Transcription inhibition by flavopiridol: mechanism of chronic lymphocytic leukemia cell death.
Flavopiridol is active against chronic lymphocytic leukemia (CLL) cells in vitro and in the treatment of advanced stage disease, but the mechanisms of these actions remain unclear. Originally developed as a general cyclin-dependent kinase inhibitor, flavopiridol is a potent transcriptional suppressor through the inhibition of positive transcription elongation factor b (P-TEFb; CDK9/cyclin T). P-TEFb phosphorylates the C-terminal domain (CTD) of RNA polymerase II to promote transcriptional elongation. Because most CLL cells are not actively cycling, and their viability is dependent upon the continuous expression of antiapoptotic proteins, we hypothesized that flavopiridol induces apoptosis in CLL cells through the transcriptional down-regulation of such proteins. This study demonstrated that flavopiridol inhibited the phosphorylation of the CTD of RNA polymerase II in primary CLL cells and reduced RNA synthesis. This was associated with a decline of the transcripts and the levels of short-lived antiapoptotic proteins such as myeloid cell leukemia 1 (Mcl-1), and resulted in the induction of apoptosis. The B-cell lymphoma 2 (Bcl-2) protein level remained stable, although its mRNA was consistently reduced, suggesting that the outcome of transcriptional inhibition by flavopiridol is governed by the intrinsic stability of the individual transcripts and proteins. The dependence of CLL-cell survival on short-lived oncoproteins may provide the biochemical basis for the therapeutic index in response to flavopiridol.
Department of Experimental Therapeutics, The University of Texas MD Anderson Cancer Center, Houston, TX 77030, USA.
Rong
R
Chen
Michael J
MJ
Keating
Varsha
V
Gandhi
William
W
Plunkett
D000970:Antineoplastic Agents / Q000494:pharmacology; D017209:Apoptosis; D016923:Cell Death; D002470:Cell Survival; D015536:Down-Regulation; D005419:Flavonoids / Q000737:chemistry / Q000494:pharmacology*; D006801:Humans; D015151:Immunoblotting; D015451:Leukemia, Lymphocytic, Chronic, B-Cell / Q000097:blood / Q000188:drug therapy* / Q000473:pathology*; D008214:Lymphocytes / Q000378:metabolism; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D009363:Neoplasm Proteins / Q000378:metabolism; D010766:Phosphorylation; D010880:Piperidines / Q000737:chemistry / Q000494:pharmacology*; D011065:Poly(ADP-ribose) Polymerases / Q000378:metabolism; D017434:Protein Structure, Tertiary; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism; D012313:RNA / Q000737:chemistry / Q000378:metabolism; D012319:RNA Polymerase II / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013997:Time Factors; D014158:Transcription, Genetic
D000970:Antineoplastic Agents; D005419:Flavonoids; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D009363:Neoplasm Proteins; D010880:Piperidines; D019253:Proto-Oncogene Proteins c-bcl-2; D012333:RNA, Messenger; C077990:alvocidib; D012313:RNA; D011065:Poly(ADP-ribose) Polymerases; D012319:RNA Polymerase II