Regulation of cathepsin L (Cath L) expression and activity by leptin and the role of cathepsin L in leptin-induced FTO upregulation.Panel A shows time-dependent increases in cathepsin L immunofluorescence intensity following leptin (Lep) addition. Control (Ctl) group depicts myocytes not treated with leptin. Panel B provides evidence for leptin-induced increase in cathepsin L activity following leptin administration and its attenuation by the JAK2 inhibitor AG490 (AG) and STAT3 inhibitor S31–201 (S31). RFUs, relative fluorescence units. Panels C and D demonstrate increased gene and protein expression, respectively, of cathespsin L in cardiomyocytes treated with leptin and their prevention by inhibitors of JAK2 and STAT3. For representative Western blots L = leptin. Panels E and F demonstrate prevention of leptin-induced FTO gene and protein upregulation by the cathepsin L peptide inhibitor. Quantitative data are presented as mean ± SEM. N = 7. *P<0.05 from all other groups in panel B and respective control group in panels C to F. +P<0.05 from the leptin alone group in panels C to F. For panels A and B, time refers to hours after leptin addition. Horizontal bar in bottom right images indicates 200 µm.