Cullin 5: A Destabilizing Force for Some Oncogenes
Cullin 5 is recruited to Hsp90-containing complexes and plays an important role in regulating the stability of the complex's client proteins such as ErbB2, which is over-expressed in 30 percent of breast cancers. Here Cul-5 stains green when only FITC stain is used (D), ErbB2 stains red when only rhodamine stain is used (E), but when both stains are present, they colocalize to same location and produce a yellow color (F). This shows that Cullin 5 and ErbB2 colocalize.
Cancer can result when cellular processes such as proliferation and cell death go haywire. Among the many mechanisms in place to regulate these critical processes are molecular chaperones, which help proteins attain their proper functional shape and also regulate protein degradation through the cell’s recycling program, called the ubiquitin/proteasome system. One molecular chaperone, heat shock protein 90 (Hsp90), is of particular interest to cancer researchers because many of its target proteins—sometimes called client proteins—have been implicated in the maintenance and progression of a number of cancers.
Len Neckers, Ph.D., and Wanping Xu, Ph.D., researchers in the CCR Urologic Oncology Branch, have been working with researchers at the Johns Hopkins Bloomberg School of Public Health to learn more about how Hsp90 and its clients contribute to tumorigenesis. A paper recently published in the Proceedings of the National Academy of Sciences describes the role of Cullin 5, an enzyme that labels proteins for degradation by the ubiqutin/proteasome system, in Hsp90 regulation of its client proteins.
In an effort to gain understanding of the cellular function of Cullin 5, the researchers did a screen to identify proteins with which it interacts. They discovered that Cullin 5 interacts both with Hsp90 and another heat shock protein, Hsp70. This led them to investigate whether Cullin 5 may have a role in regulating Hsp90 client proteins known to contribute to cancer.
One of Hsp90’s client proteins is ErbB2, which is overexpressed in approximately 30 percent of breast cancers and is required for tumor proliferation. Geldanamycin, a drug that modulates activity of Hsp90, has been shown to cause degradation of ErbB2, indicating that Hsp90 regulates the stability of this oncogene. Experiments in cell culture revealed that Cullin 5 interacts with ErbB2 at the latter’s normal cellular location, the plasma membrane. When two different types of cells were manipulated to abrogate Cullin 5 function, ErbB2 became resistant to the effects of geldanamycin, suggesting that Cullin 5 mediates Hsp90 regulation of ErbB2 stability.
Another Hsp90 client protein, HIF1α, supports tumor survival by promoting recruitment of new blood vessels and helping cancer cells thrive in low-oxygen environments. Like, ErbB2, HIF1α usually degrades upon treatment with geldanamycin. Neckers, Xu, and colleagues showed that loss of Cullin 5 function resulted in stabilization of HIF1α in the presence of geldanamycin, providing evidence that Cullin 5 participates in Hsp90 regulation of HIF1α.
Together, these data suggest that Cullin 5 is recruited to Hsp90-containing complexes and plays an important role in regulating the stability of Hsp90 client proteins, including those known to be involved in cancer. Low levels of Cullin 5—which have been observed in a number of cancers—could affect the activity of Hsp90 modulators like geldanamycin that are currently being tested in laboratory studies and clinical trials. The relationship between Cullin 5 and Hsp90 client proteins may represent an avenue for future oncology drug development.Summary Posted: 12/2009
Ehrlich ES, Wang T, Luo K, Xiao Z, Niewiadomska AM, Martinez T, Xu W, Neckers L, Yu XF. Regulation of Hsp90 client proteins by a Cullin5-RING E3 ubiquitin ligase. Proc Natl Acad Sci U S A. 2009 Dec 1;106(48):20330-5. PubMed Link