Cytoskeleton remodeling Role of PKA in cytoskeleton reorganisation
Role of PKA in cytoskeleton reorganisation A wide variety of soluble signaling mediators utilize the Proteinkinase cAMP-dependent ( PKA ) pathway to regulate cellular processes includingintermediary metabolism, ion channel conductivity, and transcription. PKA plays acentral role in cytoskeletal regulation and cell migration. Moreover, the role ofPKA in cytoskeletal organization and cell migration, exerting both negative (i.e.inhibitory) and positive (i.e. required or enhancing) effects. GNAS complex locus coupled receptor ( G-protein alpha-scoupled receptor ) interaction with the trimeric G-protein alpha-s/ Guaninenucleotide binding protein beta and gamma ( G-protein beta/gamma ) causes theexchange of GDP for GTP bound to G-protein alpha subunits and the dissociation ofthe G-protein beta/gamma heterodimers. G-protein alpha-s activatesAdenylate cyclases. Upon stimulation, Adenylate cyclases increase the level ofCyclic Adenosine 3',5'-monophosphate ( cAMP ) in cells and activate thePKA-cat and PKA-reg complex that results in PKA activation . Negative effects of PKA on cell migration have been reported forintegrin-dependent endothelial cell migration. Also, matrix-specific down-regulation ofcAMP/ PKA signaling appears to be required for collagen-inducedActin synthesis and stress fiber formation in endothelial cells . PKA -dependent phosphorylation of complex Integrin alpha 4 andbeta 1 ( Alpha-4/beta-1 integrin ) is important for migration and other integrinfunction. Phosphorylation of Alpha-4/beta-1 integrin blocks Paxillinbinding, which activates cell migration and increases lamellipodial stability . The cytoskeletal regulatory protein Vasodilator-stimulatedphosphoprotein ( VASP ) localizes to focal adhesions, largely through interactionwith proteins such as Vinculin, Zyxin, and KIAA1274. PKAphosphorylates VASP and disrupts its interaction with C-abl oncogene 1 receptortyrosine kinase ( c-Abl ) . A relative newcomer to the list, the LIM and SH3 protein 1 (LASP1 ), was identified as a potential cytoskeletal PKA substrate ingastric fibroblasts and gastric parietal cells. LASP1 regulates its translocationto areas of dynamic actin filaments synthesis. Phosphorylation of LASP1 byPKA decreases its interaction with Actin . Adducin s promote association of Spectrin non-erythrocytic (Fodrin (spectrin) ) with Actin to facilitate capping of the fast growingend of Actin filaments. PKA phosphorylates Alpha adducin and Betaadducin and reduces their affinity for Fodrin (spectrin)/ Actincomplexes as well as the activity of Adducin s in promoting binding of Fodrin(spectrin) to Actin filaments . PKA directly phosphorylates monomeric Actin, whichcauses a significant decrease in monomer 'polymerizibility'. Myosin-based contractility is important for several aspects of cellmovement, including retraction of the trailing edge and less well-defined functionswithin the leading edge, where Myosin light chain ( MELC ) interaction withActin and myosin-dependent contractility are positively regulated byphosphorylation. MELC phosphorylation is proximally controlled by the ratio ofMyosin light chain kinase ( MLCK ) and Protein phosphatase 1 catalytic subunitbeta isoform ( MLCP ) activities. The regulation of MLCK and MLCP isintensely complex, and involves cAMP -dependent PKA signaling. MLCKis activated by the Ca(2+)/ Calmodulin binding. PKA can regulateCa(2+) release through phosphorylation and inhibition of Phospholipase C (PLC-beta ), resulting in the inactivation of receptors for Inositol trisphosphate( IP3 ). cAMP and PKA can up-regulate MLCP activity throughthe Rho-associated, coiled-coil containing protein kinase ( ROCK )-dependentinhibition of MLCP phosphorylation and inhibition of Ras homolog gene family,member A ( RhoA ). The influence of PKA on MLCK and MLCP hasthe same effect on decreasing MELC phosphorylation and stress fibers formation. A kinase (PRKA) anchor protein 13 ( LBC ) activity can beinhibited by the anchoring both PKA and Tyrosine 3-monooxygenase/tryptophan5-monooxygenase activation protein beta polypeptide ( 14-3-3 beta/alpha )proteins. LBC has a close functional link with the actin cytoskeletonthrough its interaction with the RhoA and ability to promote G-protein coupledreceptors-dependent stress fiber formation. Conversely, elevation of cAMP level and activation of PKAhave been shown to be required for efficient cell migration, or hallmark steps ofmigration, in several systems as well. These include: formation of filopodia andlamellipodia, which are governed by the activation of Cell division cycle 42 (CDC42 ) and Ras-related C3 botulinum toxin substrate 1 ( Rac1 )respectively. Stimulation of PKA by cAMP results in Rho guanine nucleotideexchange factor 7 ( BETA-PIX ) phosphorylation, which in turn controlsBETA-PIX translocation to focal complexes and Rac1 and CDC42activation .