Activation of PP2A and inhibition of mTOR synergistically reduce MYC signaling and decrease tumor growth in pancreatic ductal adenocarcinoma.

In cancer, kinases are often activated and phosphatases suppressed, leading to aberrant activation of signaling pathways driving cellular proliferation, survival, and therapeutic resistance. Although pancreatic ductal adenocarcinoma (PDA) has historically been refractory to kinase inhibition, therapeutic activation of phosphatases is emerging as a promising strategy to restore balance to these hyperactive signaling cascades. In the current study, we hypothesized that phosphatase activation combined with kinase inhibition could deplete oncogenic survival signals to reduce tumor growth. We screened PDA cell lines for kinase inhibitors that could synergize with activation of PP2A, a tumor suppressor phosphatase, and determined that activation of PP2A and inhibition of mTOR synergistically increase apoptosis and reduce oncogenic phenotypes in vitro and in vivo. This combination treatment resulted in suppression of AKT/mTOR signaling coupled with reduced expression of c-MYC, an oncoprotein implicated in tumor progression and therapeutic resistance. Forced expression of c-MYC or loss of PP2A B56α, the specific PP2A subunit shown to negatively regulate c-MYC, increased resistance to mTOR inhibition. Conversely, decreased c-MYC expression increased the sensitivity of PDA cells to mTOR inhibition. Together these studies demonstrate that combined targeting of PP2A and mTOR suppresses proliferative signaling and induces cell death and implicate this combination as a promising therapeutic strategy for PDA patients.