Differential scanning calorimetry of nuclei as a test for the effects of anticancer drugs on human chromatin.

Differential scanning microcalorimetry of nuclei from cultured cells revealed differences between antitumor drugs in potency and mechanism. Scanning calorimetry of nuclei showed four structural transitions as the temperature was raised from 25 to 120 degrees C. Transitions II (76 degrees C), III (88 degrees C), and IV (105 degrees C), respectively, characterize the denaturation of the nucleosome, the unstacking of bases in nicked DNA after release from the nucleosome and unstacking in the released intact, supercoiled DNA. Nuclei from human epithelial cells treated with the DNA strand breakers bleomycin and streptonigrin showed an increase in transition III at the expense of transition IV. The effect was dose dependent. At intermediate times of treatment a substantial portion of the chromatin melted between the temperatures of transitions III and IV and this was taken to represent intact supercoiled DNA in which base-pairing had been weakened by loss of some bases. Treatment of cells with the alkylating agents N-nitroso-N-methylurea and mitomycin C gave results similar to those of the strand-breaker antitumor drugs, except that they were less potent. Irradiation by UV gave similar effects. The effects of intercalating drugs were quite distinct from those produced by strand breakers or alkylators. Nuclei from cells incubated with increasing doses of actinomycin D or ethidium bromide gave calorimetric scans that indicated progressive denaturation of the nucleosome and a concomitant stabilization of supercoiled DNA. The loss of transition IV during drug treatment was correlated with the loss of cellular capacity to divide, regardless of which drugs were used.