Checkpoint kinase 2 (Chk2) is one of the critical kinases governing the cell cycle checkpoint, DNA damage repair, and cell apoptosis in response to DNA damaging signals. In the present report, we demonstrate that Chk2 kinase is degraded at the protein level in response to cisplatin through ubiquitin-proteasome pathway. This degradation was independent of the Thr68 phosphorylation, ATM kinase, and BRCA1 tumor suppressor. Examination of Chk2 protein revealed a decreased expression of Chk2 protein in cisplatin-resistant ovarian cancer cell lines, suggesting that degradation or decreased expression of Chk2 is partially responsible for chemo-resistance. Site-directed mutation of the putative destruction box in the Chk2 protein did not affect the Chk2 degradation induced by cisplatin. Therefore, these results are the first to indicate a novel mechanism of regulating Chk2 in cisplatin-induced resistance of cancer cells. (C) 2005 Elsevier Inc. All rights reserved.