RESEARCH ARTICLE Membrane-Permeable Calpain Inhibitors Promote Rat Oral Mucosal Epithelial Cell Proliferation by Inhibiting IL-1α Signaling

To standardise regenerative medicine using cultured cells, the use of serum-free, chemi-cally defined media will be necessary. We have reported that IL-1α inhibits the growth of epi-thelial cells in culture and that recombinant IL-1 receptor antagonist (IL-1RA) significantly promotes epithelial cell growth in no feeder layer condition. In this study, we examined inhib-itors of calpain, a cysteine proteinase that plays crucial roles in various cellular functions, including IL-1αmaturation and secretion. The culturing of epithelial cells in serum-free media supplemented with a membrane-permeable calpain inhibitor significantly promoted growth while suppressing IL-1αmaturation and secretion. By contrast, non-membrane-per-meable calpain inhibitor treatment did not have these effects. Interestingly, immunoblotting analysis revealed that immature, untruncated, IL-1α expression was also downregulated by cell-permeable calpain inhibitor treatment, and the difference in IL-1α gene expression increased from day 2 to day 6. Although IL-1RA has been reported to promote epithelial cell growth, we detected no synergistic promotion of epithelial cell growth using a calpain inhibitor and IL-1RA. These findings indicate that calpain inhibitors promote epithelial cell proliferation by inhibiting IL-1αmaturation at an early phase of epithelial cell culture and by suppressing the positive feedback-mediated amplification of IL-1α signalling