2. ABPs in Cardiac Hypertrophy
2.1. Profilin-1
Profilin is widely expressed in most eukaryotes and has a molecular weight of about 17 kDa
[39][31]. There are various profilin isoforms expressed in different tissues. Profilin-1 is universally expressed, profilin-2 is specifically expressed in the brain and profilin-3 and profilin-4 are specifically expressed in kidney and testis, respectively
[40][32]. Profilin accelerates the nucleotide exchange of G-actin and delivers ATP-G-actin to the growing barbed ends of F-actin through interacting with the poly-proline motifs of formin, vasodilator-stimulated phosphoprotein (VASP) and CDC42-activated Wiskott Aldrich syndrome protein (WASP)/WASP family
[20,41,42,43][12][33][34][35].
Profilin-1 is directly associated with cardiac hypertrophy
[44][36]. Overexpression of profilin-1 in the vascular tissues of FVB/N mice leads to vascular remodeling and hypertension by increasing actin aggregation, which provides mechanical stress for the development of cardiac hypertrophy
[45,46][37][38]. It has been shown that the protein level of profilin-1 is significantly increased in mammalian hypertrophic hearts (
Figure 21). The myocardin-related transcription factor megakaryoblastic leukemia (MKL) induces the expression of the signal transducer and activator of transcription 1 (STAT1) via its SAP-domain (SAF-A/B, acinus and PIAS protein domain) activity, which upregulates
PFN expression
[47][39]. Whether this is the explanation for the increased protein level of profilin in cardiac hypertrophy remains to be investigated. In cardiomyocytes, the functional abnormality of profilin-1 can change the abundance or activity of multiple proteins associated with cardiomyopathy. For example, the overexpression of profilin-1 can contribute to decreases in the phosphorylation level of endothelial nitric oxide synthases (eNOS) at Ser1177 in the hearts of spontaneous hypertensive rats
[17][9]. Levels of atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), skeletal muscle α-actin (α-SMA) and phosphorylated ERK1/2 (active form) were significantly increased in neonatal rat ventricular myocytes (NRVMs) following stimulation by phenylephrine or endothelin 1, which can be inhibited by siRNA-directed
PFN1 silencing
[44][36]. Increased phosphorylation of ERK1/2 activates the mechanistic (mammalian) target of rapamycin complex 1 (mTORC1) that subsequently inhibits autophagy
[48,49,50][40][41][42]. It may be a potential key mechanism of cardiac hypertrophy mediated by the dysregulation of profilin-1 (
Figure 21). Additionally, the inhibition of Rho-associated coiled-coil-containing protein kinase pathway (ROCK) can suppress the upregulation of profilin-1 induced by advanced glycation end products (AEGs) in H9c2 cells
[51][43]. By comparison, overexpression of
PFN1 results in the reactivation of fetal genes (
NPPA and
NPPB), an increase in F-actin in myocardium and destruction of myofibrils
[44][36]. These processes can be reversed by inhibiting the expression of profilin-1
[17][9]. The inhibition of profilin-1 expression in H9c2 cells and Sprague–Dawley rats can attenuate cardiac hypertrophy induced by AEGs
[51,52][43][44]. In
Drosophila, myocyte-specific overexpression of profilin leads to disorders in muscle fibers and sarcomeres, which result in damaged muscle ultrastructure and function
[44][36].
Figure 21. Profilin-1 mediates cardiac hypertrophy. In normal cardiomyocytes, profilin-1 is at a basal level and the fetal genes are not activated. Pathological stimuli increase the protein level of profilin-1, which results in ERK1/2 activation, F-actin accumulation and eNOS inhibition. This results in the reactivation of hypertrophy-related genes, inhibition of autophagy and damage to sarcomere structure and, ultimately, the development of cardiac hypertrophy.
2.2. ADF/Cofilin
Actin-depolymerizing factor (ADF)/cofilin consists of a single ADF homologous domain and has a molecular weight of about 15 kDa. The ADF/cofilin family contains ADF (also known as destrin, mainly expressed in endothelial and epithelial cells) and two cofilin isoforms (cofilin-1 is universal and cofilin-2 is cardio-specific)
[53,54][45][46]. ADF/cofilin can bind to both G-actin and F-actin and can sever and depolymerize F-actin in regulating actin dynamics, which contributes to the cell contractility power
[55][47]. The activity of cofilin is regulated by phosphorylation primarily from the ROCK/Lin-11, Isl1 and MEC-3 domain kinase (LIMK)/cofilin signaling pathway (
Figure 32)
[56,57][48][49]. Cofilin is inactivated via phosphorylation.
Figure 32. Proposed roles of cofilin-2 in cardiac hypertrophy. Neurohumoral factors (e.g., Ang II, ET 1 and leptin) lead to cofilin-2 phosphorylation through the RhoA/ROCK/LIMK signaling pathway. Phosphorylated cofilin-2 can lead to F-actin accumulation, which may subsequently contribute to cardiac hypertrophy through disrupting autophagy. In addition, it promotes the activation of ERK1/2 and p38, which contributes to the inhibition of autophagy and the reactivation of hypertrophy-related genes, which subsequently cause cardiac hypertrophy.
The abundance change in cofilin-2 does not play a role in the morphogenesis of neonatal rat cardiomyocytes
[58][50], while its activity is closely associated with the development of cardiac hypertrophy. The levels of phosphorylated cofilin-2 are increased in myocardial hypertrophy through the activation of LIM-kinase (LIMK) by ROCK, which is induced by multiple neurohumoral factors, such as angiotensin II
[59,60][51][52], endothelin 1
[19][11] and leptin
[18,61][10][53]. In hypertrophic cardiomyocytes, the increase in levels of phosphorylated cofilin-2 results in an increase in F-actin/G-actin ratios and the levels of phosphorylated ERK1/2 and p38
[19,61,62,63,64][11][53][54][55][56]. Y-27632
[19][11], an inhibitor of ROCK, can reduce the levels of phosphorylated cofilin-2 through the inhibition of ROCK activity, which attenuates endothelin-1-induced neonatal cardiomyocyte hypertrophy, whereas this is achieved in ginseng (
Panax quinquefolius)
[62][54] through inhibition of p115Rho guanine nucleotide exchange factor (GEF) activity, which inhibits leptin-induced cardiac hypertrophy. In addition, WD-repeat domain 1 (WDR1), a major cofactor of the ADF/cofilin, has been reported to protect myocardium from myocardial hypertrophic stimuli
[13][5].
2.3. Formin
Formin is a type of multidomain protein consisting of 7 subfamilies and 15 members in human genes. Formins are characterized by the presence of two conserved domains: formin homology 1 (FH1) and FH2. FH1 binds to the profilin–actin complex via poly-proline sequences and brings the G-actin to FH2, which promotes actin nucleation and polymerization
[11,65][3][57].
2.4. CapZ
CapZ, a type of capping protein, anchors F-actin to the Z disc and regulates actin turnover, which contributes to sarcomere structural changes
[76,77][58][59]. PIP2, a downstream effector of RAC1, can promote the dissociation of CapZ from F-actin by weakening their binding affinity
[78,79,80][60][61][62].
Overexpression of CapZ in transgenic mice can lead to fatal cardiac hypertrophy
[77][59]. It has been shown that hypertrophic agonists, phenylephrine or endothelin can reduce the binding affinity between CapZ and F-actin via PIP2-dependent pathways in NRVMs
[81][63]. This may result in sarcomere remodeling, which induces cardiac hypertrophy. The cyclic mechanical strain activates downstream focal adhesion kinase (FAK) via the mechanotransduction of integrin, which then activates phosphatidylinositol 4-phosphate 5-kinase (PIP5K) through the RhoA/ROCK pathway. PIP5K phosphorylates phosphatidylinositol 4-phosphate (PI4P) in order to produce PIP2, which reduces the affinity of CapZ and F-actin binding, which contributes to the dysregulation of F-actin assembly and cardiac hypertrophy (
Figure 53)
[80,82,83,84][62][64][65][66].
Figure 53. CapZ regulates cardiac hypertrophy. Mechanotransduction leads to the activation of RhoA/Rho-kinase pathway through integrins, which reduce the binding affinity of CapZ and F-actin. It subsequently causes cardiac hypertrophy.