About 35.5% American men are obese and obesity has been positively associated with high-grade prostate cancer, castration-resistant prostate cancer, and prostate cancer-specific mortality. Obesity has been associated with a 50% increased risk of pathological progression in prostate cancer patients under active surveillance. However, the molecular mechanisms underlying the association between obesity and prostate cancer progression remain as a significant knowledge gap to be filled. It is known that obese people often develop type 2 diabetes (T2D) with increased blood levels of insulin and inflammatory cytokines such as interleukin-17 (IL-17). We have previously demonstrated that IL-17 promotes development of hormone-naïve and castration-resistant prostate cancer in Pten-null mice. Our preliminary studies found that prostate cancer formation rate was increased by approximately 82% in high-fat diet-induced obese Pten-null mice compared to lean Pten-null mice. We have demonstrated that insulin enhances IL-17-induced gene expression through inhibition of glycogen synthase kinase 3 (GSK3). Moreover, we have originally found that GSK3 binds to and phosphorylates IL-17 receptor A (IL-17RA) at residue T780, leading to ubiquitination and proteasome-mediated degradation of IL-17RA. In obese mice, hyperinsulinemia activated phosphoinositide 3-kinase (PI3K)/Akt to phosphorylate GSK3α at S21 and GSK3β at S9, thus inhibiting GSK3 activity; subsequently, IL-17RA phosphorylation by GSK3 is reduced, resulting in increased levels of IL-17RA protein, followed by enhanced IL-17 signaling and increased expression of multiple IL-17 downstream genes. These findings suggest that IL- 17 signaling and insulin signaling crosstalk via GSK3, which is a novel mechanism by which obesity drives prostate cancer progression. Based on these findings, we have formulated a central hypothesis that GSK3 is an intrinsic inhibitor of IL-17 signaling, and in obesity with T2D, high levels of insulin enhance IL-17-mediated responses through inhibiting GSK3 activities, thereby promoting prostate cancer progression. We propose to test our central hypothesis through achieving the following three specific aims: Aim 1) Determine GSK3’s role in regulating IL-17’s pro-tumor function in Pten-null obese mice. We will use two animal models: a) we will create Gsk3α;Pten and Gsk3β;Pten double knockout mice with wild-type IL-17ra; we expect to find a decrease in IL-17RA phosphorylation by GSK3, a decrease in IL-17RA degradation, and an increase in IL-17-mediated inflammation in mouse prostate, resulting in an increase in prostate cancer incidence; b) we will create IL-17ra T779A knockin mutant (IL-17raKI/KI) mice that will not respond to GSK3-mediated phosphorylation of IL-17RA, thus allowing assessment of IL-17-independent functions of GSK3; mouse T779 is homologous to human T780 on IL-17RA; we expect to find no significant differences in prostate cancer formation comparing Gsk3α;Pten and Gsk3β;Pten d...