Dissection of Key Molecular Events in Podocyte Response to Hyperinsulinemia and Its Implications in Prediabetic Microalbuminuria

Background: Epidemiological evidence suggests that insulin resistance, manifested as compensatory hyperinsulinemia, is an independent risk factor for chronic kidney disease. However, whether hyperinsulinemia per se is a cause for kidney injury is unknown. Methods: Pre-diabetic db/db mice were examined for renal histology, glycemia, insulinemia and albuminuria. Insulin signaling profile was estimated in glomerular podocytes in db/m and db/db mice. Immortalized podocytes were exposed to high ambient insulin, following ectopic expression of a kinase-dead mutant of GSK3β (KD), a constitutively active GSK3β mutant (S9A), or treatment with a GSK3β inhibitor tideglusib. Cell injury and insulin signaling were examined. Results: Hyperinsulinemia preceded hyperglycemia in pre-diabetic db/db mice, and was associated with microalbuminuria and early signs of podocyte impairment. In glomeruli isolated from young db/db mice, an exhausted insulin signaling was noted, marked by insulin receptor (INSR) depletion and hypoactivity of insulin signaling mediators like IRS1 and Akt. This was associated with loss of repression of the constitutively active GSK3β, as indicated by diminished inhibitory p-GSK3βS9. GSK3β hyperactivity was also confirmed in glomerular podocytes in pre-diabetic db/db mice, correlating with the degree of hyperinsulinemia or microalbuminuria. Furthermore, GSK3β co-localized and physically interacted with IRS1 in podocytes, suggesting IRS1 acts as a putative substrate for GSK3β. In cultured podocytes, prolonged exposure to high ambient insulin depleted the expression of INSR, leading to GSK3β hyperactivity and inhibitory p-IRS1S332. S9A reinforced inhibitory p-IRS1S332 and thereby desensitized insulin signaling, as evidenced by further reduction in Akt phosphorylation. This was associated with exacerbated podocyte injury. Conversely, KD mitigated the inhibitory p-IRS1S332 and thereby restored insulin signaling sensitivity, resulting in attenuated podocyte injury. This protective effect was mimicked by tideglusib. Conclusion: Hyperinsulinemia elicits podocyte injury and microalbuminuria via a series of key molecular events, involving depletion of INSR, desensitization of insulin signaling, and GSK3β hyperactivity, which promotes inhibitory p-IRS1S332 and thereby forms a vicious cycle of insulin signaling resistance and podocyte injury. Funding: NIDDK Support