Therapeutic Strategies to Increase Human β-Cell Growth and Proliferation by Regulating mTOR and GSK-3/β-Catenin Pathways.

This perspective delineates approaches to develop therapeutic strategies to stimulate the proliferative potential of adult human β-cells in vitro. Previous findings demonstrated that nutrients, through regulation of mTOR signaling, promote regenerative processes including DNA synthesis, cell cycle progression and β-cell proliferation in rodent islets but rarely in human islets. Recently, we discovered that regulation of the Wnt/GSK-3/β-catenin pathway by directly inhibiting GSK-3 with pharmacologic agents, in combination with nutrient activation of mTOR, was required to increase growth and proliferation in human islets. Studies also revealed that nuclear translocation of β-catenin in response to GSK-3 inhibition regulated these processes and was rapamycin sensitive, indicating a role for mTOR. Human islets displayed a high level of insulin resistance consistent with the inability of exogenous insulin to activate Akt and engage the Wnt pathway by GSK-3 inhibition. This insulin resistance in human islets is not present in rodent islets and may explain the differential requirement in human islets to inhibit GSK-3 to enhance these regenerative processes. Human islets exhibited normal insulin secretion but a loss of insulin content, which was independent of all treatment conditions. The loss of insulin content may be related to insulin resistance, the isolation process or culture conditions. In this perspective, we provide strategies to enhance the proliferative capacity of adult human β-cells and highlight important differences between human and rodent islets: the lack of a nutrient response, requirement for direct GSK-3 inhibition, insulin resistance and loss of insulin content that emphasize the physiological significance of conducting studies in human islets.