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Vol. 19, Issue 11, 4968-4979, November 2008

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Cyclin G2 Is Degraded through the Ubiquitin-Proteasome Pathway and Mediates the Antiproliferative Effect of Activin Receptor-like Kinase 7

Guoxiong Xu^*, Stefanie Bernaudo^*, Guodong Fu^*, Daniel Y. Lee, Burton B. Yang, and Chun Peng^*

*Department of Biology, York University, Toronto, ON, Canada M3J 1P3; and Sunnybrook and Women's College Health Science Centre and Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, ON, Canada M4N 3M5

Submitted March 11, 2008; Revised August 21, 2008; Accepted August 28, 2008
Monitoring Editor: Jonathan Chernoff

We have previously reported that Nodal, a member of the TGF-β superfamily, acts through activin receptor-like kinase 7 (ALK7) to inhibit ovarian cancer cell proliferation. To determine the mechanism underlying their effects, a cell cycle gene array was performed and cyclin G2 mRNA was found to be strongly up-regulated by Nodal and ALK7. To study the function and regulation of cyclin G2 in ovarian cancer cells, expression constructs were generated. We found that cyclin G2 protein level decreased rapidly after transfection, and this decrease was prevented by 26S proteasome inhibitors. Immunoprecipitation and pull-down studies showed that ubiquitin, Skp1, and Skp2 formed complexes with cyclin G2. Knockdown of Skp2 by siRNA increased, whereas overexpression of Skp2 decreased cyclin G2 levels. Nodal and ALK7 decreased the expression of Skp1 and Skp2 and increased cyclin G2 levels. Overexpression of cyclin G2 inhibited cell proliferation whereas cyclin G2-siRNA reduced the antiproliferative effect of Nodal and ALK7. Taken together, these findings provide strong evidence that cyclin G2 is degraded by the ubiquitin–proteasome pathway and that Skp2 plays a role in regulating cyclin G2 levels. Furthermore, our results also demonstrate that the antiproliferative effect of Nodal/ALK7 on ovarian cancer cells is in part mediated by cyclin G2.

Address correspondence to: Chun Peng (cpeng{at}yorku.ca).