Targeting ATR and Chk1 kinases for cancer treatment: a new model for new (and old) drugs

Abstract

Trying to kill cancer cells by generating DNA damage is by no means a new idea. Radiotherapy and genotoxic drugs are routinely used in cancer therapy. More recent developments also explored the potential of targeting the DNA damage response (DDR) in order to increase the toxicity of radio- and chemo- therapy. Chk1 inhibitors have pioneered studies in this regard. Interestingly, early studies noted that Chk1 inhibitors were particularly toxic for p53-deficient cells. The model proposed for this observation was that this effect was due to the simultaneous abrogation of the G2 (Chk1) and G1 (p53) checkpoints. We here challenge this view, and propose a model where the toxicity of Chk1 inhibitors is rather due to the fact that these compounds generate high loads of replicative stress (RS) during S-phase, which are further boosted by the less restrictive S-phase entry found in p53-deficient cells. This new model implies that the particular toxicity of Chk1 inhibitors might not be restricted to p53-deficient cells, but could be extended to other mutations that promote a promiscuous S-phase entry. In addition, this rationale also implies that the same effect should also be observed for other molecules that target the RS-response (RSR), such as inhibitors of the Chk1-activating kinase ATR.

Figure 1

ATR activation: From ssDNA to Chk1. ATM is directly activated by the free and unprocessed DNA ends that arise at DSB. In contrast, ssDNA is the signal for ATR activation. This can also be generated at DSB after a 5′to 3′nucleolytic degradation of one of the chains, which is also necessary to provide the substrate for homologous recombination. However, the most important source of ssDNA occurs at stalled replication forks, in what is known as RS. Upon exposure of ssDNA this is rapidly coated by RPA, which directly binds ATRIP and therefore recruits the ATRIP/ATR complex to ssDNA. At the same time, Rad17 loads the 9‐1‐1 clamp, which then brings the alosteric activator TopBP1 in close proximity to ATR unleashing its kinase activity. In order for ATR to phosphorylate Chk1, a mediator protein named Claspin is still needed that finally enables the interaction of ATR with Chk1, leading to the phosphorylation of Chk1 and a full activation of the RSR.

Figure 2

ATR or Chk1 inhibitors in cancer chemotherapy. A certain degree of RS occurs every cell division, where it is detected and suppressed by the ATR‐ and Chk1‐dependent RSR. Inhibitors of ATR or Chk1 exacerbate the levels of RS, which can ultimately promote cell killing by p53‐independent means. In this context, the rationale outlined here is rather simple: Targeting the RSR could be particularly toxic for those cells carrying higher endogenous levels of RS. The key here is that, whereas all tumors might concur with certain degree of RS, these inhibitors should only be toxic for those tumors harboring distinctly high levels of RS. In contrast, healthy tissues and tumors with minimal levels of RS might be largely non‐responsive to ATR or Chk1 inhibitors.