Endogenous Human MDM2-C Is Highly Expressed in Human Cancers and Functions as a p53-Independent Growth Activator

Abstract

Human cancers over-expressing mdm2, through a T to G variation at a single nucleotide polymorphism at position 309 (mdm2 SNP309), have functionally inactivated p53 that is not effectively degraded. They also have high expression of the alternatively spliced transcript, mdm2-C. Alternatively spliced mdm2 transcripts are expressed in many forms of human cancer and when they are exogenously expressed they transform human cells. However no study to date has detected endogenous MDM2 protein isoforms. Studies with exogenous expression of splice variants have been carried out with mdm2-A and mdm2-B, but the mdm2-C isoform has remained virtually unexplored. We addressed the cellular influence of exogenously expressed MDM2-C, and asked if endogenous MDM2-C protein was present in human cancers. To detect endogenous MDM2-C protein, we created a human MDM2-C antibody to the splice junction epitope of exons four and ten (MDM2 C410) and validated the antibody with in vitro translated full length MDM2 compared to MDM2-C. Interestingly, we discovered that MDM2-C co-migrates with MDM2-FL at approximately 98 kDa. Using the validated C410 antibody, we detected high expression of endogenous MDM2-C in human cancer cell lines and human cancer tissues. In the estrogen receptor positive (ER+) mdm2 G/G SNP309 breast cancer cell line, T47D, we observed an increase in endogenous MDM2-C protein with estrogen treatment. MDM2-C localized to the nucleus and the cytoplasm. We examined the biological activity of MDM2-C by exogenously expressing the protein and observed that MDM2-C did not efficiently target p53 for degradation or reduce p53 transcriptional activity. Exogenous expression of MDM2-C in p53-null human cancer cells increased colony formation, indicating p53-independent tumorigenic properties. Our data indicate a role for MDM2-C that does not require the inhibition of p53 for increasing cancer cell proliferation and survival.

The Roles of MDM2 and MDMX in Cancer

“…Interestingly, although the MDM2-A variant has a protective function in the p53 wild-type background, it can contribute to tumor progression in a p53-null background (114). In one study from 2013, similar p53-independent transforming activity has been found in another MDM2 splice variant, MDM2-C (115). …”

Aberrant splicing of the DMP1‐ARF‐MDM2‐p53 pathway in cancer

“…Okoro et al . reported that the MDM2‐C protein was highly expressed in human cancers and functioned as a p53‐independent growth activator . These data provide evidence that most MDM2 isoforms can contribute to tumor development in vivo .…”

“…60 Okoro et al reported that the MDM2-C protein was highly expressed in human cancers and functioned as a p53-independent growth activator. 61 These data provide evidence that most MDM2 isoforms can contribute to tumor development in vivo. The outstanding difference in the activity of MDM2 splice variants in vivo (accelerated tumorigenesis) and cultured cells (inhibition of cell growth) can be explained by the possibility that the MDM2 splice variants act as oncogenes, and their forced expression triggers growth arrest in primary cells while they promote cell proliferation in cancer cells where the Arf-p53 pathway is inactivated.…”

Inhibition of invasion and migration of prostate cancer cells by miRNA-509-5p via targeting MDM2

“…Certain miRNAs can work as specific markers for prostate cancer by contributing towards the deduction of its classification, tumor grade, or differential diagnosis against benign prostate disease (Peter, 2009;Donnem et al, 2012). MDM2 was initially cloned from BALB/3T3 cells, and is highly expressed in pulmonary, breast, and colon cancers (Okoro et al, 2013).…”