The p38/MK2 Pathway Functions as Chk1-Backup Downstream of ATM/ATR in G2-Checkpoint Activation in Cells Exposed to Ionizing Radiation

We have recently reported that in G(2)-phase cells (but not S-phase cells) sustaining low loads of DNA double-strand break (DSBs), ATM and ATR regulate the G(2)-checkpoint epistatically, with ATR at the output-node, interfacing with the cell cycle through Chk1. However, although inhibition of ATR nearly completely abrogated the checkpoint, inhibition of Chk1 using UCN-01 generated only partial responses. This suggested that additional kinases downstream of ATR were involved in the transmission of the signal to the cell cycle engine. Additionally, the broad spectrum of kinases inhibited by UCN-01 pointed to uncertainties in the interpretation that warranted further investigations. Here, we show that more specific Chk1 inhibitors exert an even weaker effect on G(2)-checkpoint, as compared to ATR inhibitors and UCN-01, and identify the MAPK p38a and its downstream target MK2 as checkpoint effectors operating as backup to Chk1. These observations further expand the spectrum of p38/MK2 signaling to G(2)-checkpoint activation, extend similar studies in cells exposed to other DNA damaging agents and consolidate a role of p38/MK2 as a backup kinase module, adding to similar backup functions exerted in p53 deficient cells. The results extend the spectrum of actionable strategies and targets in current efforts to enhance the radiosensitivity in tumor cells.

Automated summary

We have previously reported that ATM and ATR regulate the G(2)-checkpoint in G(2)-phase cells sustaining low loads of DNA double-strand break (DSBs), with ATR at the output-node through Chk1. However, the partial response generated by inhibition of Chk1 suggested involvement of additional kinases downstream of ATR. In this study, we found that specific Chk1 inhibitors have weaker effects on G(2)-checkpoint compared to ATR inhibitors, and identified p38a and MK2 as backup checkpoint effectors. These findings expand the understanding of signaling pathways involved in G(2)-checkpoint activation and provide potential targets for enhancing radiosensitivity in tumor cells.