Background: Takotsubo syndrome (TTS), also known as broken heart syndrome, is defined as a temporary cardiac ischemia associated with left ventricular ballooning, mimicking acute coronary syndrome without stenosis of coronary arteries. Typically, an acute episode of intense emotional stress precedes this left ventricular apical hypocontractility and functions as a triggering factor for a catecholamine surge. Circulating catecholamines may then lead to coronary artery spasms, constriction of the myocardial microvasculature, and/or direct cardiomyocyte toxicity which produces a transient stunning effect on the left ventricular myocardium. Whereas the pathogenic role of the excessively elevated circulating catecholamine concentrations is well known, the molecular mechanisms counteracting the impaired apical contractility need to be identified. Therefore, in the present study, we attempt to explore the influence of catecholamine exposure on cytokine-mediated STAT (signal transducer and activator of transcription) signalling using induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) obtained from five TTS patients and healthy controls.

Methods and Results: Isolated somatic cells from TTS patients and unrelated controls were reprogrammed into iPSCs, differentiated into cardiomyocytes and finally exposed to interferon-γ (IFNγ) in the presence or absence of epinephrine. Western blotting using a phosphotyrosine-specific STAT1 antibody demonstrated an unaltered phosphorylation status of STAT1 upon co-stimulation with epinephrine. In contrast, simultaneous exposure to epinephrine and IFNγ showed an elevated tyrosine phosphorylation of its homolog STAT3 when compared to cells treated with IFNγ alone in both control and TTS iPSC-CMs. Despite normal STAT1 phosphorylation in co-treated TTS iPSC-CMs, we found an impaired nuclear accumulation of STAT1 in co-treated iPSC-CMs compared to cells stimulated exclusively with IFNγ. Gelshift experiments using radioactively labelled double-stranded oligonucleotides containing a high-affinity DNA binding site, termed gamma-activated sequence (GAS), showed no specific STAT1 binding in untreated or epinephrine-treated iPSC-CMs. While IFNγ treatment resulted in prominent GAS binding, co-stimulation of IFNγ with epinephrine showed a significantly reduced STAT1 DNA-binding activity compared to cells exclusively stimulated with IFNγ under the same conditions. The impaired DNA binding in the combined presence of cytokine and epinephrine was observed in both control and TTS iPSC-CMs. Tetramer formation on two GAS sites aligned in tandem orientation was normal in cellular extracts from both IFNγ- and IFNγ/epinephrine-treated iPSC-CMs, respectively. However, transcriptional activation was reduced on numerous STAT target genes in IFNγ-treated iPSC-CMs exposed additionally to epinephrine as compared to cells treated with the same concentration of IFNγ alone.

Conclusions: Based on these results, we conclude that adrenergic pathway stimulation alters the selectivity of STAT proteins to respond to cytokine stimulation from STAT1 to STAT3. In contrast to STAT1, its homolog STAT3 is known to execute protective functions in myocardial infarction, ischemia-reperfusion and non-ischemic cardiomyopathies. A similar cardio-protective function mediated by the hyper-active STAT3 may ameliorate the adverse hemodynamic consequences of the catecholamine-induced vasoconstriction observed in TTS patients.