PKC and PKN in heart disease

Abstract

The protein kinase C (PKC) and closely related protein kinase N (PKN) families of serine/threonine protein kinases play crucial cellular roles. Both kinases belong to the AGC subfamily of protein kinases that also include the cAMP dependent protein kinase (PKA), protein kinase B (PKB/AKT), protein kinase G (PKG) and the ribosomal protein S6 kinase (S6K). Involvement of PKC family members in heart disease has been well documented over the years, as their activity and levels are mis-regulated in several pathological heart conditions, such as ischemia, diabetic cardiomyopathy, as well as hypertrophic or dilated cardiomyopathy. This review focuses on the regulation of PKCs and PKNs in different pathological heart conditions and on the influences that PKC/PKN activation has on several physiological processes. In addition, we discuss mechanisms by which PKCs and the closely related PKNs are activated and turned-off in hearts, how they regulate cardiac specific downstream targets and pathways, and how their inhibition by small molecules is explored as new therapeutic target to treat cardiomyopathies and heart failure.

Keywords: Cardiomyopathy; Kinase; PKC; PKN; Phosphorylation.

Fig. 1

Schematic overview of the PKC and PKN families of kinases. Evolutionary analysis of human PKN and PKC kinase domains. Domain layout of conventional, novel and atypical PKCs, as well as PKN isozymes is shown, in addition to ligands, agonists or binding partners for their activation. Abbreviations: DAG – diacylglycerol; FA – fatty acid; HR1 – polybasic coiled-coil homology region 1; PB1 – Phox and Bem1; PIP₂ – phosphatidylinositol 4,5-bisphosphate; PS – phosphatidylserine; Rho - Ras homology.

Fig. 2

Activation of PKCs and PKNs. A-B. Schematic steps for the canonical activation of prototypical conventional PKC isozymes (A) or PKNs (B) are illustrated. C. Non-canonical activation by protease activity or tyrosine phosphorylation.

Fig. 3

Phospholamban phosphorylation at Ser10. A. Verification of the specificity of the phospholamban (PLN) Ser10 antibody using in vitro kinase assay-mediated phosphorylation of wildtype (wt) or Ser10Ala mutant (S10A) GST-PLN fusion proteins. Only phosphorylation at Ser10, but not S10A mutant PLN is recognized by the antibody. B. Analysis of SERCA2, total and phospho-Ser10 PLN levels in total cardiac extracts of control (Ctl), Ankrd1 knockout (Ankrd1-ko), MLP knockout (MLP-ko) or Ankrd1/MLP double knockout (dKO) mice indicates increased monomeric PLN phosphorylation at Ser10 in MLP-ko. Cardiac actin was used as loading control. C. Analysis of SERCA2, total and phospho-Ser10 PLN levels in cardiac extracts from non-failing individuals and dilated cardiomyopathy patients. Molecular weights of monomeric and pentameric phospholamban species are indicated, and reveal increased monomeric phosphorylation of PLN at Ser10 in patients with pathologically decreased SERCA2 levels. GAPDH was used as loading control.