Regulation of HTLV-1 Transcription

Masison, Cynthia

Abstract

The human retrovirus HTLV-1 is the etiologic agent of adult T-cell leukemia. The viral transforming protein, Tax, plays a critical role in viral replication and transformation by regulating the expression of viral and cellular genes. Previous studies have demonstrated that Tax interacts with the cellular CREB protein and facilitates the binding of the coactivator and histone acetyltransferase CBP and PCAF, forming a multimeric complex on the CRE-like sites in the HTLV-1 promoter. The studies presented in this report focus on the ability of Tax to interact with cellular transcription factors and chromatin modifying proteins to regulate the CREB transcription pathway, which is essential for viral gene expression. We have demonstrated that arginine methyltransferase CARM1 enhances Tax transcription of the HTLV-1 LTR through a direct interaction between CARM1 and Tax and this binding promotes methylation of H3 in vitro and in vivo. Unexpectedly, we found that the general methyltransferase inhibitor adenosine dialdehyde (AdOx) potently inhibited the growth of HTLV-1-transformed cells. Further investigation revealed that AdOx also inhibited the Tax-activated NF-kappaB pathway, resulting in reactivation of p53 and induction of p53 target genes. Analysis of the NF-kappaB pathway demonstrated that AdOx treatment resulted in degradation of the IkappaB kinase complex and inhibition of NF-kappaB through stabilization of the NF-kappaB inhibitor IkappaBalpha. Our data further demonstrated that AdOx induced G(2)/M cell cycle arrest and cell death in HTLV-1-transformed but not control lymphocytes. These studies demonstrate that protein methylation plays an important role in NF-kappaB activation and survival of HTLV-1-transformed cells as well as mediating viral transcriptional activation. Mechanisms through which Tax interacts and communicates with RNA polymerase II and cyclin dependent kinases are not clearly understood. We have recently demonstrated that Tax recruits P-TEFb, a critical cellular elongation factor, to the viral promoter. The recruitment likely involves protein-protein interactions since Tax associates with P-TEFb in vitro as demonstrated by GST protein pull-down assays and in vivo as shown by co-immunoprecipitation assay. Functionally, a siRNA directed toward CDK9 inhibited Tax transactivation in transient assays. Consistent with these findings, depletion of CDK9 from nuclear extracts inhibited Tax transactivation in vitro. Reconstitution of the reaction with WT P-TEFb, but not a kinase dead mutant, recovered HTLV-1 transcription. Addition of the CDK9 inhibitor flavopiridol blocked Tax transactivation in vitro and in vivo. Our studies further demonstrate that Tax regulates CDK9 kinase activity through a novel autophosphorylation pathway by inducing autophosphorylation of threonine 29, the first experimental evidence of an inhibitory phosphorylation site in CDK9. Cellular P-TEFb is found in two major complexes, the inactive form which is associated with inhibitory subunits 7SK snRNA and HEXIM1 and the active form which is associated with, at least in part, Brd4. Our recent results suggest that Tax competes and functionally substitute for Brd4 in P-TEFb regulation. In vitro binding studies demonstrate that Tax and Brd4 compete for binding to P-TEFb through direct interaction with cyclin T1. Tax interacts with cyclin T1 amino acids 426-533 which overlaps the region responsible for Brd4 binding. In addition, overexpression of Tax decreased the amount of 7SK snRNA associated with P-TEFb and stimulates Serine 2 phosphorylation of the RNA Pol II CTD, suggesting Tax regulates the functional activity of P-TEFb. Overexpression of Brd4 repressed Tax transactivation of the HTLV-1 LTR in a dose dependent manner. More recently, we show in glycerol gradient sedimentation assays that Tax/P-TEFb complexes migrate with active LMW P-TEFb in complexes distinct from BRD4/P-TEFb complexes. We demonstrate that Tax can play a role in regulating the amount of HMW complex present in the cell by decreasing the binding of 7SK snRNP/HEXIM1 to P-TEFb. This is seen both in vitro using purified Tax protein and in vivo in cells transduced with Tax expression constructs. Further, we find that a peptide of cyclin T1 spanning the Tax binding domain, inhibits the ability of Tax to disrupt HMW P-TEFb complexes. These results suggest that the direct interaction of Tax with cyclin T1 can dissociate P-TEFb from the P-TEFb/7SK snRNP/HEXIM1 complex for activation of the viral LTR. We also show that Tax competes with Brd4 for P-TEFb binding. ChIP assays demonstrated that Brd4 and P-TEFb are associated with the basal HTLV-1-LTR while Tax and P-TEFb are associated with the activated template. Furthermore, the knockdown of Brd4 by siRNA activates the HTLV-1 LTR promoter which results in an increase in viral expression and production. Our studies have identified Tax as a regulator of P-TEFb capable of affecting the balance between its association with the large inactive complex and the small active complex. These results suggest that the direct interaction of Tax with cyclin T can compete P-TEFb regulation and provides a molecular target for antiviral therapy.