Deoxyribonucleotide metabolism in hydroxyurea-resistant V79 hamster cells

Hsp90 inhibitors and drug resistance in cancer: the potential benefits of combination therapies of Hsp90 inhibitors and other anti-cancer drugs

Hsp90 is a chaperone protein that interacts with client proteins that are known to be in the cell cycle, signaling and chromatin-remodeling pathways. Hsp90 inhibitors act additively or synergistically with many other drugs in the treatment of both solid tumors and leukemias in murine tumor models and humans. Hsp90 inhibitors potentiate the actions of anti-cancer drugs that target Hsp90 client proteins, including trastuzumab (Herceptin™) which targets Her2/Erb2B, as Hsp90 inhibition elicits the drug effects in cancer cell lines that are otherwise resistant to the drug. A phase II study of the Hsp90 inhibitor 17-AAG and trastuzumab showed that this combination therapy has anticancer activity in patients with HER2-positive metastatic breast cancer progressing on trastuzumab. In this review, we discuss the results of Hsp90 inhibitors in combination with trastuzumab and other cancer drugs. We also discuss recent results from yeast focused on the genetics of drug resistance when Hsp90 is inhibited and the implications that this might have in understanding the effects of genetic variation in treating cancer in humans.

Cytosolic high K(m) 5'-nucleotidase and 5'(3')-deoxyribonucleotidase in substrate cycles involved in nucleotide metabolism

5'-Nucleotidases are the catabolic members of the substrate cycles postulated to be involved in the regulation of intracellular deoxyribonucleoside triphosphate pools. Here, we attempt to identify the nature of the nucleotidases. Earlier, we constructed various mammalian cell lines that can be induced to overproduce the high K(m) 5'-nucleotidase (hkm-NT) or the 5'(3')-deoxynucleotidase (dNT-1). Now we labeled control and induced human 293 cells and hamster V79 cells with radioactive hypoxanthine or uridine and during a chase measured quantitatively the metabolism of ribo- and deoxyribonucleotides, DNA replication, and excretion of nucleosides into the medium. Overproduction of hkm-NT greatly increased excretion of inosine and guanosine but did not affect adenosine or deoxyribonucleosides. dNT-1 overproduction increased excretion of deoxycytidine, thymidine, and in particular deoxyuridine but also uridine and cytidine. We conclude that the hkm-NT is not involved in the regulation of deoxyribonucleotide pools but affects IMP and GTP pools. dNT-1, instead, appears to be the catabolic arm of substrate cycles regulating pyrimidine nucleotide pools.