Inhibition of human telomerase by rubromycins: Implication of spiroketal system of the compounds as an active moiety

We found that a group of rubromycins and their analogues, a class of quinone antibiotics that possesses benzofuran and benzodipyran rings to form a. spiroketal system, strongly inhibited human telomerase as assessed with a modified telomeric repeat amplification protocol. beta- and gamma-Rubromycins and purpuromycin appeared to be the most potent telomerase inhibitors, with 50% inhibitory concentrations (IC50) of about 3 mu M, and griseorhodins A and C also showed comparable potencies for the inhibition (IC50 = 6-12 mu M). In contrast, opening of the spiroketal system of beta-rubromycin, giving rise to alpha-rubromycin, substantially decreased its inhibitory potency toward telomerase (IC50 > 200 mu M), indicating the essential role of the spiroketal system in telomerase inhibition. A kinetic study of the inhibition by beta-rubromycin revealed a competitive interaction with respect to the telomerase substrate primer, with a K-i of 0.74 mu M, whereas a mixed type inhibition was observed with respect to the nucleotide substrate. beta-Rubromycin was also potent in inhibiting retroviral reverse transcriptases but had virtually no effect on other DNA/RNA-modifying enzymes including DNA and RNA polymerases, deoxyribonuclease, and topoisomerase. Although beta-rubromycin showed nonspecific cytotoxicities, reducing proliferation of cancer cells (IC50 similar to 20 mu M), we conclude that beta-rubromycin appears to be a lead structure for the development of more potent and selective inhibitors of human telomerase.