Protein kinase D: coupling extracellular stimuli to the regulation of cell physiology

Abstract

Protein kinase D (PKD) mediates the actions of stimuli that promote diacylglycerol (DAG) biogenesis. By phosphorylating effectors that regulate transcription, fission and polarized transport of Golgi vesicles, as well as cell migration and survival after oxidative stress, PKDs substantially expand the range of physiological processes controlled by DAG. Dysregulated PKDs have been linked to pathologies including heart hypertrophy and cancer invasiveness. Our understanding of PKD regulation by trans- and autophosphorylation, as well as the subcellular dynamics of PKD substrate phosphorylation, have increased markedly. Selective PKD inhibitors provide new, powerful tools for elucidating the physiological roles of PKDs and potentially treating cardiac disease and cancer.

Figure 1

Domain organization and regulatory phosphorylation sites of protein kinase D isoforms. Mammalian PKD1, PKD2 and PKD3 have highly conserved DAG/PMA-binding (C1a, C1b), PH and kinase domains. The locations of regulatory serine and tyrosine phosphorylation sites are indicated. The text explains the way that these amino acids are phosphorylated and regulate PKD activity. Amino-acid sequences of C1a, C1b and kinase domains of Caenorhabditis elegans (DKF-2A and DKF-2B) and mammalian PKDs are more than 70% identical. The number of amino acids comprising individual PKD isoforms is shown on the right. DAG, diacylglycerol; DKF, D-kinase family, C. elegans PKD; PKD, protein kinase D; PMA, phorbol 12-myristate 13-acetate; PH, pleckstrin homology.

Figure 2

Isoforms of protein kinase D regulate crucial aspects of cell physiology. (A) Cell survival after oxidative stress. Mitochondria-derived ROS leads to DAG generation and PKD1 recruitment and activation. PKD1 then promotes the activation and translocation of NF-κB and co-activators from cytoplasm to nucleus. NF-κB-dependent transcription induces MnSOD, which eliminates ROS. (B) Inhibition of cell migration. PKD phosphorylates SSH1L and cortactin in the F-actin cytoskeleton, leading to inhibition of actin-severing and polymerization activities that enable lamellipodium formation and, thus, inhibition of cell migration. (C) Golgi-vesicle fission and transport. PKD1 activation leads to PI4P production, which enables the delivery of endoplasmic-reticulum-derived cholesterol and ceramide to Golgi membranes by the docking of transfer proteins with PI4P. There, ceramide and phosphocholine are converted to sphingomyelin and DAG. Sphingomyelin and cholesterol are crucial for packaging and sorting of TGN vesicles and DAG increases the curvature of the TGN membrane, thereby facilitating fission and transport of vesicles to the plasma membrane (inset). PKD1 prevents excessive, potentially toxic accumulation of cholesterol and ceramide through a negative-feedback loop. (D) Gene transcription. Activated PKDs phosphorylate HDAC5 and HDAC7 in the nucleus. Phosphorylated HDACs dissociate from the transcription activator MEF2, leading to their cytoplasmic accumulation. De-repressed MEF2 recruits co-activators and drives cell-specific programmes of gene transcription. Specific pathways are detailed in the text. ARP, actin-related protein; CERT, ceramide transfer protein; HDAC, histone deacetylase; MEF-2, myocyte enhancer factor 2; MnSOD, Mn-dependent superoxide dismutase; mRNA, messenger RNA; NF-κB, nuclear factor κB; OSBP, oxysterol-binding protein; PAP, phosphatidic acid phosphatase; PI, phosphatidylinositol; PI4K, phosphatidylinositol 4-kinase; PI4P, phosphatidylinositol 4-phosphate; PKC, protein kinase C; PKD, protein kinase D; PLD, phospholipase D; ROS, reactive oxygen species; SFK, Src family kinase; SMS sphingomyelin synthase; SSH1L, Slingshot 1L protein phosphatase; TGN, trans-Golgi network; WAVE-2, Wiskott–Aldrich verprolin homology domain protein 2; 7-TM, seven-transmembrane.

Figure 3

Intracellular distribution of protein kinase D effectors. PKDs provide integrated physiological responses to extracellular stimuli by disseminating signals carried by DAG to distinct groups of effectors located in several cell compartments. Details are provided in the text. CERT, ceramide transfer protein; DAG, diacylglycerol; HDAC, histone deacetylase; NF-κB, nuclear factor κB; OSBP, oxysterol-binding protein; PI4K, phosphatidylinositol 4-kinase; PKD, protein kinase D; RIN1, Ras and Rab interactor 1; SSH1L, Slingshot 1L protein phosphatase.

Ya Fu & Charles S Rubin