Intracellular signaling pathways that transduce hypertrophic responses involve specific G-protein isoforms, low-molecular-weight GTPases (Ras, RhoA, and Rac), MAPK, AKT, PKC, Calcineurin-NFAT system, GP130-STAT, IGF1, FGF and TGF-β.Within the cardiovascular system, three functional classes of GPCRs are of primary importance owing to their acute hemodynamic and chronic myotrophic effects. All G-proteins consist of the subunits G-α and G-β γ, which upon activation dissociate and independently modulate the activity of downstream signaling effectors, typically adenylate cyclase (AC) or phospholipase C (PLC). In addition, free G-βγ subunits directly enhance MAPK signaling, PI3K activity, and Ras signaling in the heart.
Activation of PLC leads to increased hydrolysis of membrane PIP2 to produce IP3 and DAG. DAG binds to and activates PKC, which phosphorylates numerous substrates. IP3 binds to IP3R on the surface of the ER leading to release of Ca2+ ions. The increased intracellular free Ca2+ levels activate the protein phosphatase Calcineurin by disrupting the inhibitory effects of Calmodulin. Calcineurin activation leads to the dephosphorylation of NFATc4, allowing it to enter the nucleus, where it cooperates with other transcription factors such as GATA4, E12, CBP, p300, MEF2 to induce transcription of genes essential for cardiac development and hypertrophy. The ADSS gene expression is the outcome of the activation of the enhancer region that contains binding sites for NKX2.5, GATA4, MEF2, E12, HAND1, and HAND2.
Receptor-mediated activation of the G-αi subunit results in the direct attenuation of AC in the heart. AC catalyzes the formation of cAMP, which augments myocardial contractility through a PKA signaling pathway. Activation of PKA directly inhibits phospholamban and promotes increased SERCA activity and augments Ca2+ handling in the heart. Because G-αi inhibits AC activity, increased expression of G-αi contributes to the pathology of cardiac hypertrophy and heart failure.
The Rho, Rac, and Cdc42 subfamilies regulate the cytoskeletal organization of cardiomyocytes. AgtII-stimulated RhoA activation in cardiac myocytes results in the formation of premyofibrils. RhoA also activates ROCK, which in turn promotes activation of MLCK. MLCK, which is also activated by Ca2+/calmodulin, is sufficient to induce sarcomeric organization, one of the hallmarks of the hypertrophic phenotype. Ras activation initiated by membrane-bound receptors promotes activation of Raf1 and PI3K. In addition, Ras activity is important in all three MAPK signaling branches, all of which participate in the hypertrophic response. MAPK pathways are activated in cardiomyocytes by GPCRs, RTKs, TGF-βR, PKC, Ca2+, or stress stimuli. TGF-βR regulates MEKs by signaling through TAK1/TAB1. The ERKs are activated by MEK1/2, the JNKs are activated by MKK4/7, and the p38-MAPKs are activated by MKK3/6. MAPKAPK1 is involved in transcriptional regulation. In addition, MAPKAPK2/3 phosphorylate Hsp25/27 and thereby confer cytoprotection. Activated MAPKs directly phosphorylate serine and threonine residues in a wide array of cytoplasmic proteins and transcription factors, including MAPKAPK2 and 3, MEF2, ATF2, ATF6Elk1 and c-Jun.
Induction of GP130, a promiscuous receptor for several cytokines, including IL-6, IL-11, LIF, and CT1 leads to activation of MAPK, PI3K and STAT3 pathways, which results in the induction of genes involved in hypertrophy and survival pathways. Activation of IGF1R phosphorylates IRS1, leading to signal transduction through GRB2-SOS complex and resulting in Ras activation. PI3K is also activated by IRS1, which then leads to Akt activation. Two well-defined direct downstream targets of Akt are GSK3β and mTOR. mTOR speeds up the process of protein synthesis by activating its downstream targets p70S6K and eIF4E.