ABSTRACT: Elevation of activity and mRNA level of a cytosolic aldehyde dehydrogenase-1 (ALDH1), which oxidizes aldophosphamide, was previously observed in a cyclophosphamide-resistant murine leukemia cell line. However, changes in other enzyme(s) which may detoxify the drug or produce anti-alkylating agent(s), have not been examined. The human leukemia cell line, K562, was made 30-fold resistant against 4-hydroperoxyphosphamide (4HC) by exposing the cells to increasing concentrations of the drug. Resistance against cisplatin was also increased by about 3-fold. Activities of glucose-6-phosphate dehydrogenase (G6PD) and ALDH1 were elevated more than 7-fold in the resistant cells. The mRNA level of the two enzymes was also proportionally elevated. The concentration of reduced glutathione (GSH) was higher in the resistant cells (i.e., 21.1 versus 4.68 nmole per 10(6) cells), while activities of gamma-glutamylcysteine synthetase and glutathione synthetase, and the expressions of other human ALDH genes were not increased in the resistant cells. These findings suggest that the acquired resistance against 4HC is a consequence of transcriptional activation of two genes, i.e., one encoding the G6PD, a major enzyme regenerating anti-alkylating GSH, and the other encoding ALDH1, which has a high activity for oxidation of aldophosphamide derived from 4HC.
Keywords: Aldehyde dehydrogenase, glucose-6-phosphate dehydrogenase, glutathione, drug-resistance, leukemia cells.
Reprint requests to: Akira Yoshida, Ph.D., Department of Biochemical Genetics, Beckman Research Institute of the City of Hope,1450 E. Duarte Road, Duarte, CA 91010, phone: (626) 359-8111 ext. 3692, fax: (626) 358-7703, e-mail: ayoshida@smtplink.coh.org.
ABSTRACT: The process of hematopoiesis is critically dependent on correct interactions of multiple regulatory molecules and transcription factors. We have studied the interactions of the v-Myb and retinoic acid receptor proteins which have opposing effects on hematopoiesis. While v-myb acts as a transforming oncogene preventing differentiation of monoblasts to macrophages, RARalpha functions as an anti-oncogene arresting the growth of v-myb-transformed cells and allowing their final myeloid differentiation steps to occur. We found that the retinoic acid receptor alpha inhibits v-Myb transformation by suppressing the expression of v-Myb target genes typified by the mim-1 gene. Conversely, v-Myb protein interferes with RARalpha transactivation function as well as with retinoic acid-induced apoptosis of HL-60 cells. These results demonstrate that retinoic acid receptor and v-Myb proteins act in antagonistic ways and reciprocally modify each other's functions.
Keywords: Hematopoiesis, apoptosis, Myb, retinoic acid receptor, mim-1.
Reprint requests to: Jan Smarda, Ph.D., Department of Genetics and Molecular Biology, Faculty of Science, Masaryk University, Kotlarska 2, 611 37 Brno, Czech Republic, phone: 420-5-41129538, fax: 420-5-41211214, email: smarda@sci.muni.cz e-mail: smarda@sci.muni.cz.