The study shows that the natural peptide LL‑37 (called CRAMP in mice) can protect heart cells from a type of iron‑driven cell death called ferroptosis, which contributes to heart damage after a heart attack. In mouse cells and a mouse heart‑attack model, adding LL‑37 or increasing its production reduced cell death and improved heart function by blocking an enzyme (cathepsin L) that otherwise lowers a protective protein (PDIA4).

Ferroptosis is an important cause of cardiomyocyte loss and cardiac dysfunction. Cathelicidin-related antimicrobial peptide (CRAMP) is an endogenous polypeptide that regulates oxidative stress in the body and is involved in ferroptosis. However, its specific role and mechanism in ferroptosis are unclear. To analyze the role of CRAMP in ferroptosis, we first analyzed its expression in infarcted myocardial tissues, and verified its role in ferroptosis in vitro through overexpression and knock-down techniques. The activity and expression of cathepsin L (CTSL) and its effect on ferroptosis were analyzed to verify whether CTSL participated in ferroptosis as a downstream of CRAMP. Protein disulfide isomerase family A member 4 (PDIA4) was screened as an interacting protein of CTSL by using the database, and the role of PDIA4 in ferroptosis was analyzed by gene knockdown and overexpression. Finally, the regulatory mechanism of CRAMP in ferroptosis was verified in vivo by mouse myocardial infarction model. CRAMP levels were reduced in both infarcted cardiac tissues and cardiomyocytes exposed to ferroptosis inducers. The overexpression of CRAMP or pretreatment of LL-37 alleviated cardiomyocyte ferroptosis, whereas CRAMP knockdown exacerbated cell death. Under ferroptotic stress, the expression of CTSL was elevated. CRAMP inhibited ferroptosis by antagonizing the CTSL activity. Abnormal increase in CTSL activity and levels caused PDIA4 to decrease. Overexpression of PDIA4 inhibited ferroptosis induced by CTSL, while knocking down PDIA4 counteracted the protection of CRAMP. In vivo, both CRAMP overexpression and administration of CRAMP peptide significantly ameliorated myocardial injury and improved cardiac function. CRAMP increases PDIA4 levels by inhibiting the activity of CTSL and antagonizes ferroptosis in cardiomyocytes. Targeting CRAMP offers innovative therapeutic strategies and insights for the prevention and management of myocardial injury.