The stem cell factor–c-KIT pathway must be inhibited to enable apoptosis induced by BCR–ABL inhibitors in chronic myelogenous leukemia cells

Abstract

Imatinib is an effective first-line therapy for chronic myelogenous leukemia (CML) that acts by targeting the tyrosine kinase activity of BCR-ABL. To overcome resistance, secondgeneration inhibitors of BCR-ABL have been developed. Among these, nilotinib is more potent against BCR-ABL than imatinib, and is effective against many imatinib-resistant BCR-ABL mutants. In this study, an in vitro flow cytometry assay to analyze imatinib-and nilotinib-induced apoptosis in CML cells has been developed. Both the drugs induced significant apoptosis in CD34 þ cells from 36 CML bone marrow samples (Po10 À4 ), whereas CD34 þ cells from BCR-ABL negative samples were unaffected. When the experiments were carried out in the presence of a cocktail of cytokines, nilotinib-but not imatinib-induced apoptosis was inhibited. This differential inhibition was confirmed on K562 cells. A blocking anti-CD117 antibody alleviated the antiapoptotic effect of cytokines against nilotinib. Moreover, using short hairpin RNA against BCR-ABL, we showed that K562 cells were not dependent on BCR-ABL signaling as long as the stem cell factor (SCF) receptor pathway was activated. We conclude that the c-KIT pathway may substitute for BCR-ABL tyrosine kinase to activate survival signals, and that c-KIT must be inhibited besides Bcr-Abl to allow apoptosis of CML cells.

Physiologic hypoxia promotes maintenance of CML stem cells despite effective BCR-ABL1 inhibition

“…Given that cells within the stem cell niche communicate through cytokines, and cytokines can induce BCR-ABL1-independent imatinib resistance, 16,17,55 it would be of interest to determine the combined effect of hypoxia and cytokines on CML progenitors. Our unpublished data also indicate that hypoxia and stromal cell coculture enhance colony formation, with or without imatinib.…”

“…Indeed, recent studies indicate that physiologic growth factor signaling, by stimulating prosurvival pathways in CML progenitors, may render them independent of BCR-ABL1-mediated survival signals. [15][16][17] Physiological hypoxia is said to occur when the oxygen (O 2 ) tension falls below 2%. 18 In humans, including patients with leukemia, the average BM O 2 has been measured at 6% to 7%, 19,20 whereas in mouse, the oxygen tension is estimated to be below 1% in the stem cell niche.…”

Principles and Current Topics Concerning Management of Tyrosine Kinase Inhibitor Therapy for Chronic Myelogenous Leukemia

“…While dasatinib and bosutinib exert multikinase inhibitory effects and bafetinib potently inhibits Lyn, a member of the Src kinase proteins, bosutinib does not inhibit c-KIT. Whether this insensitivity of bosutinib for c-KIT influences its anti-CML effect has not been clarified, but caution is advisable because the findings of several basic studies suggest that the concomitant blockade of Bcr-Abl and c-KIT is essential for suppressing Bcr-Abl-positive leukemias [35,36]. All SGIs possess different affinity (in vitro kinase inhibitory) profiles for mutated Abl KD.…”

Phosphorylation of Serine Palmitoyltransferase Long Chain-1 (SPTLC1) on Tyrosine 164 Inhibits Its Activity and Promotes Cell Survival

“…Imatinib mesylate (Gleevec, Novartis, Basel, Switzerland) was dissolved in DMSO at a stock concentration of 250 mM, stored at Ϫ20°C, and subsequently diluted with serum-free RPMI medium prior to use. Down-regulation of ABL expression in K562 cells was obtained by lentiviral expression of shRNA as described previously (14). Transduction efficiency was verified by FACS monitoring of GFP expression.…”