Small molecule modulation of insulin receptor-insulin like growth factor-1 receptor heterodimers in human endothelial cells

Objectives: The insulin receptor (IR) and insulin like growth factor-1 receptor (IGF-1R) are heterodimers consisting of two extracellular α-subunits and two transmembrane β -subunits. Insulin αβ and insulin like growth factor-1 αβ hemi-receptors can heterodimerize to form hybrids composed of one IR αβ and one IGF-1R αβ. The function of hybrids in the endothelium is unclear. We sought insight by developing a small molecule capable of reducing hybrid formation in endothelial cells.

Methods: We performed a high-throughput small molecule screening, based on a homology model of the apo hybrid structure. Endothelial cells were studied using western blotting and qPCR to determine the effects of small molecules that reduced hybrid formation.

Results: Our studies unveil a first-in-class quinoline-containing heterocyclic small molecule that reduces hybrids by >50% in human umbilical vein endothelial cells (HUVECs) with no effects on IR or IGF-1R. This small molecule reduced expression of the negative regulatory p85α subunit of phosphatidylinositol 3-kinase, increased basal phosphorylation of the downstream target Akt and enhanced insulin/insulin-like growth factor-1 and shear stress-induced serine phosphorylation of Akt. In primary saphenous vein endothelial cells (SVEC) from patients with type 2 diabetes mellitus undergoing coronary artery bypass (CABG) surgery, hybrid receptor expression was greater than in patients without type 2 diabetes mellitus. The small molecule significantly reduced hybrid expression in SVEC from patients with type 2 diabetes mellitus.

Conclusions: We identified a small molecule that decreases the formation of IR: IGF-1R hybrid receptors in human endothelial cells, without significant impact on the overall expression of IR or IGF-1R. In HUVECs, reduction of IR: IGF-1R hybrid receptors leads to an increase in insulin-induced serine phosphorylation of the critical downstream signalling kinase, Akt. The underpinning mechanism appears, at least in part to involve the attenuation of the inhibitory effect of IR: IGF-1R hybrid receptors on PI3-kinase signalling.