Decreased Mdm2 expression inhibits tumor development induced by loss of ARF

Abstract

The tumor suppressor p14/p19 ARF regulates Mdm2, which is known for controlling the p53 tumor suppressor. Here we report that loss of one allele of Mdm2 in cells that lack ARF resulted in a decreased rate of proliferation, fewer chromosomal aberrations, and suppression of Ras-induced transformation. Moreover, a haploinsufficiency of Mdm2 inhibited spontaneous tumor development in ARF-null mice. Remarkably, Mdm2 þ /À ARF À/À mice survived an average of 6 months longer than Mdm2 þ / þ ARF À/À mice. The spectrum of tumors that arose in Mdm2 þ /À ARF À/À mice did not significantly differ from those that developed in mice lacking only ARF. However, the extended tumor latency allowed for the emergence of multiple primary tumors in a third of the Mdm2 þ /À ARF À/À mice, as compared to the single tumor type that arose in ARFnull only mice. Therefore, a decrease in Mdm2 levels restored regulation of critical cellular processes that are altered during transformation and that occur in the absence of ARF. Our findings also indicate that Mdm2 can function independently from ARF and imply that targeting Mdm2 in tumors that lack ARF expression should be an effective therapeutic approach.

Elevated Mdm2 expression induces chromosomal instability and confers a survival and growth advantage to B cells

“…Therefore, our results in B cells indicate that Mdm2 alone can induce chromosomal instability and a greater transformation potential. In support of this idea, loss of one allele of Mdm2 in ARF-null mouse embryo fibroblasts, which are predisposed to transformation, resulted in increased chromosomal stability with fewer breaks, fusions and exchanges and a decreased transformation ability (Wang et al, 2006). Moreover, Mdm2 overexpression in breast epithelial cell-specific Mdm2 transgenic mice in the presence or absence of p53, led to multinucleated polyploid epithelial cells (Lundgren et al, 1997).…”

“…Fragmented (sub-G1) DNA was quantified following propidium iodide (PI) staining and flow cytometry. BrdU incorporation was performed as we previously described (Wang et al, 2006). 3 H-thymidine incorporation was determined daily in triplicate following co-culture with 1 mCi 3 H-thymidine.…”

“…Spleens or B-cell lymphomas were lysed and proteins western blotted, genomic DNA Southern blotted, and RNA sequenced, as described previously (Eischen et al, 1999;Alt et al, 2003;Wang et al, 2006). Relative protein expression was determined with densitometry.…”

“…Metaphase spreads were prepared and analysed with standard protocols as we previously described (Wang et al, 2006). Each metaphase was evaluated for chromosomal aberrations.…”

Mdm2 Affects Genome Stability Independent of p53

“…+/+ mice (10). These studies illustrate the influence Mdm2 levels have on the regulation of tumorigenesis, yet the contribution of p53-independent pathways to this process is incompletely understood.…”