siRNA depletion of 7SK snRNA induces apoptosis but does not affect expression of the HIV-1 LTR or P-TEFb-dependent cellular genes

P-TEFb is a general transcriptional elongation factor composed of Cdk9 and either cyclin T1, T2, or K. A substantial portion of P-TEFb is associated with the 7SK small nuclear RNA (7SK) and the HEXIM1 or HEXIM2 proteins; this complex has reduced kinase activity in vitro relative to free P-TEFb. Here we report that 7SK and HEXIM1 levels are induced inactivated lymphocytes concomitantly with increased P-TEFb activity and global transcription. We used siRNA-mediated depletion to probe the function of 7SK in HeLa cells. Depletion of 7SK caused a large reduction in the association of HEXIM1 with Cdk9 and cyclin T1, and greatly reduced the amount of the cyclin T1 present in the 7SK/HEXIM1/P-TEFb complex. Similar to previous studies, siRNA-mediated depletion of 7SK resulted in increased expression of several reporter plasmids tested, including a plasmid lacking promoter elements. However, in contrast to previous studies, which did not examine the effects of 7SK depletion on endogenous gene expression, depletion of 7SK did not appear to affect the expression of the corresponding endogenous genes. Moreover, 7SK depletion had no effect on expression from the integrated HIV-1 provirus or the c-myc and MCL-1 genes, three transcription units known to be highly dependent upon P-TEFb. Importantly, depletion of 7SK was found to cause apoptosis by 72 h post-transfection in HeLa cells. These results suggest that 7SK may provide an essential cellular function whose relation to P-TEFb function is unclear.