Knowledge Bank

Heart failure acts as a major source of morbidity and mortality in the U.S., with 870,000 new cases annually and a total cost of $30.7 billion to the healthcare system. Cardiac fibrosis, a common finding in heart failure, has been associated with increased risk for arrhythmias and cardiac dysfunction. Transition of resident cardiac fibroblast (CF) phenotype from a resting, quiescent state to an active state with increased proliferation, contractility and deposition of extracellular matrix is a critical step in the fibrotic response to chronic stress. Recently, our group has identified a novel complex involving the cytoskeletal protein βIV-spectrin and the multifunctional transcription factor STAT3, important for the regulation of normal gene transcription and maintenance of the quiescent phenotype in CFs. Furthermore, we found that loss of βIV-spectrin function promotes STAT3 dysregulation, CF activation and fibrosis in the setting of chronic pressure overload. However, the upstream extracellular stress signals that promote βIV-spectrin/STAT3 dysfunction and subsequent CF activation remain unknown. We hypothesized that the pro-fibrotic cytokine TGF-β promotes βIV-spectrin degradation in CFs and that spectrin-deficient CFs release an extracellular stress signal to propagate cardiac dysfunction. Primary adult cardiac fibroblasts were isolated and cultured from wildtype (WT) and qv4J mice expressing truncated βIV-spectrin lacking interaction with STAT3. Specifically, WT and qv4J CFs were treated with TGF-β (10ng/mL) or vehicle (1xPBS) in culture for 48 hrs. to evaluate STAT3-related gene expression via quantitative polymerase chain reaction (qPCR). In parallel, conditioned media experiments were performed on WT CFs to assess their proliferation rates and gene expression. Our results show that WT CFs treated with TGF-β demonstrate ~40% decrease in βIV-spectrin gene expression (i.e. Sptnb4) compared to vehicle control. Interestingly, WT CFs treated with conditioned media from spectrin-deficient (qv4J) CFs displayed increased proliferation rates along with higher STAT3 differentially regulated gene expression compared to conditioned media from WT CFs. These data identify TGF-β as a potential stress signal for dysregulation of the βIV-spectrin/STAT3 complex and suggest that βIV-spectrin deficient CFs transmit spectrin-based dysfunction to neighboring cells, although the exact signal(s) remain to be determined. Our findings elucidate a novel pathway for regulating CF function and cell-to-cell communication in heart with implications for chronic disease.