Topoisomerase I inhibition suppresses nuclear blebbing via RNA Pol II stalling and nuclear stiffening.

Abnormal nuclear blebbing occurs in many human diseases and causes nuclear rupture and dysfunction. Nuclear blebbing is caused by chromatin motion via RNA Pol II transcriptional activity and nuclear mechanical weakening. Camptothecin, a topoisomerase I inhibitor, rapidly suppresses nuclear blebbing within hours. We find that camptothecin does not decrease RNA Pol II phosphorylation, but does decrease newly synthesized RNA, likely by stalling RNA Pol II. However, camptothecin treatment suppresses nuclear blebbing more drastically than inhibition of transcription activity by alpha amanitin, suggesting a second mechanism of nuclear blebbing suppression. Dual micromanipulation nuclear force measures revealed camptothecin treatment increased chromatin-based nuclear stiffness but not lamin-based strain stiffening. Thus, inhibition of topoisomerase I via camptothecin drastically suppresses nuclear blebbing by both stalling RNA Pol II and increasing chromatin-based nuclear stiffness.