Molecular cloning and expression of a functionally different alternative splice variant of prointerleukin-1 alpha from the rat testis

We report here the characterization of an alternative splice Variant of pro interleukin-1 alpha (proIL-1 alpha), constitutively expressed by the normal adult rat testis. In addition to the classical 32K proIL-1 alpha: (32proIL -1 alpha) messenger RNA, the testis produced a shorter variant encoding a p utative protein of 24K (24proIL-1 alpha). In situ hybridization demonstrate d constitutive expression of the splice transcript in the seminiferous tubu les. This alternative complementary DNA lacked the fifth exon, harboring th e calpain cleavage site essential for generation of mature 17K IL-1 alpha. This was verified by calpain treatment, producing the expected cleavage pro ducts of recombinant 32proIL-1 alpha, but not of 24proIL-1 alpha. Similarly , expression in COS-7 cells demonstrated processing of 32proIL-1 alpha to t he mature 17K form and secretion, whereas 24proIL-1 alpha remained unproces sed. Both 32proIL-1 alpha: and 24proIL-1 alpha showed a dose-dependent stim ulatory effect in a thymocyte proliferation assay, although at lower potenc y than mature 17K IL-1 alpha. In contrast, when tested on hCG-stimulated Le ydig cells in vitro, a dose-dependent inhibition of testosterone production was obtained with mature 17K IL-la and at a lower potency with 32proIL-1 a lpha, whereas 24proIL-1 alpha was inactive. In conclusion, the three IL-I b ioactive proteins described here contribute to IL-1 protein heterogeneity a nd may serve as constitutive paracrine mediators in the testis.