Atrogin-1/MAFbx Enhances Simulated Ischemia/Reperfusion-induced Apoptosis in Cardiomyocytes through Degradation of MAPK Phosphatase-1 and Sustained JNK Activation

Abstract

Atrogin-1/MAFbx is a major atrophy-related E3 ubiquitin ligase that is expressed specifically in striated muscle. Although the contribution of atrogin-1 to cardiac and muscle hypertrophy/atrophy has been examined extensively, it remains unclear whether atrogin-1 plays an essential role in the simulated ischemia/reperfusion-induced apoptosis of primary cardiomyocytes. Here we showed that atrogin-1 markedly enhanced ischemia/ reperfusion-induced apoptosis in cardiomyocytes via activation of JNK signaling. Overexpression of atrogin-1 increased phosphorylation of JNK and c-Jun and decreased phosphorylation of Foxo3a. In addition, atrogin-1 decreased Bcl-2, increased Bax, and enhanced the activation of caspases. Furthermore, JNK inhibitor SP600125 markedly blocked the effect of atrogin-1 on cell apoptosis and the expression of apoptotic-related proteins and caspases. Importantly, atrogin-1 induced sustained activation of JNK through a mechanism that involved degradation of MAPK phosphatase-1 (MKP-1) protein. Atrogin-1 interacted with and triggered MKP-1 for ubiquitin-mediated degradation. In contrast, proteasome inhibitors markedly blocked the degradation of MKP-1. Taken together, these results demonstrate that atrogin-1 promotes degradation of MKP-1 through the ubiquitin-proteasome pathway, thereby leading to persistent activation of JNK signaling and further cardiomyocyte apoptosis following ischemia/reperfusion injury.

The E-box Binding Factors Max/Mnt, MITF, and USF1 Act Coordinately with FoxO to Regulate Expression of Proapoptotic and Cell Cycle Control Genes by Phosphatidylinositol 3-Kinase/Akt/Glycogen Synthase Kinase 3 Signaling

“…Thus, regulation of MITF by PI 3-kinase/Akt/GSK3 signaling also plays a critical role in the control of apoptosis. siRNA knockdowns of either ATROGIN-1 or TXNIP also significantly protected cells from apoptosis following PI 3-kinase inhibition, consistent with previous studies showing that both Atrogin-1 (55) and TXNIP (56) promote apoptosis. These results indicate that E-box binding as well as FoxO transcription factors, regulated by Akt and GSK3, function to regulate the expression of genes downstream of PI 3-kinase signaling.…”

Muscle RING finger-1 attenuates IGF-I-dependent cardiomyocyte hypertrophy by inhibiting JNK signaling

“…34). In the JNK signaling pathway alone, at least six proteins with ubiquitin ligase activity have been described, including MEKK1, Fbw7, DCX, itch, MuRF1, and atrogin-1/MAFbx (17,23,25,31,50,58,59). Interestingly, MEKK1 is a MAPK3K in the JNK signaling pathways that phosphorylates MAP2K, which in turn phosphorylates JNK, which then phosphorylates c-Jun (37).…”

The Polycomb group (PcG) protein EZH2 supports the survival of PAX3-FOXO1 alveolar rhabdomyosarcoma by repressing FBXO32 (Atrogin1/MAFbx)

“…This hypothesis is supported by the concomitant protein downregulation of Myogenin and MyoD seen in EZH2-depleted cells, the latter known as directly targeted by FBXO32 for degradation in myoblasts, [57][58][59] testifying the inability of our cells to undergo differentiation. 32 In addition, a decrease in BCL2, which has been shown to be degraded in cardiomyoblasts through a mechanism that indirectly involves FBXO32, 60 could support the triggering of an apoptotic process. These observations are in keeping with recently published data showing that the FBXO32 overexpression is not capable to induce apoptosis in the absence of chemotherapeutic drugs, although it is able to enhance the drug-induced pro-apoptotic effect in an 'activated' context.…”