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 (IC(50)) of about 3 microM, and griseorhodins A and C also showed comparable potencies for the inhibition (IC(50) = 6-12 microM). In contrast, opening of the spiroketal system of beta-rubromycin, giving rise to alpha-rubromycin, substantially decreased its inhibitory potency toward Telomerase (IC(50) > 200 microM), 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 microM, 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 (IC(50) approximately 20 microM), we conclude that beta-rubromycin appears to be a lead structure for the development of more potent and selective inhibitors of human Telomerase.