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Oncotarget. 2017 Aug 1;8(35):59476-59491. doi: 10.18632/oncotarget.19761. eCollection 2017 Aug 29.

Positive transcription elongation factor b (P-TEFb) is a therapeutic target in human multiple myeloma.

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Division of Hematology/Oncology, Department of Medicine, Virginia Commonwealth University and The Massey Cancer Center, Richmond, VA, USA.
Department of Hematology, Beijing Chaoyang Hospital of Capital Medical University, Beijing, China.
Cancer Center, The First Hospital of Jilin University, Changchun, China.
Department of Myeloma and Lymphoma, MD Anderson Cancer Center, Houston, TX, USA.
Virginia Institute of Molecular Medicine, Virginia Commonwealth University, Richmond, VA, USA.
Department of Biochemistry, Virginia Commonwealth University, Richmond, VA, USA.
Department of Pharmacology Virginia Commonwealth University, Richmond, VA, USA.
Contributed equally


The role of the positive RNA Pol II regulator, P-TEFb (positive transcription elongation factor b), in maintenance of the anti-apoptotic protein Mcl-1 and bortezomib (btz) resistance was investigated in human multiple myeloma (MM) cells. Mcl-1 was up-regulated in all MM lines tested, including bortezomib-resistant lines, human MM xenograft mouse models, and primary CD138+ MM cells. Mcl-1 over-expression significantly reduced bortezomib lethality, indicating a functional role for Mcl-1 in bortezomib resistance. MM cell lines, primary MM specimens, and murine xenografts exhibited constitutive P-TEFb activation, manifested by high CTD (carboxy-terminal domain) S2 phosphorylation, associated with a) P-TEFb subunit up-regulation i.e., CDK9 (42 and 55 kDa isoforms) and cyclin T1; and b) marked CDK9 (42 kDa) T186 phosphorylation. In marked contrast, normal hematopoietic cells failed to exhibit up-regulation of p-CTD, CDK9, cyclin T1, or Mcl-1. CDK9 or cyclin T1 shRNA knock-down dramatically inhibited CTD S2 phosphorylation and down-regulated Mcl-1. Moreover, CRISPR-Cas CDK9 knock-out triggered apoptosis in MM cells and dramatically diminished cell growth. Pan-CDK e.g., dinaciclib or alvocidib and selective CDK9 inhibitors (CDK9i) recapitulated the effects of genetic P-TEFb disruption. CDK9 shRNA or CDK9 inhibitors significantly potentiated the susceptibility of MM cells, including bortezomib-resistant cells, to proteasome inhibitors. Analogously, CDK9 or cyclin T1 knock-down or CDK9 inhibitors markedly increased BH3-mimetic lethality in bortezomib-resistant cells. Finally, pan-CDK inhibition reduced human drug-naïve or bortezomib-resistant CD138+ cells and restored bone marrow architecture in vivo. Collectively, these findings implicate constitutive P-TEFb activation in high Mcl-1 maintenance in MM, and validate targeting the P-TEFb complex to circumvent bortezomib-resistance.


CDK inhibitors; MCL-1; P-TEFb; bortezomib resistance; myeloma

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