Chemical screen identifies shikonin as a broad DNA damage response inhibitor that enhances chemotherapy through inhibiting ATM and ATR

by Selleckchem | Jul 17 2023

DNA damage response (DDR) is a highly conserved genome surveillance mechanism that preserves cell viability in the presence of chemotherapeutic drugs. Hence, small molecules that inhibit DDR are expected to enhance the anti-cancer effect of chemotherapy. Through a recent chemical library screen, we identified shikonin as an inhibitor that strongly suppressed DDR activated by various chemotherapeutic drugs in cancer cell lines derived from different origins. Mechanistically, shikonin inhibited the activation of ataxia telangiectasia mutated (ATM), and to a lesser degree ATM and RAD3-related (ATR), two master upstream regulators of the DDR signal, through inducing degradation of ATM and ATR-interacting protein (ATRIP), an obligate associating protein of ATR, respectively. As a result of DDR inhibition, shikonin enhanced the anti-cancer effect of chemotherapeutic drugs in both cell cultures and in mouse models. While degradation of ATRIP is proteasome dependent, that of ATM depends on caspase- and lysosome-, but not proteasome. Overexpression of ATM significantly mitigated DDR inhibition and cell death induced by shikonin and chemotherapeutic drugs. These novel findings reveal shikonin as a pan DDR inhibitor and identify ATM as a primary factor in determining the chemo sensitizing effect of shikonin. Our data may facilitate the development of shikonin and its derivatives as potential chemotherapy sensitizers through inducing ATM degradation.

Comments:

The DNA damage response (DDR) is a crucial mechanism that cells employ to maintain genome integrity and survive in the presence of DNA-damaging agents such as chemotherapeutic drugs. Inhibition of DDR has been proposed as a strategy to enhance the effectiveness of chemotherapy in treating cancer. Recently, in a chemical library screen, a compound called shikonin was identified as a potent inhibitor of DDR activation induced by various chemotherapeutic drugs in cancer cell lines derived from different tissue origins.

The mechanism by which shikonin inhibits DDR involves targeting two key upstream regulators of the DDR signal: ataxia telangiectasia mutated (ATM) and ATM and RAD3-related (ATR). Shikonin induces the degradation of ATM and ATR-interacting protein (ATRIP), an essential protein that associates with ATR. This leads to the inhibition of ATM and ATR activation and subsequent downstream DDR signaling.

Interestingly, the degradation of ATRIP is dependent on the proteasome, a cellular machinery responsible for protein degradation. On the other hand, the degradation of ATM relies on caspases (enzymes involved in programmed cell death) and lysosomes, but not on the proteasome. These distinct degradation pathways suggest different regulatory mechanisms for ATRIP and ATM in response to shikonin treatment.

Furthermore, the study demonstrated that shikonin enhanced the anti-cancer effects of chemotherapeutic drugs both in cell cultures and in mouse models. Importantly, overexpression of ATM significantly reduced the inhibitory effects of shikonin on DDR and cell death induced by shikonin and chemotherapeutic drugs. This highlights the critical role of ATM in determining the sensitizing effect of shikonin on chemotherapy.

Overall, these findings identify shikonin as a broad-spectrum DDR inhibitor and highlight ATM as a primary determinant of shikonin's chemo sensitizing effect. These discoveries provide valuable insights for the potential development of shikonin and its derivatives as chemotherapy sensitizers through the induction of ATM degradation. Further research and development efforts may focus on optimizing shikonin-based compounds for potential clinical applications in enhancing the efficacy of chemotherapy in cancer treatment.