1. ¹CNRS, Institut de Pharmacologie et de Biologie Structurale, UMR5089, Toulouse, France
2. ²Inserm, U466, Toulouse, France
3. ³Université Toulouse III Paul Sabatier, Toulouse, France
4. ⁴Department of Haematology, Imperial College London, Hammersmith Hospital, London, UK
5. ⁵Daiichi Sankyo Co. Ltd, Tokyo, Japan

Correspondence: Dr O Cuvillier, CNRS UMR 5089, Toulouse 31077, France. E-mail: olivier.cuvillier@ipbs.fr

⁶Current address: Weatherall Institute of Molecular Medicine, Oxford, UK

⁷Current address: Institute of Medical and Veterinary Science, Adelaide, South Australia, Australia

Received 12 November 2007; Revised 13 March 2008; Accepted 18 March 2008; Published online 10 April 2008.

Abstract

We examined the involvement of sphingosine kinase-1 (SphK1), which governs the ceramide/sphingosine-1-phosphate balance, in susceptibility to imatinib of either sensitive or resistant chronic myeloid leukemia cells. Imatinib-sensitive LAMA84-s displayed marked SphK1 inhibition coupled with increased content of ceramide and decreased pro-survival sphingosine-1-phosphate. Conversely, no changes in the sphingolipid metabolism were observed in LAMA84-r treated with imatinib. Overcoming imatinib resistance in LAMA84-r with farnesyltransferase or MEK/ERK inhibitors as well as with cytosine arabinoside led to SphK1 inhibition. Overexpression of SphK1 in LAMA84-s cells impaired apoptosis and inhibited the effects of imatinib on caspase-3 activation, cytochrome c and Smac release from mitochondria through modulation of Bim, Bcl-xL and Mcl-1 expression. Pharmacological inhibition of SphK1 with F-12509a or its silencing by siRNA induced apoptosis of both imatinib-sensitive and -resistant cells, suggesting that SphK1 inhibition was critical for apoptosis signaling. We also show that imatinib-sensitive and -resistant primary cells from chronic myeloid leukemia patients can be successfully killed in vitro by the F-12509a inhibitor. These results uncover the involvement of SphK1 in regulating imatinib-induced apoptosis and establish that SphK1 is a downstream effector of the Bcr-Abl/Ras/ERK pathway inhibited by imatinib but upstream regulator of Bcl-2 family members.