Ubiquitous calpains show differential dynamics and function during adipocyte differentiation

Adipose tissue homeostasis is disrupted either, by decreased adipocyte number or increased adipogenesis, leading to endocrine dysfunction, developmental anomalies, or tumour progression. Calpains are calcium-dependent proteases that modulate adipogenesis, yet the distinct roles of ubiquitously expressed isoforms calpain-1 (CAPN1) and calpain-2 (CAPN2) remain unclear. This study investigates calpains isoform- and phase-specific expression, distribution, and function during hormonally induced 3T3-L1 preadipocyte differentiation. Both isoforms showed progressive upregulation, increased activity (Nt/Ct-CAPN1 ratio, calpastatin levels and enzymatic activity), and dynamic isoform-specific redistribution throughout differentiation phases. Depletion of CAPN1 or CAPN2 disrupted the transition from mitotic clonal expansion (MCE) to early differentiation (ED) stage, preventing cell cycle exit, as indicated by impaired C/EBPα expression, increased S-phase preadipocytes, and elevated p53 levels. Immunofluorescence staining revealed distinct N-terminal/C-terminal histone H3 patterns at the MCE-to-ED transition. Calpain inhibition or depletion prevented histone H3 cleavage, and proximity ligation assays confirmed in vivo CAPN1/Nt-H3 interaction. Although the precise CAPN2 targets during adipocyte differentiation remain unknown, CAPN1 was identified as the main isoform responsible for chromatin remodelling by histone H3 cleavage at MCE-to-ED transition. These findings underscore the critical role of phase-specific calpains in the adipogenic program, revealing them as potential therapeutic targets for obesity and metabolic disorders.