Limb-Bud and Heart (LBH) Upregulation in Cardiomyocytes under Hypoxia Promotes the Activation of Cardiac Fibroblasts via Exosome Secretion.

The activation of cardiac fibroblasts (CFs) after myocardial infarction (MI) is essential for post-MI infarct healing, during which the regulation of transforming growth factor beta1 (TGF- β 1) signaling is predominant. We have demonstrated that TGF- β 1-mediated upregulation of LBH contributes to post-MI CF activation via modulating α B-crystallin (CRYAB), after being upregulated by TGF- β 1. In this study, the effect of LBH-CRYAB signaling on the cardiac microenvironment via exosome communication and the corresponding mechanisms were investigated. The upregulation of LBH and CRYAB was verified in both cardiomyocytes (CMs) and CFs in hypoxic, post-MI peri-infarct tissues. CM-derived exosomes were isolated and identified, and LBH distribution was elevated in exosomes derived from LBH-upregulated CMs under hypoxia. Treatment with LBH+ exosomes promoted cellular proliferation, differentiation, and epithelial-mesenchymal transition-like processes in CFs. Additionally, in primary LBH KO CFs, western blotting showed that LBH knockout partially inhibited TGF- β 1-induced CF activation, while LBH-CRYAB signaling affected TGF- β 1 expression and secretion through a positive feedback loop. The administration of a Smad3 phosphorylation inhibitor to LBH KO CFs under TGF- β 1 stimulation indicated that Smad3 phosphorylation partially accounted for TGF- β 1-induced LBH upregulation. In conclusion, LBH upregulation in CMs in post-MI peri-infarct areas correlated with a hypoxic cardiac microenvironment and led to elevated exosomal LBH levels, promoting the activation of recipient CFs, which brings new insights into the studies and therapeutic strategies of post-MI cardiac repair.