Michael L. McDaniel, PhD
Professor of Pathology
A major focus of diabetes research in my laboratory is to physiologically enhance the growth & proliferative capacity of primary adult human b-cells by appropriately regulating both the mTOR and canonical Wnt signaling pathways. Stimulating islet β-cells to proliferate is a clinical goal that could have a beneficial impact on both Type 1 diabetes for ex vivo transplantation protocols and in Type 2 diabetes for approaches that generate β-cell replication in vivo. Mammalian Target of Rapamycin (mTOR) is a unique protein kinase that integrates signals derived from nutrients and growth factors to physiologically regulate cell growth and proliferation that occurs in a rapamycin sensitive manner. Rapamycin acts by forming an inhibitory complex with its intracellular receptor, which then binds to a region in the C terminus, thereby specifically inhibiting mTOR activity. Our previous studies demonstrated that nutrient and growth factor regulation of mTOR signaling stimulated protein translation and promoted regenerative processes including DNA synthesis, cell cycle progression and β-cell proliferation in rodent islets that occurred in a rapamycin sensitive manner. However, these positive effects rarely occurred in human islets. Our recent findings demonstrated that human islets require direct inhibition of GSK-3 by lithium or 1-azakenpaullone (1-Akp) which then engages the Wnt signaling pathway resulting in the translocation of β-catenin to the nucleus of human β-cells. Lithium is a well documented direct inhibitor of GSK-3 that is used clinically, although it also targets other signaling pathways, whereas 1-Akp is a highly selective and potent inhibitor of GSK-3. This engagement of Wnt signaling mimics all the positive regenerative effects as observed with rodent islets. Unexpectedly, all of these positive effects also occurred in a rapamycin sensitive manner. To explain this rapamycin sensitivity, we discovered that mTOR facilitates the translocation of β-catenin in the Wnt pathway to the nucleus of human β-cells, and this nuclear translocation is blocked by rapamycin. Another significant difference between human and rodent islets in vitro is that human islets display a high level of insulin resistance even at basal glucose due to the engagement of a negative feedback that is associated with the chronic activation of mTOR and the inhibition of upstream IRS proteins that produces insulin resistance. This negative feedback may explain the requirement to directly inhibit GSK-3 in human islets. Our ongoing studies will utilize molecular and genetic approaches as well as physiologically relevant Wnt agonists to further understand the transcriptional and translational signals that facilitate GSK-3 and mTOR-mediated regenerative processes necessary to expand functional human β-cells.