MIF coordinates the cell cycle with DNA damage checkpoints. Lessons from knockout mouse models

Abstract

Macrophage migration inhibitory factor (MIF) is a ubiquitously expressed pro-inflammatory mediator that has also been implicated in the process of oncogenic transformation and tumor progression. We used a genetic approach to show that deletion of the MIF gene in mice has several major consequences for the proliferative and transforming properties of cells. MIF-deficient cells exhibit increased resistance to oncogenic transformation. The transformation defects associated with MIF deficiency can be overcome through concomitant inactivation of the p53 and Rb/E2F tumor suppressor pathways. We have produced compelling evidence that the effects of MIF on cell survival and tumorigenesis are mediated through overlapping pathways, wherein MIF and p53 functionally antagonize each other in the cell. However, the involvement of MIF in p53 function is secondary to p53-independent mechanisms controlling protein stability, DNA damage checkpoints, and the integrity of the genome. Given the broad spectrum of cell types that normally express MIF and its elevated levels at sites of chronic inflammation, this pathway may be generic for many early stage tumors.

Figure 1

MIF loss promotes skin tumorigenesis. 6–10 week old male MIF^-/- or MIF^+/+ C57Bl/6 mice (n = 20 per group) were treated with 200 μg of the carcinogen benzo[α]pyrene (B[α]P) in 100 μl acetone topically on their backs once per week for 20 weeks. Skin tumors started to appear after week 14, and increased in number during the course of B[α]P treatment. MIF^-/- mice developed nearly twice as many tumors per mouse as MIF^+/+ controls. Based on histological evaluation by a blinded pathologist, this primarily reflected an increase of non-invasive tumors. By contrast, the number of invasive tumors was similar between the genotypes. Likewise, we found no significant difference with respect to tumor size or vascularization. * = statistically significant with p < 0.01 in a Student's t-test.

Figure 2

SCF activity is sustained by dynamic cycles of assembly and disassembly. (A). DNA damage checkpoint pathways feed into the proteolytic degradation of key cell cycle regulators, mediated by SCF, to stop the cell cycle. The scheme indicates the elements that make up signal transducers (ATM, ATR, Chk1, Chk2) and effectors (SCF, CSN and 26S proteasome). MIF binds to Jab1/CSN5 and prevents it from interacting with proteins targeted by CSN, notably the Cullins. (B). Multi-subunit structure of the SCF class of E3 ubiquitin ligases. All SCFs consist of Cullins, Skp1, Rbx1 and F-box proteins which associate to form an enzymatically active complex. The posttranslational modification of Cullin (Cul1) by Nedd8 renders SCF active, though unstable. The removal of Nedd8 from Cullin is catalyzed by Jab1/CSN5. Following deneddylation of Cullins, Skp1 and F-box proteins are replaced by the inhibitory protein Cand1.