Suppressor of cytokine signaling 3 (SOCS3) is a critical attenuator of the JAK-STAT signaling pathway, and it is involved in mediating the intensity and duration of STAT3 activation in the process of myocardial protection. Nuclear factor-κB (NF-κB) has emerged as a decisive transcription factor in cardiac myocyte compensatory responses to stress that enhance survival. However, the expression, activation and regulation of this signaling molecule in response to hypoxic stress have not been elucidated. We investigated 40 infants with cyanotic or acyanotic cardiac defects, as well as H9c2 embryonic rat cardiomyocytes, to examine the effect of hypoxia on the expression or activation of SOCS3, STAT3 and NF-κB in vivo and in vitro. We found an increase in endogenous cardiac SOCS3, p-STAT3 and AC-RelA activation in the myocardium of infants with cyanotic cardiac defects. In hypoxic cultivated H9c2 cells, SOCS3, STAT3 and AC-RelA activity slowly increased and then reached a stable expression. We evaluated the interaction of SOCS3 with STAT3 and NF-κB by transfecting the SOCS3 plasmid to hypoxic cultured H9c2 cells. Forced expression of SOCS3suppressed tyrosine phosphorylation of STAT3 and transcription of the C-myc and interleukin-6 genes. AC-RelA activation was also suppressed by over expression of SOCS3. These findings suggest that the mechanism of a positive transactivation loop that maintains higher levels of NF-κB and p-STAT3 and the negative feedback factor SOCS3, which maintains balanced NF-κB and p-STAT3 activities, is important in the process of myocardial adaptation to chronic hypoxia. SOCS3 is a rapid hypoxia inducible gene and acts to inhibit activation of the cellular signaling pathway in a classical negative feedback loop. Upregulated SOCS3 might play an important role in cardiocytes during chronic hypoxia as SOCS3 regulates cell signaling crosstalking between NF-κB and p-STAT3 under stressful conditions.
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