PHLPPing the Script: Emerging Roles of PHLPP Phosphatases in Cell Signaling

Abstract

Whereas protein kinases have been successfully targeted for a variety of diseases, protein phosphatases remain an underutilized therapeutic target, in part because of incomplete characterization of their effects on signaling networks. The pleckstrin homology domain leucine-rich repeat protein phosphatase (PHLPP) is a relatively new player in the cell signaling field, and new roles in controlling the balance among cell survival, proliferation, and apoptosis are being increasingly identified. Originally characterized for its tumor-suppressive function in deactivating the prosurvival kinase Akt, PHLPP may have an opposing role in promoting survival, as recent evidence suggests. Additionally, identification of the transcription factor STAT1 as a substrate unveils a role for PHLPP as a critical mediator of transcriptional programs in cancer and the inflammatory response. This review summarizes the current knowledge of PHLPP as both a tumor suppressor and an oncogene and highlights emerging functions in regulating gene expression and the immune system. Understanding the context-dependent functions of PHLPP is essential for appropriate therapeutic intervention.

Keywords: Akt; PHLPP; PKC; cancer; inflammation; phosphatase; phosphorylation; transcription.

Figure 1.. Phylogeny and Domain Architecture of PHLPP Phosphatases

(A) PHLPP1 and PHLPP2 are PP2C-type Ser/Thr protein phosphatases, belonging to the protein phosphatase magnesium/manganese (PPM)-dependent shrub of the protein phosphatase phylogenetic tree [115]. (B) Domain structure of ancestral and mammalian PHLPP. Yeast CYR1 contains a Gα domain, 26 Leucine rich repeats (LRR), a PP2C phosphatase domain (PP2C) fused to adenylate cyclase (AC), and C-terminal cyclase-associated protein 1 binding domain (CAP). Mammalian PHLPP retains the LRR domain (18 in PHLPP1 and 19 in PHLPP2) and PP2C phosphatase domain in addition to a PH domain (PH), N-terminal extension (NTE) containing a bipartite Nuclear Localization Signal (NLS), and C-terminal PDZ ligand. Scale bar denotes 100 amino acids (a.a.).

Figure 2.. PHLPP Function in Growth Factor Signaling.

Schematic showing some of the substrates and signaling pathways regulated by PHLPP. In general, PHLPP suppresses proliferative and survival pathways through direct dephosphorylation of Akt, S6K1, Mst1, and Raf (red sector). PHLPP dephosphorylates the hydrophobic motif (green square with P) of Akt and S6K1 to inactivate them, while PHLPP activates the pro-apoptotic kinase, Mst1, via removal of an inhibitory phosphorylation (white hexagon with P). PHLPP also dephosphorylates PKC isozymes on the hydrophobic motif, which destabilizes them and reduces their cellular levels. Filled in circles on Akt, S6K1, and PKC denote other key phosphorylation sites, including the PDK-1 site on the activation loop (magenta circle) and mTORC2-regulated turn motif (orange circle). In addition, PHLPP suppresses the transcription and hence steady-state protein levels of RTKs such as the EGFR, thus dampening PI3K survival and ERK proliferative pathways, by an epigenetic mechanism (blue sector). PHLPP itself is under feedback regulation by components of the PI3K pathway (tan sector). High Akt activity increases PHLPP protein levels by suppressing GSK3β-dependent degradation. Furthermore, high mTORC1 activity increases S6K1-dependent translation of PHLPP (left). Moreover, S6K1 activity also activates a negative feedback loop that suppresses Akt activity.

Figure 3.. Emerging Roles for PHLPP in Inflammatory Signaling.

Binding of IFNγ to interferon receptors activates inflammatory transcriptional programs. Among the mediators of this inflammatory response are components of the JAK/STAT pathway: phosphorylation of STAT1 transcription factors on Tyr701 by Janus kinases (JAKs) induces STAT1 dimerization, nuclear translocation, and activation to promote transcription of target genes. This activity is enhanced by phosphorylation on Ser727. PHLPP1 opposes STAT1-mediated transcription to promote resolution of the inflammatory response by directly desphosphorylating Ser727 to tune STAT1 activity [6].

Figure 4.. Pharmacological Targeting of PHLPP in Cancer.

Despite the discovery of PHLPP as a bona fide tumor suppressor by opposing PI3K and growth factor signaling (left), complex roles in cancer involving novel oncogenic functions present therapeutic opportunities for PHLPP inhibitors (right). In pancreatic cancer, PHLPP1 suppresses levels of the tumor suppressor PKC through quality control dephosphorylation of the hydrophobic motif, where the loss of PKC correlates inversely with patient survival [32]. In prostate cancer, PHLPP2 stabilization of oncogenic Myc is critical for tumor progression and metastatic disease [106]. Thus, future studies should explore the potential therapeutic benefit of PHLPP inhibition in these specific contexts.