Maintaining glucose homeostasis in response to aging and stress: The role of Pcif1, Bmi1, and Pdx1

A sufficient number of functioning beta cells is necessary for maintaining glucose homeostasis. Reduction of beta cell mass or function leads to diabetes. Investigation into the maintenance of both beta cell mass and function is important for the development of therapies to prevent and/or restore functional beta cells. Here, the networks surrounding three proteins in the beta cell, Pcif1, Bmi1, and Pdx1, were studied as they relate to beta cell function and number. The Polycomb protein, Bmi1, has been shown to influence beta cell replication via epigenetic repression of the Ink4a/Arf locus, resulting in suppression of p16 protein translation. The adapter protein, Pcif1, facilitates the ubiquitination of Bmi1 and influences beta cell replication, as Pcif1 heterogyzous mice have increased rates of beta cell proliferation. I hypothesized that Pcif1 regulates beta cell proliferation through a Bmi1-dependent mechanism. Analysis of Pcif1 heterozygous islets revealed that p16 protein levels were indistinguishable from controls, thus making a p16-dependent mechanism unlikely. Further investigation of Bmi1 targets may reveal another pathway by which Pcif1 and Bmi1 influence beta cell replication. The role of Bmi1 has not been well-described in adult animals. Analysis of Bmi1 heterozygous animals revealed increased insulin sensitivity, as compared to wildtype. This was found to be due to an enhancement of Akt phosphorylation, with the upstream insulin signaling pathway unaffected. Bmi1 also appears to play a role in the development of insulin resistance, as Bmi1 levels are high in insulin-resistant animals. I also began to explore the possibility that the action of Pcif1 on Bmi1 is responsible for the role Bmi1 plays in insulin signaling. The transcription factor, Pdx1, regulates numerous processes specific to the beta cell, including multiple pathways regulating translation. Pdx1 levels have been shown to affect the ability of beta cells to respond to ER stress. A global analysis of translational efficiencies using the TRAP methodology indicated that Pdx1 activity may result in repression of translation of some transcripts. Further analysis of these transcripts will help determine how Pdx1 regulates the translatome of the beta cell and, potentially, how Pdx1 influences the beta cell stress response