Interleukin-24: a molecule with potential anti-cancer activity and a cytokine in search of a function.

The gene encoding Melanoma-differentiation antigen-7 (MDA-7) was discovered more than 10 years ago. Its potential anti-cancer activity was surmised because its expression is inversely correlated with the cell proliferation status. Indeed adenoviral delivery of this gene proved to be efficient in killing several cancer cell lines and great strides have been made concerning its molecular ways of action. Later it was shown that mda7 encoded a secreted cytokine which belongs to the IL-10, class-II family of cytokines. We recently found that this molecule exerted apoptotic activity on stimulating but not on resting lymphocytes from a B cell leukaemia. This activity is distinct from that of intracellular MDA-7, and may pave the way for using the cytokine in cancers provided that they express the IL-24 Receptors; in this respect, melanomas are insensitive to the recombinant cytokine due to the lack of IL-24 receptors at their surface. In contrast to its anti-cancer activity, the immunological role of IL-24 is still unclear, with differences between mice and human. If however it is demonstrated that IL-24 can inhibit the function of STAT3 in normal lymphocytes as it is the case in leukemic cells, and given that STAT3 is needed for the differentiation of several lymphocyte subsets, this will give us hints as to the potential role of this cytokine in the immune system.