The Phosphatase PHLPP1 Regulates Akt2, Promotes Pancreatic Cancer Cell Death, and Inhibits Tumor Formation

Abstract

BACKGROUND & AIMS The kinase Akt mediates resistance of pancreatic cancer (PaCa) cells to death and is constitutively active (phosphorylated) in cancer cells. Whereas the kinases that activate Akt are well characterized, less is known about phosphatases that dephosporylate and thereby inactivate it. We investigated regulation of Akt activity and cell death by the phosphatases PHLPP1 and PHLPP2 in PaCa cells, mouse models of PaCa, and human pancreatic ductal adenocarcinoma (PDAC). METHODS We measured the effects of PHLPP overexpression or knockdown with small interfering RNAs on Akt activation and cell death. We examined regulation of PHLPPs by growth factors and reactive oxygen species, as well as associations between PHLPPs and tumorigenesis. RESULTS PHLPP overexpression inactivated Akt, whereas PHLPP knockdown increased phosphorylation of Akt in PaCa cells. Levels of PHLPPs were greatly reduced in human PDAC and in mouse genetic and xenograft models of PaCa. PHLPP activities in PaCa cells were down-regulated by growth factors and Nox4 reduced nicotinamide adenine dinucleotide phosphate oxidase. PHLPP1 selectively dephosphorylated Akt2, whereas PHLPP2 selectively dephosphorylated Akt1. Akt2, but not Akt1, was up-regulated in PDAC, and Akt2 levels correlated with mortality. Consistent with these results, high levels of PHLPP1, which dephosphorylates Akt2 (but not PHLPP2, which dephosphorylates Akt1), correlated with longer survival times of patients with PDAC. In mice, xenograft tumors derived from PaCa cells that overexpress PHLPP1 (but not PHLPP2) had inactivated Akt, greater extent of apoptosis, and smaller size. CONCLUSIONS PHLPP1 has tumor suppressive activity and might represent a therapeutic or diagnostic tool for PDAC.

Downregulation of PHLPP Expression Contributes to Hypoxia-Induced Resistance to Chemotherapy in Colon Cancer Cells

“…In our current study, we demonstrated that the level of PHLPP expression regulates drug sensitivity in colon cancer cells and that downregulation of PHLPP contributes to hypoxiainduced chemoresistance. These findings are consistent with the established role of PHLPP in promoting apoptosis and inhibiting proliferation in various cancer cells (18,20,23,24,39). In summary, we have identified PHLPP as a critical factor in determining drug sensitivity under hypoxic conditions.…”

“…As a direct regulator of several critical protein kinases, including Akt, protein kinase C (PKC), mitogen-activated protein kinase (MAPK), and MST1, PHLPP plays a critical role in maintaining the balance of signaling and homeostasis in cells (13)(14)(15)(16). Recently, an increasing numbers of studies have revealed that loss of PHLPP expression is highly associated with tumor progression in both human cancers and mouse models, thus further confirming the role of PHLPP as a tumor suppressor (17)(18)(19)(20). Our previous studies have shown that PHLPP is degraded via the ubiquitin-proteasome pathway, and the turnover of PHLPP proteins is mediated by the E3 ubiquitin ligase ␤-TrCP (21).…”

microRNA-224 Promotes Cell Proliferation and Tumor Growth in Human Colorectal Cancer by Repressing PHLPP1 and PHLPP2

“…As Ser/Thr protein phosphatases, PHLPP1 and PHLPP2 can directly dephosphorylate AKT to inhibit AKT signaling activity, which promotes apoptosis (15,30,33). Overexpression of PHLPP1 and PHLPP2 in cancer cell lines decreases cell proliferation and tumorigenesis both in vitro and in xenograft models (26,30,(33)(34)(35)(36)(37). In colorectal cancer cells, stable overexpression of PHLPP1 or PHLPP2 blocked the G 2 -M transition or induced G 1 cell-cycle arrest, thus decreasing the rate of cell proliferation (26).…”

The regulatory roles of phosphatases in cancer

“…PH domain and Leucine rich repeat protein phosphatases (PHLPP) levels are markedly reduced in human PDAC that have elevated AKT phosphorylation (71). Studies have shown that PHLPP1 and PHLPP2 are able to terminate AKT signaling by directly dephosphorylating and inactivating AKT resulting in great suppression of tumour growth (72,73).…”