Loss of Foxp3 and Tsc1 Accelerate Prostate Cancer Progression through a Synergistic Transcriptional and Post-translational Regulation of c-MYC

Although c-MYC and mTOR are frequently activated proteins in prostate cancer, any interaction between the two is largely untested. Here we characterize the functional crosstalk between FOXP3-c-MYC and TSC1-mTOR signaling during tumor progression. Deletion of Tsc1 in mouse embryonic fibroblasts (MEF) decreased phosphorylation of c-MYC at threonine 58 (pT58) and increased phosphorylation at serine 62 (pS62), an observation validated in prostate cancer cells. Conversely, inhibition of mTOR increased pT58 but decreased pS62. Loss of both Foxp3 and Tsc1 in prostate cancer cells synergistically enhanced c-MYC expression via regulation of c-Myc transcription and protein phosphorylation. This crosstalk between FOXP3 and TSC1 appeared to be mediated by both the mTOR-4EBP1-c-MYC and FOXP3-c-MYC pathways. In mice, Tsc1 and Foxp3 double-deletions in the prostate led to prostate carcinomas at an early age; this did not occur in these mice with an added c-Myc deletion. In addition, we observed synergistic anti-tumor effects of co-treating mice with inhibitors of mTOR and c-MYC in prostate cancer cells as well as in Foxp3 and Tsc1 double-mutant mice. In human prostate cancer, loss of nuclear FOXP3 is often accompanied by low expression of TSC1. Since loss of Foxp3 transcriptionally induces c-Myc expression and loss of Tsc1 activates mTOR signaling, these data suggest crosstalk between FOXP3-c-MYC and TSC1-mTOR signaling that converges on c-MYC to regulate tumor progression. Co-administration of c-Myc and mTOR inhibitors may overcome the resistance to mTOR inhibition so commonly observed prostate cancer cells.

http://bit.ly/2HYMGLB