Our data suggest that enhancing Akt activity by inhibiting its PHLPP1-mediated dephosphorylation promotes processes associated with physiological hypertrophy that may be beneficial in attenuating the development of pathological hypertrophy.

Crucial cellular decisions that lead to cell growth, metabolism, proliferation, and survival are all dependent on the precise control of the phosphorylation state of proteins. The serine/threonine phosphatase, PHLPP (PH domain leucine-rich repeat protein phosphatase) has been shown to directly dephosphorylate several members of the AGC family of kinases. Knockdown of PHLPP1 by siRNA in neonatal cardiomyocytes potentiates Akt activity and phosphorylation specifically at Ser473 basally and following agonist stimulation while, the removal of PHLPP2 in cardiomyocytes does not affect Akt phosphorylation as previously reported in other cells. We hypothesize that PHLPP2 may target other AGC kinases in cardiomyocytes to regulate cardiac hypertrophy. Preliminary data suggests that removal of PHLPP2 activates fetal gene re-expression at baseline and potentiates phenylephrine (PE) induced gene expression 2 fold over siControl. Recently, G protein-coupled receptor kinase 5 (GRK5), which is an AGC kinase, has been shown to regulate cardiac hypertrophy through HDAC5 phosphorylation and de-repression of gene transcription. We wanted to determine whether PHLPP2 regulates GRK5 phosphorylation and localization in cardiomyocytes. GRK5 translocates to the nucleus following hypertrophic stimulation and we found that removal of PHLPP2 increased GRK5 translocation to the nucleus at baseline and with PE treatment compared to siControl cells. Also, removal of PHLPP2 increased nuclear export of HDAC5 at baseline and following PE treatment. Conversely, overexpression of PHLPP2 blocked nuclear translocation of GRK5 following PE treatment. Ongoing studies will determine whether PHLPP acts as a scaffold or if its phosphatase activity is necessary for inhibition of GRK5 translocation by directly measuring the phosphorylation of GRK5 in the presence and absence of PHLPP2 following hypertrophic stimulation. Our preliminary data is the first to uncover GRK5 as a novel PHLPP2 target in cardiomyocytes. Since little is known about the non-canonical regulation of GRK5, understanding whether phosphorylation and localization is regulated within the cardiomyocyte by PHLPP has potential for new therapeutic targets in the treatment of cardiac hypertrophy and failure.