| Drug resistance to anti-cancer chemotherapy is a significant barrier to the treatment of leukemia patients. Many times, resistance results from molecular adaptation to drug exposure, such as genetic mutation of key enzymes, upregulation of pro-survival compensatory signaling pathways, and altered drug transport. In this dissertation, we describe two examples of molecular drug resistance in cell models of (1) Ara-C-resistant acute lymphoblastic leukemia (ALL) and (2) imatinib-resistant chronic myelogenous leukemia (CML). First, we determined that nucleoside transport is deficient in the Ara-C-resistant T-cell ALL CCRF-CEM cell line (Ara-C/8C) in comparison to drug-sensitive parental CCRF-CEM cells. Further study found a single point mutation in glycine residue 24 (G24) within equilibrative nucleoside transporter 1 (ENT1), a protein responsible for nucleoside uptake in these cells. Therefore, we tested the ability of G24A, G24R, and G24E ENT1 mutants to transport uridine and Ara-C and localize to the plasma membrane. Our data suggest that mutation of G24 disrupts ENT1 transport activity without altering localization; thus, expression of mutant ENT1 may confer Ara-C resistance in CCRF-CEM AraC/8C cells. In the second portion of the dissertation, we studied the role of Lyn tyrosine kinase (Lyn) in imatinib-resistant CML MYL-R cells. In comparison to drug-sensitive parental MYL cells, Lyn was hyper-active, and loss of Lyn activity sensitized cells to imatinib treatment. We determined that Lyn inhibited miR181 microRNA (miRNA) expression in MYL-R cells at the transcriptional level. In addition, we determined that miR181b targeted the 3' UTR of Mcl-1, a pro-survival protein associated with drug-resistance, resulting in Mcl-1 degradation. Thus, we defined a molecular signaling axis by which Lyn may confer drug resistance in imatinib-resistant CML. We attempted to elucidate the mechanism of Lyn-dependent miR181 expression and found that (1) the transcription factors CREB and STAT5 do not regulate miR181a/b and (2) Lyn may regulate miR181c/d expression through modification of histone acetylation. These studies contribute to our knowledge of drug resistance mechanisms in leukemia and provide novel biomarkers for the identification of drug-resistant cancer. |
