Methods to identify compounds useful for tumor sensitization to DNA damage
Cdc25A is herein identified as a substrate for β-TrCP1- or β-TrCP2-mediated ubiquitination and subsequent degradation via the ubiquitin-proteasome pathway. In particular, it has been found that interfering with β-TrCP expression or function, or increasing β-TrCP degradation, leads to accumulation of Cdc25A in a cell. Since degradation of Cdc25A is a key feature of the response to DNA damage, leading to a stall in the cell cycle during which the cell can repair the damage, Cdc25A accumulation can abolish this response, thereby sensitizing the cell to DNA damage. Described herein are assays for identifying β-TrCP inhibitors, and method of using such inhibitors for modulating Cdc25A degradation, sensitization of tumor cells, and as adjuvants in cancer therapy based on DNA damaging agents.
1. A method of screening for an agent useful for sensitizing a cancer cell to DNA damage by a second agent, which comprises:
(i) contacting a human β-transducin repeat containing protein (β-TrCP) β-TrCP1 or β-TrCP2 with a test compound and a phosphorylated cell division cycle 25A (Cdc25A) protein or a phosphorylated fragment thereof of at least 8 amino acids in length;
(ii) detecting a reduced binding of the β-TrCP1 or β-TrCP2 to the phosphorylated Cdc25A protein or fragment thereof in the presence of the test compound as compared to a control; and
(iii) selecting the test compound that reduces the binding as an agent useful for sensitizing a cancer cell to DNA damage by the second agent;
wherein the control is the binding of the β-TrCP1 or β-TrCP2 to the Cdc25A protein or fragment thereof in the absence of the test compound.
2. The method of claim 1 , wherein the β-TrCP1 comprises the sequence of SEQ ID NO:3.
3. The method of claim 1 , wherein the β-TrCP2 comprises the sequence of SEQ ID NO:6.
4. The method of claim 1 , wherein the Cdc25A protein or fragment thereof comprises the sequence of SEQ ID NO:29.
5. The method of claim 4 , wherein the Cdc25A protein comprises the sequence of SEQ ID NO:8.
6. The method of claim 1 , wherein the second agent is ionizing radiation or an alkylating agent.
7. The method of claim 1 , wherein the β-TrCP1 or β-TrCP2 and Cdc25A protein or fragment thereof are expressed by a cell, and the test compound is added to the cell.
8. The method of claim 1 , wherein the β-TrCP1 or β-TrCP2, test compound, and phosphorylated Cdc25A protein or fragment thereof are in a reconstituted system.
9. The method of claim 7 , wherein the cell further expresses Skp1 and Cul1.
10. The method of claim 8 , wherein the reconstituted system further comprises Skp1 and Cul1.