Ingenane Diterpenoidsfrom <i>Euphorbia peplus</i> as Potential New CHK1 InhibitorsThat Sensitize Cancer Cells toChemotherapy

Phosphorylation of checkpoint kinase 1 at Ser-345 (p-CHK1(S345)) mediates the replication stress response in cancer cells, leading to chemotherapy resistance. Therefore, finding inhibitors of p-CHK1(S345) is currently a promising strategy to prevent acquired drug resistance. In this study, 14 ingenane diterpenoids (<b>1</b>–<b>14</b>), involving two undescribed compounds possessing an unprecedented exocyclic double bond Δ⁶⁽²⁰⁾, were identified from <i>Euphorbia peplus</i>. The inhibitory effects of the isolated compounds on p-CHK1(S345) and their structure–activity relationship (SAR) were investigated. Compounds <b>7</b> and <b>8</b> presented the strongest inhibitory effects, abrogated cell cycle arrest, and caused the accumulation of DNA damage, improving the sensitivity of cancer cells to chemotherapeutic drugs. An <i>in vivo</i> assay confirmed the enhancement of <b>8</b> on the anticancer effect of topotecan via blocking of p-CHK1(S345). Mechanistically, <b>8</b> increased CHK1 ubiquitination to inhibit p-CHK1(S345) via activation of protein kinase C (PKC). PKC activation was first found to enhance CHK1 ubiquitination to block p-CHK1(S345). Above all, this finding not only indicates that compound <b>8</b> could be developed as a novel CHK1 inhibitor but also reveals a previously unrecognized role of PKC in regulating cancer chemotherapy sensitivity.