Tumour growth environment modulates Chk1 signalling pathways and Chk1 inhibitor sensitivity

Clinical development of Chk1 inhibitors is currently focussed on evaluating activity as monotherapy and as potentiators of chemotherapy. To aid translation of pre-clinical studies, we sought to understand the effects of the tumour growth environment on Chk1 signalling and sensitivity to small molecule Chk1 inhibition. Spheroid culture altered Chk1 signalling to a more xenograft like state but decreased sensitivity to Chk1 inhibition. Growth in low serum did not alter DDR signalling but increased the sensitivity of A2058 and U2OS tumour cells to Chk1 inhibition. An analysis of the expression levels of replication associated proteins identified a correlation between Cdc6 and pChk1 (S296) as well as total Chk1 in xenograft derived samples and between Cdc6 and total Chk1 in anchorage-dependent growth derived protein samples. No apparent correlation between Chk1 or Cdc6 expression and sensitivity to Chk1 inhibition in vitro was observed. A database analysis revealed upregulation of CDC6 mRNA expression in tumour compared to normal tissue and a correlation between CDC6 and CHEK1 mRNA expression in human cancers. We suggest that Cdc6 overexpression in human tumours requires a concomitant increase in Chk1 to counterbalance the deleterious effects of origin hyperactivation-induced DNA damage.