat islets overexpressing ucp2 had significantly lower GSIS than controls. Acutely, the protein kinase A (PKA) and epac pathway stimulant forskolin normalized insulin secretion in ucp2-OE rat islets and b-TC-6f7 b-cells, an effect blocked by specific PKA inhibitors but not mimicked by epac agonists. However, there was no effect of ucp2-OE on cAMP concentrations or PKA activity. In ucp2-OE islets, forskolin inhibited ATP-dependent potassium (K ATP ) channel currents and 86 Rb C efflux, indicative of K ATP block. Likewise, forskolin application increased intracellular Ca 2C , which could account for its stimulatory effects on insulin secretion. Chronic exposure to forskolin increased ucp2 mRNA and exaggerated basal secretion but not GSIS. In mice deficient in UCP2, there was no augmentation of either cAMP content or cAMP-dependent insulin secretion. Thus, elevating cellular cAMP can reverse the deficiency in GSIS invoked by ucp2-OE, at least partly through PKA-mediated effects on the K ATP channel.

Mutated ATP synthase induces oxidative stress and impaired insulin secretion in β‐cells of female BHE/cdb rats

“…Thus, the impairment of GSIS in BHE/cdb rats reported earlier is confirmed [5] and can be attributed, at least partially, to reduced ATP production. Forskolin, which normalizes insulin secretion in other models of reduced ATP [20] also promoted insulin secretion in BHE/cdb islets. Forskolin increases protein kinase A activity via cyclic adenosine monophosphate to modify the ability of ATP to stimulate K ATP channels [21].…”

Amniotic Fluid Oxidative and Nitrosative Stress Biomarkers Correlate with Fetal Chronic Hypoxia in Diabetic Pregnancies

“…Hence, transcription factors may directly induce the expression of pro-inflammatory cytokines such as interleukin-6, tumor necrosis factor-α or monocyte chemoattractant protein-1 which will cause insulin resistance or indirectly through the activation of serine/threonine kinases which will interfere with key components of the insulin signaling pathway [14]. In addition, increased flux of glucose through the tricarboxylic acid cycle in the mitochondria will result in increased oxygen consumption and ROS production as well as enhanced uncoupling protein activation causing excessive proton leak and exhaustion of ATP reserves leading to cell death by apoptosis [15]. …”

Impact of uncoupling protein-2 overexpression on proinsulin processing

“…When the supply of intracellular Ca 2C is reduced, proinsulin processing is impaired (Guest et al 1997). UCP2-OE reduces glucose-stimulated Ca 2C influx (McQuaid et al 2006), which may directly impact Ca 2C availability in the granule. Interestingly, in this study, proinsulin processing was not altered solely by UCP2-OE and consequent reduction of ATP.…”

“…Interestingly, in this study, proinsulin processing was not altered solely by UCP2-OE and consequent reduction of ATP. Possibly, cells unable to secrete by virtue of UCP2-OE effects on K ATP and voltagedependent Ca 2C channel activity (Chan et al 2001, McQuaid et al 2006) maintained a normal proinsulin:insulin ratio because of prolonged retention of insulin granules despite a sub-optimal granule environment. Therefore, a strategy to depolarize INS-1 cells and promote b-cell exhaustion was devised.…”