Tumor necrosis factor receptor associated factor 4 (
TRAF4) is a RING domain
E3 ubiquitin ligase that mediates the ubiquitination of various
proteins and plays an important role in driving
tumor progression. By studying the relationship between
TRAF4 and Eg5, a member of the
kinesin family that plays a critical role in spindle assembly, we demonstrated that
TRAF4 regulated Eg5 ubiquitination and contributed to Eg5-mediated
breast cancer proliferation and inhibited
breast cancer apoptosis.
TRAF4 and Eg5 were both highly expressed in
breast cancer and their
protein level was positively correlated. Relying on its Zinc fingers domain,
TRAF4 interacted with Eg5 in the cytoplasm of
breast cancer cells.
TRAF4 was a mitosis-related
protein, and by up-regulating the
protein level of Eg5
TRAF4 participated in spindle assembly. Loss of
TRAF4 resulted in monopolar spindles formation, but loss of function could be rescued by Eg5. Relying on its RING domain,
TRAF4 up-regulated Eg5
protein levels by inhibition of Eg5 ubiquitination, thus stabilizing Eg5
protein level during mitosis. Furthermore, we found that Smurf2, a TRAF4-targeted ubiquitination substrate, mediated the regulation of Eg5 ubiquitination by
TRAF4.
TRAF4 inhibited the interaction between Smurf2 and Eg5, and down-regulated the
protein level of Smurf2 by promoting its ubiquitination, thereby inhibited the Smurf2-catalyzed ubiquitination of Eg5 and up-regulated Eg5
protein levels. We also demonstrate that
TRAF4 plays an important role in promoting cell proliferation and in inhibiting cell apoptosis induced by Eg5. In summary, our study suggests a new direction for investigating the role of
TRAF4 in driving
breast cancer progression.