Mild cold exposure mitigates ischemic heart injury through indole-3-acetamide–AHR-mediated SMAD2 proteolysis

Ambient temperature profoundly influences organismal metabolism, physiology, and disease. While cold exposure is a well-recognized risk factor for the onset of myocardial infarction (MI), its impact on the injured heart after MI remains unexplored. Here, we demonstrate that a mild cold exposure (MCE) regimen at 16oC improved mice systolic function post-MI compared to the controls kept at room temperature. Integrative multi-omics analyses revealed a critical link between a cold-inducible tryptophan metabolite indole-3-acetamide (I3AM) and cardiac fibrosis in post-MI hearts. MCE elevated I3AM levels in serum and heart tissue and upregulated gut indole-related metabolic pathways; which was necessary to attenuate cardiac fibrotic remodeling by inhibiting SMAD2/3 signaling and fibroblast activation. Mechanistically, I3AM acts as a ligand for the aryl hydrocarbon receptor (AHR). I3AM-bound AHR promoted ubiquitination and proteasomal degradation of SMAD2. Treatment with I3AM recapitulated the cardioprotective effects of MCE, while genetic or pharmacological inhibition of AHR abolished the benefits of both MCE and I3AM in MI mice. In patients with and without heart failure, serum I3AM levels correlated positively with cardiac function, underscoring its clinical relevance. Our findings unveil a previously unrecognized metabolic-proteolytic axis triggered by MCE and identify the temperature responsive I3AM-AHR-SMAD pathway as a potential therapeutic target for ischemic hearts.