AKAPLbc and the Regulation of Rho and PKD

AKAP-Lbc is the second AKAP that is abundant in cardiac tissue and has been implicated in hypertrophic signaling pathways involving Rho and PKD (Fig. 4). AKAP-Lbc is the full-length form of the PKA-binding fragment Ht31 that was first isolated from human thyroid and has been widely used as a prototype to study the structural basis of PKA-AKAP interactions (and the physiologic importance of AKAP-mediated PKA anchoring in physiologic responses). AKAP-Lbc is a large 320-kDa modular protein with an N-terminal regulatory region that contains ankyrin repeats, a leucine zipper motif, sites for PKA and 14-3-3 binding and a C1 domain (homologous to the highly conserved compact DAG/PMA-binding site that was first identified in PKCs), followed by adjacent DH and PH domains [a characteristic structural feature of Rho guanine nucleotide exchange factors (GEFs)] and a second regulatory region that binds PKD at the C-terminus. AKAP-Lbc Rho-GEF activity is increased by GPCRs that couple to the heterotrimeric G12 proteins, Ga12 overexpression, or by serum (Diviani et al. 2001, 2004). Recent studies implicate AKAP-Lbc in a1-AR-dependent activation of Rho and induction of the hypertrophic phenotype in neonatal cardiomyocyte cultures (Appert-Collin et al. 2007). The observation that chronic a1-AR activation leads to increased AKAP-Lbc expression (which augments a1-AR-dependent activation of Rho) has been taken as tentative evidence that dynamic changes in AKAP-Lbc expression might amplify the early molecular events that contribute to the pathogenesis of cardiac hypertrophy/failure (i.e. that AKAP-Lbc might constitute a rather attractive target for therapeutic intervention).

AKAP-Lbc-Rho-GEF activity is autoinhibited in the basal state through intramolecular interactions with N- and/or C-terminal sequences; a truncation form

Fig. 4 mAKAP binds PKA, anchors PDE4D3, and serves as an adaptor to recruit Epac1 and ERK5. (a) Schematic representation and protein domain organization of AKAP-Lbc, which includes two ankyrin repeats, a PKA-binding domain, a C1 domain, and both Dbl (DH) and pleck-strin (PH) homology domains. (b) Various Ga12-coupled receptors enhance the Rho-GEF activity of AKAP-Lbc, leading to increased GDP-GTP exchange on RhoA and the activation of downstream effectors that remodel the actin cytoskeleton and influence gene transcription. AKAP-Lbc also localizes PKD with its upstream activator PKC (which interacts with the AKAP-Lbc-PH domain). (c) PKA anchored to AKAP-Lbc executes phosphorylations that inhibit AKAP-Lbc-Rho GEF activity and release active PKD from the AKAP-Lbc complex

Fig. 4 mAKAP binds PKA, anchors PDE4D3, and serves as an adaptor to recruit Epac1 and ERK5. (a) Schematic representation and protein domain organization of AKAP-Lbc, which includes two ankyrin repeats, a PKA-binding domain, a C1 domain, and both Dbl (DH) and pleck-strin (PH) homology domains. (b) Various Ga12-coupled receptors enhance the Rho-GEF activity of AKAP-Lbc, leading to increased GDP-GTP exchange on RhoA and the activation of downstream effectors that remodel the actin cytoskeleton and influence gene transcription. AKAP-Lbc also localizes PKD with its upstream activator PKC (which interacts with the AKAP-Lbc-PH domain). (c) PKA anchored to AKAP-Lbc executes phosphorylations that inhibit AKAP-Lbc-Rho GEF activity and release active PKD from the AKAP-Lbc complex of AKAP-Lbc encoding the DH- and PH domains without regulatory sequences (termed Onco-Lbc) is an oncogene in NIH 3T3 cells (Sterpetti et al. 1999). Recent studies implicate leucine zipper-mediated AKAP-Lbc homo-oligomerization and PKA-dependent phosphorylation of AKAP-Lbc at S1565 as a mechanism that decreases AKAP-Lbc-Rho-GEF activity (Baisamy et al. 2005; Diviani et al. 2004). While the AKAP-Lbc-^RHSi5tf5WGPGK site diverges somewhat from a canonical 14-3-3 binding site [RSXpSXP or RXXXpSXP], it nevertheless mediates 14-3-3 protein binding. Since 14-3-3 proteins typically assume a dimeric structure, 14-3-3 binding promotes AKAP-Lbc homo-dimerization and imposes a conformational constraint that inhibits Rho-GEF activity (Baisamy et al. 2005). In this manner, AKAP-Lbc anchored PKA antagonizes Rho signaling in cells.

AKAP-Lbc also has been implicated in the activation of PKD, a serine/threonine kinase that has recently emerged as a biologically important mediator of cardiac hypertrophy/remodeling. PKD phosphorylates class II HDACs (effectively neutralizing HDAC5's repressive effects on MEF2-dependent transcription and inhibiting HDAC5's antihypertrophic actions), cardiac troponin I, and CREB (Vega et al. 2004; Haworth et al. 2004; Harrison et al. 2006). PKD activation is via translocation to membranes and PKC-dependent phosphorylation of S744/S748 in the activation loop. There is recent evidence that the AKAP-Lbc-PH domain binds certain PKC isoforms (PKCa and PKCn) (Carnegie et al. 2004) and that AKAP-Lbc coordinates PKC-dependent PKD activation in cells (Carnegie et al. 2004). PKA contributes to the activation of AKAP-Lbc-anchored PKD by phosphorylating AKAP-Lbc at S2737 in the PKD-binding region that releases newly phosphorylated PKD to the cytosol.

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