Altered metabolism in cancer cells is suspected to contribute to chemoresistance but the precise mechanisms are unclear. Here we show that intracellular ATP levels are a core determinant in the development of acquired cross-drug resistance of human colon cancer cells that harbor different genetic backgrounds. Drug-resistant cells were characterized by defective mitochondrial ATP production, elevated aerobic glycolysis, higher absolute levels of intracellular ATP and enhanced HIF-1α-mediated signaling. Interestingly, direct delivery of ATP into cross-chemoresistant cells destabilized HIF-1α and inhibited glycolysis. Thus, drug-resistant cells exhibit a greater “ATP debt” defined as the extra amount of ATP needed to maintain homeostasis of survival pathways under genotoxic stress. Direct delivery of ATP was sufficient to render drug-sensitive cells drug resistant. Conversely, depleting ATP by cell treatment with an inhibitor of glycolysis, 3-bromopyruvate, was sufficient to sensitize cells cross-resistant to multiple chemotherapeutic drugs. In revealing intracellular ATP levels are a core determinant of chemoresistance in colon cancer cells, our findings may offer a foundation for new improvements to colon cancer treatment.

Sequence homology and structural similarity between cytochrome b of mitochondrial complex III ABSTRACTThe amino acid sequences of cytochrome b of complex III from five different mitochondrial sources (human, bovine, mouse, yeast, and Aspergilus nidulans) and the chloroplast cytochrome b6 from spinach show a high degree of homology. Calculation of the distribution of hydrophobic residues with a "hydropathy" function that is conserved in this family of proteins implies that the membrane-folding pattern of the 42-kilodalton (kDa) mitochondrial cytochromes involves 8-9 membrane-spanning domains. The smaller 23-kDa chloroplast cytochrome appears to fold in five spanning domains that are similar to the first five of the mitochondria. Four highly conserved histidines are considered to be the likely ligands for the two hemes. The positions of the histidines along the spanning segments and in a cross section of the membrane-spanning a helices implies that two ligand pairs, His-82-His-

Optical, resonance Raman, and electron paramagnetic resonance spectroscopies have been used to characterize the ligands and spin state of the chloroplast cytochrome b-559. The protein was isolated from both maize and spinach in a low-potential form. The spectroscopic data indicate that the heme iron in both ferric and ferrous cytochrome b-559 is in its low-spin state and ligated in its fifth and sixth coordination positions by histidine nitrogens. Electron paramagnetic resonance data for the purified spinach cytochrome are in good agreement with those determined by Bergström and Vänngård [Bergström, J., & Vänngård, T. (1982) Biochim. Biophys. Acta 682, 452-456] for a low-potential membrane-bound form of cytochrome b-559. The g values of high-potential cytochrome b-559 are shifted from those of its low-potential forms; this shift is interpreted as arising from a deviation of the planes of the two axial histidine imidazole rings from a parallel orientation. The model is consistent with the physical data and may also account for the facility with which cytochrome b-559 can be converted between low- and high-potential forms. Recent biochemical and molecular biological data [Widger, W. R., Cramer, W. A., Hermodson, M., Meyer, D., & Gullifor, M. (1984) J. Biol. Chem. 259, 3870-3876; Herrmann, R. G., Alt, J., Schiller, D., Cramer, W. A., & Widger, W. R. (1984) FEBS Lett. 179, 239-244] have shown that two polypeptides, one with 83 residues and a second with 39 residues, most likely constitute the protein of the cytochrome.(ABSTRACT TRUNCATED AT 250 WORDS)

Bicyclomycin is a novel, commercially important antibiotic. Information concerning the site of bicyclomycin inhibition in Escherichia coli has been obtained by the production of bicyclomycin resistant mutants by UV irradiation. Selection by growth in the presence of bicyclomycin of a plasmid clone library generated from a highly resistant mutant in recipient antibiotic-sensitive host cells (E. coli strain W3350) has led to the characterization of three different plasmids that confer drug resistance, which contained the gene encoding the transcription termination factor, rho. These mutant rho genes contained single base changes at nucleotide positions 656, 796, and 1009. Preliminary mechanistic information has been obtained by monitoring the polyC-dependent ATPase activity of rho in the absence and presence of bicyclomycin and dihydrobicyclomycin. Addition of bicyclomycin to aqueous solutions containing rho and ATP led to a decrease in the release of inorganic phosphate with an I50 value of 60-70 microM bicyclomycin. This inhibition is comparable to the drug concentration needed to inhibit bacterial growth on plates. No loss of activity was observed when a similar concentration of dihydrobicyclomycin was used in place of bicyclomycin, while use of 10-fold higher concentrations of this derivative led to partial rho inhibition. PolyC-dependent ATPase activity from partially purified rho isolated from the mutant BCMr108 was not inhibited by bicyclomycin at concentrations (200 microM) found to completely inhibit wild-type rho. These cumulative findings are consistent with the notion that bicyclomycin expresses its activity by interfering with the polyC-dependent ATPase activity of rho.

The interaction of Rho and the antibiotic bicyclomycin was probed using in vitro transcription termination reactions, poly(C) binding assays, limited tryptic digestions, and the bicyclomycin inhibition kinetics of ATPase activity in the presence of poly(dC) and ribo(C) 10