The Cyclin-dependent Kinase Inhibitor Flavopiridol Induces Apoptosis in Multiple Myeloma Cells through Transcriptional Repression and Down-Regulation of Mcl-1

The Cyclin-dependent Kinase Inhibitor Flavopiridol Induces Apoptosis in Multiple Myeloma Cells through Transcriptional Repression and Down-Regulation of Mcl-11

Greenebaum Cancer Center, University of Maryland Medical System, Baltimore, Maryland 21201

Abstract

Multiple myeloma (MM) is a B-cell malignancy characterized by the accumulation of malignant plasma cells with slow proliferative rate but enhanced survival. MM cells express multiple Bcl-2 family members, including Bcl-2, Bcl-XL, and Mcl-1, which are thought to play a key role in the survival and drug resistance of myeloma. The cyclin-dependent kinase inhibitor flavopiridol has antitumor activity against hematopoietic malignancies, including CLL, in which induction of apoptosis was associated with reduced expression of antiapoptotic proteins. Therefore, we sought to characterize the effect of flavopiridol on the proliferation and survival of myeloma cells and to define its mechanisms of action. Treatment of MM cell lines (8226, ANBL-6, ARP1, and OPM-2) with clinically achievable concentrations of flavopiridol resulted in rapid induction of apoptotic cell death that correlated temporally with the decline in Mcl-1 protein and mRNA levels. Levels of other antiapoptotic proteins did not change. Overexpression of Mcl-1 protected MM cells from flavopiridol-induced apoptosis. Additional analysis demonstrated that flavopiridol treatment resulted in a dose-dependent inhibition of phosphorylation of the RNA polymerase II COOH-terminal domain, thus blocking transcription elongation. These data indicate that Mcl-1 is an important target for flavopiridol-induced apoptosis of MM that occurs through inhibition of Mcl-1 mRNA transcription coupled with rapid protein degradation via the ubiquitin-proteasome pathway.

Footnotes

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↵1 Supported by a Translational Research Grant from the Leukemia and Lymphoma Society of America (to R. G. F.).
↵2 These authors contributed equally to this work.
↵3 To whom requests for reprints should be addressed, at University of Maryland Greenebaum Cancer Center, Bressler Research Building, 655 West Baltimore Street, Room 7–023, Baltimore, MD, 21201. Phone:(410) 328-0372; Fax: (410) 328-6559; E-mail: rfent001{at}umaryland.edu
↵4 The abbreviations used are: MM, multiple myeloma; Mcl-1, myeloid cell factor-1; CDK, cyclin-dependent kinase; CLL, chronic lymphocytic leukemia; CTD, COOH-terminal domain; BM, bone marrow; IL, interleukin; STAT, signal transducer and activator of transcription; P-TEFb, positive transcription elongation factor b; DRB, 5,6-dichloro-1-β-d-ribofuranosylbenzimidazole; ABCG2 (BCRP), ATP-binding cassette half-transporter (breast cancer resistance protein); IAP, inhibitor of apoptosis; TUNEL, terminal deoxynucleotidyl transferase-mediated nick end labeling; Ab, antibody.
- Accepted July 5, 1902.
- Received March 21, 1902.
- Revision received June 25, 1902.