Lytic peptide-mediated sensitization of TRAIL-resistant prostate cancer cells to death receptor agonists.

Tumor Necrosis Factor-alpha Related Apoptosis Inducing Ligand (TRAIL) and agonistic antibodies to death receptors (DR) 4 and 5 have attracted significant attention in recent years due to their ability to selectively induce apoptosis in malignant cells while demonstrating little cytotoxicity in normal cells. Although these candidates are promising in cancer therapy, a number of tumor cells are resistant to TRAIL-mediated apoptosis. We describe the use of a cationic amphipathic lytic peptide, KLA (single letter sequence HHHHHKLAKLAKKLAKLAKC), for the chemosensitization of TRAIL-resistant LNCaP and PC3-PSMA human prostate cancer cells to DR agonistic antibodies. 'Single-agent' treatment with DR agonistic antibodies did not result in loss of viability of these cells confirming the resistance of these cells. However, the combination treatment of KLA followed by DR agonists resulted in greater cell death compared to the individual treatments acting alone, indicating synergistic action between the two components of the combination treatment. The combination of lytic peptide and DR agonists resulted in a significant increase in activated caspase-3 cleavage and cytochrome-C protein levels in cells, indicating a role for the caspase-mediated apoptotic pathway. In addition, KLA treatment also resulted in increased localization of DR5 and lipid rafts in LNCaP cells. Our results demonstrate, for the first time, that lytic peptides can be employed for sensitizing TRAIL-resistant prostate cancer cells to DR-mediated apoptosis resulting in novel combination treatments for the ablation of advanced cancer cells.

Tax interacts with P-TEFb in a novel manner to stimulate human T-lymphotropic virus type 1 transcription.

Human T-lymphotropic virus type 1 (HTLV-1) encodes a transcriptional activator, Tax, whose function is essential for viral transcription and replication. Tax transactivates the viral long-terminal repeat through a series of protein-protein interactions which facilitate CREB and CBP/p300 binding. In addition, Tax dissociates transcription repressor histone deacetylase 1 interaction with the CREB response element. The subsequent events through which Tax interacts and communicates with RNA polymerase II and cyclin-dependent kinases (CDKs) are not clearly understood. Here we present evidence that Tax recruits positive transcription elongation factor b (P-TEFb) (CDK9/cyclin T1) to the viral promoter. This recruitment likely involves protein-protein interactions since Tax associates with P-TEFb in vitro as demonstrated by glutathione S-transferase fusion protein pull-down assays and in vivo as shown by co-immunoprecipitation assays. Functionally, small interfering RNA directed toward CDK9 inhibited Tax transactivation in transient assays. Consistent with these findings, the depletion of CDK9 from nuclear extracts inhibited Tax transactivation in vitro. Reconstitution of the reaction with wild-type P-TEFb, but not a kinase-dead mutant, recovered HTLV-1 transcription. Moreover, the addition of the CDK9 inhibitor flavopiridol blocked Tax transactivation in vitro and in vivo. Interestingly, we found that Tax regulates CDK9 kinase activity through a novel autophosphorylation pathway.

Tax relieves transcriptional repression by promoting histone deacetylase 1 release from the human T-cell leukemia virus type 1 long terminal repeat.

Expression of human T-cell leukemia virus type 1 (HTLV-1) is regulated by the viral transcriptional activator Tax. Tax activates viral transcription through interaction with the cellular transcription factor CREB and the coactivators CBP/p300. In this study, we have analyzed the role of histone deacetylase 1 (HDAC1) on HTLV-1 gene expression from an integrated template. First we show that trichostatin A, an HDAC inhibitor, enhances Tax expression in HTLV-1-transformed cells. Second, using a cell line containing a single-copy HTLV-1 long terminal repeat, we demonstrate that overexpression of HDAC1 represses Tax transactivation. Furthermore, a chromatin immunoprecipitation assay allowed us to analyze the interaction of transcription factors, coactivators, and HDACs with the basal and activated HTLV-1 promoter. We demonstrate that HDAC1 is associated with the inactive, but not the Tax-transactivated, HTLV-1 promoter. In vitro and in vivo glutathione S-transferase-Tax pull-down and coimmunoprecipitation experiments demonstrated that there is a direct physical association between Tax and HDAC1. Importantly, biotinylated chromatin pull-down assays demonstrated that Tax inhibits and/or dissociates the binding of HDAC1 to the HTLV-1 promoter. Our results provide evidence that Tax interacts directly with HDAC1 and regulates binding of the repressor to the HTLV-1 promoter.