Pleckstrin Homology Domain Leucine-rich Repeat Protein Phosphatase (PHLPP): A New Player in Cell Signaling

Abstract

Precise balance between phosphorylation, catalyzed by protein kinases, and dephosphorylation, catalyzed by protein phosphatases, is essential for cellular homeostasis. Deregulation of this balance leads to pathophysiological states that drive diseases such as cancer, heart disease, and diabetes. The recent discovery of the PHLPP (pleckstrin homology domain leucinerich repeat protein phosphatase) family of Ser/Thr phosphatases adds a new player to the cast of phosphate-controlling enzymes in cell signaling. PHLPP isozymes catalyze the dephosphorylation of a conserved regulatory motif, the hydrophobic motif, on the AGC kinases Akt, PKC, and S6 kinase, as well as an inhibitory site on the kinase Mst1, to inhibit cellular proliferation and induce apoptosis. The frequent deletion of PHLPP in cancer, coupled with the development of prostate tumors in mice lacking PHLPP1, identifies PHLPP as a novel tumor suppressor. This minireview discusses the structure, function, and regulation of PHLPP, with particular focus on its role in disease. StructureThe PHLPP (pleckstrin homology domain leucine-rich repeat protein phosphatase) enzymes are novel members of the protein phosphatase 2C (PP2C) 3 grouping in the protein phosphatase metal-dependent (PPM) family of Ser/Thr phosphatases (1). The catalytic core of PPMs has a signature set of conserved Asp residues that coordinate Mg 2ϩ or Mn 2ϩ , metals required for catalytic activity (2). Like other PPMs, but in contrast to the more abundant and better characterized PPPs (phosphoprotein phosphatase), PHLPP contains regulatory modules within the same polypeptide as the phosphatase domain.The PHLPP family comprises three isozymes, the alternatively spliced PHLPP1␣ and PHLPP1␤ (also referred to as suprachiasmatic nucleus circadian oscillatory protein (SCOP) (3)) and PHLPP2, a separate gene product. All three isozymes share a similar domain structure (Fig. 1A): an N-terminal PH domain, a leucine-rich repeat (LRR) region, a PP2C phosphatase domain, and a C-terminal PDZ-binding motif. In addition, both PHLPP1␤ and PHLPP2 have a predicted N-terminal Ras association domain that is not present in PHLPP1␣. The PH domain contains only the middle Arg of the RXRSF motif required for phosphoinositide binding and, as such, is a weak membrane-binding module (4). The PDZ ligands are type 1 PDZ ligands (TPL for PHLPP1 and TAL for PHLPP2). PHLPP is evolutionarily conserved, with the PH domain and PDZ ligands being later additions. Curiously, the yeast homolog, Cyr1, is fused to the N terminus of adenylate cyclase (5). FunctionBased on the reasoning that so many players in the Akt/PKB pathway have a PH domain (notably Akt itself and its upstream kinase, PDK1), a rational search of the NCBI database was performed to identify a gene predicted to encode a PH domain and phosphatase (6). This led to the discover of PHLPP, which was shown to dephosphorylate a key regulatory site on the C terminus of Akt, the hydrophobic motif, and thus inactivate the kinase (6). PHLPP was subsequently shown to serve as the ...

Comparative microRNA Profiling of Prostate Carcinomas with Increasing Tumor Stage by Deep Sequencing

“…PHLPP1 is recognized as a tumor suppressor with a prominent role in suppressing oncogenic signaling pathways, including the AKT pathway (for review, see refs. 22,23). Interestingly, another member of this family of phosphatases, PHLPP2, is a potential target of miR15a, which we found to be upregulated in deep sequencing and Northern blotting in all prostate carcinoma samples.…”

“…A downregulation of PHLPP1 or PHLPP2 could be detected in 11% of primary prostate carcinoma and in 37% of metastases (23,36). Recently, it has been reported that both PHLPP1 and PHLPP2 are deleted or downregulated in combination with highly metastatic prostate carcinoma (22) or glioblastoma (37). PHLPPs belong to a family of phosphatases that regulate protein kinases A, B (known as Akt), and C and ribosomal protein S6 kinase (RPS6K) negatively (23,38).…”

“…2 and 3). miRNAs are short, noncoding RNAs of approximately [19][20][21][22][23][24][25] nucleotides that bind preferentially to specific sequences in the 3 0 -untranslated region (3 0 UTR) of mRNAs, resulting in either translational repression or mRNA degradation. Interaction of the miRNAs with their mRNA target ultimately leads to reduced protein synthesis via association with the Argonaute (Ago-) containing RNAinduced silencing complex (for a review, see ref.…”

Transcription factor C/EBP-β induces tumor-suppressor phosphatase PHLPP2 through repression of the miR-17–92 cluster in differentiating AML cells

“…The PHLPP2 is a member of this family. 2,3 To date four PHLPP substrates, Akt, PKC, Mst1 and S6K1, have been identified, all kinases involved in cell survival or apoptosis. [4][5][6][7] Dephosphorylation inactivates Akt, S6K1 and PKC, and subsequently cell survival signaling, whereas dephosphorylation of Mst1 activates its apoptotic function.…”

PHLPP: a putative cellular target during insulin resistance and type 2 diabetes

“…PHLPP directly dephosphorylates AKT at its hydrophobic motif (Ser473). AKT is a crucial player to execute intracellular responses post IRS-1 (insulin receptor substrate-1) phosphorylation (Machado-Neto et al 2011, Warfel & Newton 2012.…”