On the PHLPPside: Emerging roles of PHLPP phosphatases in the heart

Abstract

PH domain leucine-rich repeat protein phosphatase (PHLPP) is a family of enzymes made up of two isoforms (PHLPP1 and PHLPP2), whose actions modulate intracellular activity via the dephosphorylation of specific serine/threonine (Ser/Thr) residues on proteins such as Akt. Recent data generated in our lab, supported by findings from others, implicates the divergent roles of PHLPP1 and PHLPP2 in maintaining cellular homeostasis since dysregulation of these enzymes has been linked to various pathological states including cardiovascular disease, diabetes, ischemia/reperfusion injury, musculoskeletal disease, and cancer. Therefore, development of therapies to modulate specific isoforms of PHLPP could prove to be therapeutically beneficial in several diseases especially those targeting the cardiovascular system. This review is intended to provide a comprehensive summary of current literature detailing the role of the PHLPP isoforms in the development and progression of heart disease.

Keywords: Akt; Cardiac hypertrophy; Cardiovascular disease; PHLPP; Phosphatase; Serine/threonine.

Figure 1.. PHLPP Isoform Structure.

PH Domain Leucine-Rich Repeat Protein Phosphatase (PHLPP) has two isoforms, PHLPP1, which contains two splice variants, PHLPP1α and PHLPP1β, and PHLPP2. PHLPP1α has a molecular weight of about 133kDa, followed by PHLPP2 at 147 kDa, and PHLPP1β with 185 kDa. The PHLPP isoforms have similar domain structures, which consist of an N-terminal Ras association (RA) domain (PHLPP1β and PHLPP2), PH domain, Leucine-rich repeat region (LRR), PP2C phosphatase domain, and a PDZ ligand domain. Figure created in BioRender.com.

Figure 2.. PHLPP Signaling in Cancer.

The PHLPP isoforms are tumor suppressors in cancer and inhibit cell growth and survival. The first identified substrate of PHLPP was the AGC kinase Akt, in which PHLPP selectively dephosphorylates the Akt isoforms on its hydrophobic motif. Additional PHLPP targets include Protein Kinase C (PKC), Macrophage Stimulating Protein 1 (MST1), p70S6 kinase 1 (p70S6K), and the RAF/MEK/ERK cascade, which PHLPP dephosphorylates in order to inhibit cell survival. Additionally, PHLPP dephosphorylates Signal Transducer and Activator of Transcription 1 (STAT1) and Inhibitor of Nuclear Factor Kappa B Kinase Subunit Beta (IKKB) to regulate inflammation. Two negative feedback loops regulate PHLPP expression. High levels of PHLPP expression reduce p70S6K activity and therefore reduce PHLPP protein translation and expression. Additionally, increased PHLPP levels reduces Akt activity and releases the inhibitory brake on Glycogen Synthase Kinase 3 (GSK3), leading to phosphorylation and degradation by the proteasome. Figure created in BioRender.com.

Figure 3.. PHLPP 2 Regulates Cardiac Hypertrophy Though a Novel Interaction with GRK 5.

(panel A) PHLPP 2 binds GRK 5 in cardiomyocytes. Following α - adrenergic stimulation with phenylephrine (PE), interaction between P HLPP 2 and GRK 5 decreases, allowing calmodulin (CaM) binding and translocation of GRK 5 to the nucleus. In the nucleus, GRK 5 acts as an HDAC kinase to phosphorylate HDAC 5 and relieve repression of transcription of genes involved in cardiac hypertrophy. (panel B) Removal of PHLPP 2 increased hypertrophic growth of cardiomyocytes through increased basal and PE-induced nuclear occupancy of GRK5. Figure created in BioRender.com.

Figure 4.. PHLPP1 Removal Accentuates Physiological Hypertrophy and Attenuates Pathological Hypertrophy.

(A) In wild-type (WT) mice, exercise induces physiological hypertrophy whereas pressure-overload via transverse aortic constriction (TAC) induces pathological hypertrophy, which leads to heart failure. (B) Removal of PHLPP1 increased Akt activity and capillary density in the heart and upregulation of angiogenic markers VEGF and Angpt2, without an increase in hypertrophy. Phlpp1^−/− mice subjected to exercise showed accentuated physiological hypertrophy relative to WT mice as seen by an increase in heart size and myocyte cell area without hypertrophic gene expression or fibrosis. Following pressure overload (TAC), Phlpp1^−/− mice had an attenuated pathological hypertrophic response relative to WT mice as demonstrated by reduced fibrosis, cell death, and preserved function. Figure created in BioRender.com.

Figure 5.. PHLPP in Disease.

Initially found as a tumor suppressor to play a key role in cancer, dysregulation of PHLPP isoforms have since been discovered to be involved in various pathophysiological states. PHLPP has been linked to several diseases including cancer, metabolic disorders (diabetes), brain injury and memory formation, cardiovascular disease (cardiac hypertrophy and heart failure), musculoskeletal disease, and atherosclerosis.