GSK3-β Stimulates Claspin Degradation via β-TrCP Ubiquitin Ligase and Alters Cancer Cell Survival

Claspin is essential for activating the DNA damage checkpoint effector kinase Chk1, a target in oncotherapy. Claspin functions are tightly correlated to Claspin protein stability, regulated by ubiquitin-dependent proteasomal degradation. Here we identify Glycogen Synthase Kinase 3-beta (GSK3-beta) as a new regulator of Claspin stability. Interestingly, as Chk1, GSK3-beta is a therapeutic target in cancer. GSK3-beta inhibition or knockdown stabilizes Claspin, whereas a GSK3-beta constitutively active form reduces Claspin protein levels by ubiquitination and proteasome-mediated degradation. Our results also suggest that GSK3-beta modulates the interaction of Claspin with beta-TrCP, a critical E3 ubiquitin ligase that regulates Claspin stability. Importantly, GSK3-beta knock down increases Chk1 activation in response to DNA damage in a Claspin-dependent manner. Therefore, Chk1 activation could be a pro-survival mechanism that becomes activated upon GSK3-beta inhibition. Importantly, treating triple negative breast cancer cell lines with Chk1 or GSK3-beta inhibitors alone or in combination, demonstrates that Chk1/GSK3-beta double inhibition restrains cell growth and triggers more apoptosis compared to individual treatments, thereby revealing novel possibilities for a combination therapy for cancer.