The p53-dependent effects of macrophage migration inhibitory factor revealed by gene targeting

Macrophage migration inhibitory factor (MIF) is a mediator of host immunity and functions as a high, upstream activator of cells within the innate and the adaptive immunological systems. Recent studies have suggested a potentially broader role for MIF in growth regulation because of its ability to antagonize p53-mediated gene activation and apoptosis. To better understand MIF's activity in growth control, we generated and characterized a strain of MIF-knockout (MIF-KO) mice in the inbred, C57BL/6 background. Embryonic fibroblasts from MIF-KO mice exhibit p53-dependent growth alterations, increased p53 transcriptional activity, and resistance to ras-mediated transformation. Concurrent deletion of the p53 gene in vivo reversed the observed phenotype of cells deficient in MIF. In vivo studies showed that fibrosarcomas induced by the carcinogen benzoalphapyrene are smaller in size and have a lower mitotic index in MIF-KO mice relative to their WT counterparts. The data provide direct genetic evidence for a functional link between MIF and the p53 tumor suppressor and indicate an important and previously unappreciated role for MIF in carcinogenesis.