Molecular basis for inhibiting human glucose transporters by exofacial inhibitors.

Human glucose transporters (GLUTs) are responsible for cellular uptake of hexoses. Elevated expression of GLUTs, particularly GLUT1 and GLUT3, is required to fuel the hyperproliferation of cancer cells, making GLUT inhibitors potential anticancer therapeutics. Meanwhile, GLUT inhibitor-conjugated insulin is being explored to mitigate the hypoglycemia side effect of insulin therapy in type 1 diabetes. Reasoning that exofacial inhibitors of GLUT1/3 may be favored for therapeutic applications, we report here the engineering of a GLUT3 variant, designated GLUT3exo, that can be probed for screening and validating exofacial inhibitors. We identify an exofacial GLUT3 inhibitor SA47 and elucidate its mode of action by a 2.3 Å resolution crystal structure of SA47-bound GLUT3. Our studies serve as a framework for the discovery of GLUTs exofacial inhibitors for therapeutic development.

Differentiation of pancreatic endocrine progenitors reversibly blocked by premature induction of MafA.

Specification and maturation of insulin(+) cells accompanies a transition in expression of Maf family of transcription factors. In development, MafA is expressed after specification of insulin(+) cells that are expressing another Maf factor, MafB; after birth, these insulin(+) MafA(+) cells stop MafB expression and gain glucose responsiveness. Current differentiation protocols for deriving insulin-producing ß-cells from stem cells result in ß-cells lacking both MafA expression and glucose-stimulated insulin secretion. So driving expression of MafA, a ß-cell maturation factor in endocrine precursors could potentially generate glucose-responsive MafA(+) ß cells. Using inducible transgenic mice, we characterized the final stages of ß-cell differentiation and maturation with MafA pause/release experiments. We found that forcing MafA transgene expression, out of its normal developmental context, in Ngn3(+) endocrine progenitors blocked endocrine differentiation and prevented the formation of hormone(+) cells. However, this arrest was reversible such that with stopping the transgene expression, the cells resumed their differentiation to hormone(+) cells, including α-cells, indicating that the block likely occurred after progenitors had committed to a specific hormonal fate. Interestingly, this delayed resumption of endocrine differentiation resulted in a greater proportion of immature insulin(+)MafB(+) cells at P5, demonstrating that during maturation the inhibition of MafB in ß-cell transitioning from insulin(+)MafB(+) to insulin(+)MafB(-) stage is regulated by cell-autonomous mechanisms. These results demonstrate the importance of proper context of initiating MafA expression on the endocrine differentiation and suggest that generating mature Insulin(+)MafA(+) ß-cells will require the induction of MafA in a narrow temporal window to achieve normal endocrine differentiation.

GW8510 increases insulin expression in pancreatic alpha cells through activation of p53 transcriptional activity.

BACKGROUND: Expression of insulin in terminally differentiated non-beta cell types in the pancreas could be important to treating type-1 diabetes. Previous findings led us to hypothesize involvement of kinase inhibition in induction of insulin expression in pancreatic alpha cells. METHODOLOGY/PRINCIPAL FINDINGS: Alpha (αTC1.6) cells and human islets were treated with GW8510 and other small-molecule inhibitors for up to 5 days. Alpha cells were assessed for gene- and protein-expression levels, cell-cycle status, promoter occupancy status by chromatin immunoprecipitation (ChIP), and p53-dependent transcriptional activity. GW8510, a putative CDK2 inhibitor, up-regulated insulin expression in mouse alpha cells and enhanced insulin secretion in dissociated human islets. Gene-expression profiling and gene-set enrichment analysis of GW8510-treated alpha cells suggested up-regulation of the p53 pathway. Accordingly, the compound increased p53 transcriptional activity and expression levels of p53 transcriptional targets. A predicted p53 response element in the promoter region of the mouse Ins2 gene was verified by chromatin immunoprecipitation (ChIP). Further, inhibition of Jun N-terminal kinase (JNK) and p38 kinase activities suppressed insulin induction by GW8510. CONCLUSIONS/SIGNIFICANCE: The induction of Ins2 by GW8510 occurred through p53 in a JNK- and p38-dependent manner. These results implicate p53 activity in modulation of Ins2 expression levels in pancreatic alpha cells, and point to a potential approach toward using small molecules to generate insulin in an alternative cell type.