βIV-spectrin/STAT3 complex regulates the orientation of hypertrophic growth

Background: Cardiac hypertrophy, defined as a stress-induced increase in heart mass/size, is a major risk factor for adverse cardiovascular events including heart failure and arrhythmia. Within this general definition, the orientation of cell and organ growth varies considerably depending on stress type and duration with important implications for cardiac function, yet, little is known regarding the mechanisms that regulate hypertrophic orientation. Here, we evaluated the role of the cytoskeletal protein βIV-spectrin and associated pro-hypertrophic signal transducer and activator of transcription (STAT3) to direct the orientation of hypertrophic growth. Methods: Transgenic mouse models with altered STAT3 signaling through modified interaction with its scaffolding partner βIV-spectrin, or phospho-regulation of STAT3 directly, were evaluated at baseline, and following transaortic constriction (TAC), or aortocaval fistula (ACF). Unbiased screening of gene expression from these structurally divergent states were evaluated for pathways responsible for directing myocyte length/width. These pathways were tested in vitro using primary mouse myocytes and in vivo to tune growth patterns for therapeutic intervention. Results: Loss of βIV-spectrin or direct STAT3 activation promoted a preferential increase in myocyte length over width, resulting in dilation of the left ventricular (LV) chamber (eccentric hypertrophy) and decreased systolic function. Conversely, preservation of βIV-spectrin favored an increase in myocyte width without LV dilation (concentric hypertrophy) and preserved systolic function in response to TAC or ACF. Differential expression of genes associated with microtubules, including the trafficking kinesin motor, KIF20A, were identified in concentric vs. eccentric hypertrophic states. In vitro assays revealed a relationship between βIV-spectrin/STAT3 signaling, KIF20A expression, microtubule density, and spatial distribution of mRNA for the sarcomeric gene actc1. Finally, intervention with pharmacologic STAT3 inhibition following chronic 6-week TAC successfully recovered concentric growth with improved systolic function. Conclusions: These data identify a novel and pivotal role for βIV-spectrin/STAT3 to direct myocyte geometry in response to chronic stress. These studies further illustrate the unique separation of hypertrophic growth and orientation as distinct pathways in cardiac remodeling.