Differential sensitivity of interleukin-1 alpha and -beta precursor proteins to cleavage by calpain, a calcium-dependent protease

In view of the observations that the calcium ionophores, A23187 and ionomycin, enhance the processing and secretion of interleukin-1 (IL-1 alpha) and IL-1 beta from macrophages, and IL-1 alpha processing is mediated by calpain, a calcium-dependent protease, we evaluated the possibility that calpain might also play a role in the processing of IL-1 beta. Whereas calpain-containing P388D1 macrophage lysates and purified calpain processed precursor IL-1 alpha to its mature 17-kDa form, precursor IL-1 beta was degraded by both sources of calpain. However, the activation of calpain in P388D1 cells that were transiently transfected with a cDNA expression vector encoding the precursor form of IL-1 beta did not result in the degradation of precursor IL-1 beta, but did result in the processing and secretion of IL-1 alpha, implying that precursor IL-1 beta is protected from calpain degradation in vivo. Furthermore, calpain did not enhance the processing of the IL-1 beta precursor by the IL-1 beta-converting enzyme. These results indicate that calpain is not involved in the processing of precursor IL-1 beta in vitro or in vivo. The IL-1 beta precursor may be protected from calpain degradation by a sequestering mechanism that involves a cytoplasmic factor(s) that reduces the sensitivity of IL-1 beta to attack by calpain or localizes IL-1 beta to a site that precludes any interaction with the protease. Although MDL 28,170, a calpain inhibitor, prevented the ionomycin-induced processing of precursor IL-1 alpha to the mature protein in P388D1 cells, it did not inhibit the ionomycin-induced secretion of the mature IL-1 alpha and -beta proteins expressed in these cells. These results indicate that a calcium-dependent factor other than calpain is involved in the secretion of the mature IL-1 proteins.