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2017 ; 8
(35
): 59476-59491
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Positive transcription elongation factor b (P-TEFb) is a therapeutic target in
human multiple myeloma
#MMPMID28938651
Zhang Y
; Zhou L
; Leng Y
; Dai Y
; Orlowski RZ
; Grant S
Oncotarget
2017[Aug]; 8
(35
): 59476-59491
PMID28938651
show ga
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.