Nuclear Co-translocation of Myotrophin and p65 Stimulates Myocyte Growth

Abstract

Myotrophin, a 12-kDa ankyrin repeat protein, stimulates protein synthesis and cardiomyocyte growth to initiate cardiac hypertrophy by activating the NF-B signaling cascade. We found that, after internalization into myocytes, myotrophin cotranslocates into the nucleus with p65 to stimulate myocyte growth. We used structure-based mutations on the hairpin loops of myotrophin to determine the effect of the loops on myotrophin and p65 localization, induction of protein synthesis, and cardiac hypertrophy. Loop mutants, most prominently glutamic acid 33 3 alanine (E33A), stimulated protein synthesis much less than wild type. Myotrophin-E33A internalized into myocytes but did not translocate into the nucleus and failed to promote nuclear translocation of p65. In addition, two cardiac hypertrophy marker genes, atrial natriuretic factor and ␤-myosin heavy chain, were not up-regulated in E33A-treated cells. Myotrophin-induced myocyte growth and initiation of hypertrophy thus require nuclear co-translocation of myotrophin and p65, in a manner that depends crucially on the myotrophin hairpin loops.Myotrophin, a 12-kDa protein, is the smallest known ankyrin (ANK) 2 repeat protein (1) and contains four ANK repeats (2, 3). Myotrophin, also known as V-1 protein, was identified in spontaneously hypertensive rat hearts, cardiomyopathic human hearts (4), and neonatal rat cerebella (5). Myotrophin expression is ubiquitous in mammalian tissues (6). It is known to participate in the catecholamine synthesis signaling pathway (7), in regulation of actin polymerization (8), and in regulation of capping and uncapping by actin capping protein at the barbed ends of actin filaments (9). Myotrophin plays an important role in stimulation of cardiac myocyte growth and cardiac hypertrophy (4, 10). Sil et al. (11) reported an increase in myotrophin levels in human cardiomyopathic heart compared with normal heart. Overexpression of myotrophin in transgenic mice initiates cardiac hypertrophy that progresses toward heart failure (12, 13). Significantly, an increase in myotrophin level in blood plasma is observed after acute myocardial infarction (14) and in patients with heart failure (15). These data have established myotrophin as an initiator of cardiac hypertrophy and eventual heart failure.Myotrophin has been shown to interact with NF-B proteins (6, 16), and myotrophin-induced myocyte growth appears to be mediated through activation of the NF-B pathway (10), although the details of these interactions are poorly characterized. Knuefermann et al. (16) showed that myotrophin physically associates with p65 and prevents the reassociation of the p65-p50 heterodimer, resulting in activation of the NF-B pathway. Separate studies have established the involvement of NF-B signaling in cardiac hypertrophy and dilated cardiomyopathy (17-19) and the importance of NF-B activation in the pathogenesis of cardiac remodeling and heart failure (20 -22). Recently, Gupta et al. (13) showed that progression of cardiac hypertrophy to heart failure is associated with...

Retracted: Influence of p53 in the transition of myotrophin-induced cardiac hypertrophy to heart failure

“…Neonatal rat (Rattus norvegicus) ventricular cardiomyocytes were isolated from 3-to 4-day-old American Wistar rat pups according to the procedure described by Sen et al 10 2.4 Confocal imaging to detect activation of p53 by myo and quantitation of cell surface area Isolated rat neonatal ventricular myocytes were stimulated with myo and 5-fluorouracil (5-FU, a potent activator of p53) 18 for 24 h; cell fixation and permeabilization and confocal imaging was performed as described previously 12,19 (see Supplementary material online, Methods). Stimulation of cardiomyocytes with 100 nm Angiotensin II (Sar 1 ) 20 was performed as positive control for the measurement of cell surface area using Image-Pro Plus 6.1 software (see Supplementary material online, Methods).…”

“…Total RNA was isolated and reverse transcription (RT) PCR analysis was performed following the protocol described previously 12 (see Supplementary material online, Methods).…”

Capping protein regulators fine-tune actin assembly dynamics

“…Consistent with the proposed steric mechanism of CP regulation by V-1, point mutations in the basic patch of CP disrupt both V-1 binding and barbed-end capping 62 . The binding sites on V-1 / myotrophin for CP and for NFkappaB are essentially identical 53,60 , suggesting that V-1 / myotrophin might interact with CP or NFkappaB, but not both. If so, competition between CP and NFkappaB for binding V-1/myotrophin could direct cross-talk between signaling and control of actin assembly.…”

“…Originally identified as a protein upregulated in the cerebellum during mouse development 54,55 , V-1 was independently linked to cardiac hypertrophy and thus called myotrophin 56-59 . V-1 / myotrophin also interacts with NF-kappaB and co-translocates to the nucleus with p65, which is involved in inducing cardiac hypertrophy 60,61 …”

ATO /miRNA-885-5p/MTPN axis induced reversal of drug-resistance in cholangiocarcinoma

“…We have only covered a bit of the biological functions of MTPN. It has been found that MTPN and p65 are co-transported to the nucleus of cardiac myocytes based on the structure of MTPN hairpin, which induces protein synthesis and myocardial growth [35]. Owing to the mechanism of MTPN leading to cell drug resistance is still not clear, it may be the next research direction in CCA.…”