Smac mimetics: implications for enhancement of targeted therapies in leukemia.

Drug resistance is a growing concern with clinical use of tyrosine kinase inhibitors. Utilizing in vitro models of intrinsic drug resistance and stromal-mediated chemoresistance, as well as functional mouse models of progressive and residual disease, we attempted to develop a potential therapeutic approach designed to suppress leukemia recurrence following treatment with selective kinase inhibitors. The novel IAP inhibitor, LCL161, [corrected] was observed to potentiate the effects of tyrosine kinase inhibition against leukemic disease both in the absence and presence of a stromal-protected [corrected] environment. LCL161 enhanced the proapoptotic effects of nilotinib and PKC412, against leukemic disease in vitro and potentiated the activity of both kinase inhibitors against leukemic disease in vivo. In addition, LCL161 synergized in vivo with nilotinib to reduce leukemia burden significantly below the baseline level suppression exhibited by a moderate-to-high dose of nilotinib. Finally, LCL161 displayed antiproliferative effects against cells characterized by intrinsic resistance to tyrosine kinase inhibitors as a result of expression of point mutations in the protein targets of drug inhibition. These results support the idea of using IAP inhibitors in conjunction with targeted tyrosine kinase inhibition to override drug resistance and suppress or eradicate residual disease.

Identification of c-MYC as a target of the APC pathway.

The adenomatous polyposis coli gene (APC) is a tumor suppressor gene that is inactivated in most colorectal cancers. Mutations of APC cause aberrant accumulation of beta-catenin, which then binds T cell factor-4 (Tcf-4), causing increased transcriptional activation of unknown genes. Here, the c-MYC oncogene is identified as a target gene in this signaling pathway. Expression of c-MYC was shown to be repressed by wild-type APC and activated by beta-catenin, and these effects were mediated through Tcf-4 binding sites in the c-MYC promoter. These results provide a molecular framework for understanding the previously enigmatic overexpression of c-MYC in colorectal cancers.

Characterization of human FAST-1, a TGF beta and activin signal transducer.

We have identified a human homolog of the Xenopus forkhead activin signal transducer-1 (xFAST-1). Although significantly different in sequence from its Xenopus counterpart, hFAST-1 shared with xFAST-1 the ability to bind to human Smad2 and activate an activin response element (ARE). The hFAST-1-dependent activation of ARE was completely dependent on endogenous Smad4 and stimulation by a TGF beta-like ligand. The hFAST-1 protein was shown to bind to a novel DNA motif, TGT (G/T) (T/G)ATT, an exact copy of which was present within the ARE. A single copy of this motif could activate a reporter in a TGF beta-dependent fashion but only when an adjacent Smad-binding element was present in the construct. These data suggest that responses to TGF beta family members may be mediated by a DNA-binding complex formed by hFAST-1, hSmad2, and hSmad4.

Human Smad3 and Smad4 are sequence-specific transcription activators.

Mounting evidence indicates that Smad proteins are required for TGF beta signaling, but the way(s) in which Smad proteins propagate this signal is unclear. We found that two human Smad proteins (Smad3 and Smad4) could specifically recognize an identical 8 bp palindromic sequences (GTCTAGAC). Tandem repeats of this palindrome conferred striking TGF beta responsiveness to a minimal promoter. This responsiveness was abrogated by targeted deletion of the cellular Smad4 gene. These results define a novel biochemical property of Smad proteins that is likely to play a direct role in the biologic responses to TGF beta and related ligands.

Targeted deletion of Smad4 shows it is required for transforming growth factor beta and activin signaling in colorectal cancer cells.

Smad4 (DPC4) is a candidate tumor suppressor gene that has been hypothesized to be critical for transmitting signals from transforming growth factor (TGF) beta and related ligands. To directly test this hypothesis, the Smad4 gene was deleted through homologous recombination in human colorectal cancer cells. This deletion abrogated signaling from TGF-beta, as well as from the TGF-beta family member activin. These results provide unequivocal evidence that mutational inactivation of Smad4 causes TGF-beta unresponsiveness and provide a basis for understanding the physiologic role of this gene in tumorigenesis.

Recycling of the general transcription factors during RNA polymerase II transcription.

We have analyzed the fate of the RNA polymerase II (RNAPII) general transcription factors during the transition from initiation to elongation using multiple approaches. We demonstrate that all of the basal factors coexist in mature initiation complexes but that following nucleotide addition, this complex becomes disrupted. During this transition, TFIID remains promoter-bound whereas TFIIB, TFIIE, TFIIF, and TFIIH are released. Upon release, TFIIB reassociates with TFIID, reforming the RNAPII docking site, the DB complex. TFIIE is released before formation of the tenth phosphodiester bond. This precedes TFIIH release, which occurrs after the transcription complex reaches +30. TFIIF is unique in that it is the only basal factor detected in the RNAPII elongation complex. Following its release from the initiation complex, TFIIF has the ability to reassociate with a stalled RNAPII.

Visualization of TBP oligomers binding and bending the HIV-1 and adeno promoters.

The binding of the 28 kDa yeast TATA binding protein (yTBP) to the HIV and adeno major late promoters has been examined by electron microscopy (EM). Three different EM preparative methods were employed: direct mounting and shadowcasting of fixed samples, cryofixation and freeze-drying followed by shadowcasting, and negative staining of unfixed samples. Excellent agreement among the three methods was obtained. With ten yTBP monomers/DNA fragment, up to 25% of the DNA molecules contained easily distinguished protein particles at the TATA box and, less frequently, smaller particles were observed. Non-specific binding to DNA ends was common. The mass of the easily distinguished particles measured 63(+/- 5) kDa (cryofixation and shadowcasting) and 48(+/- 6) kDa (negative staining) indicating TBP dimerization. With 22 and 44 yTBP monomers/DNA, yTBP polymerization produced DNA-protein rods 9 nm wide and 20 to 30 nm long, frequently with two DNA strands exiting one end. Bending analysis revealed that yTBP dimers bend the DNA about the TATA box by 80 to 90 degrees. Although these protein ratios are relatively high, the structures formed demonstrate the propensity of yTBP to engage in protein-protein interactions.

Common themes in assembly and function of eukaryotic transcription complexes.

Eukaryotes contain three distinct RNA polymerase enzymes, each responsible for the transcription of a subclass of nuclear genes. Despite this division of labor, each RNA polymerase system follows a common blueprint to execute the loading of the polymerase onto the relevant promoter region. The RNA polymerase II system appears unique in that after RNA polymerase II has loaded onto the DNA, two auxiliary factors, TFIIE and TFIIH, are necessary for its escape from the promoter region. The complexity of the RNA polymerase II initiation pathway provides a multitude of potential targets for transcriptional activators. Tight control over transcription initiation levels is afforded by multiple cofactors that both enhance and repress.

Dual role of TFIIH in DNA excision repair and in transcription by RNA polymerase II.

The RNA polymerase II general transcription factor TFIIH is composed of several polypeptides. The observation that the largest subunit of TFIIH is the excision-repair protein XPB/ERCC3 (ref. 1), a helicase implicated in the human DNA-repair disorders xeroderma pigmentosum (XP) and Cockayne's syndrome, suggests a functional link between transcription and DNA repair. To understand the connection between these two cellular processes, we have extensively purified and functionally analysed TFIIH. We find that TFIIH has a dual role, being required for basal transcription of class II genes and for participation in DNA-excision repair. TFIIH is shown to complement three different cell extracts deficient in excision repair: XPB/ERCC3, XPC and XPD/ERCC2. The complementation of XPB and XPD is a consequence of ERCC3 and ERCC2 being integral subunits of TFIIH, whereas complementation of XPC is due to an association of this polypeptide with TFIIH. We found that the general transcription factor IIE negatively modulates the helicase activity of TFIIH through a direct interaction between TFIIE and the ERCC3 subunit of TFIIH.

Identification of a minimal set of proteins that is sufficient for accurate initiation of transcription by RNA polymerase II.

In eukaryotes, initiation of mRNA synthesis is a multistep process that is carried out by RNA polymerase II and auxiliary factors that are commonly referred to as basal or general factors. In this study accurate initiation of transcription was reconstituted with purified, Escherichia coli-synthesized TFIIB, TBP (the TATA box-binding polypeptide of the TFIID complex), and the 30-kD subunit of TFIIF (also known as RAP30), along with purified, native RNA polymerase II from Drosophila embryos, calf thymus, or HeLa cells. This minimal set of factors was able to transcribe a subset of the promoters tested. The addition of both subunits of TFIIE and the 74-kD subunit of TFIIF increased the efficiency of transcription by a factor of 2 to 4. In contrast, the inclusion of a crude TFIID fraction from Drosophila embryos in place of recombinant TBP resulted in a strong dependence on TFIIE. By gel mobility-shift analysis, TFIIB, TBP, RAP30, and polymerase were able to assemble into DB and DBPolF30 complexes with transcriptionally competent (wild type or initiator mutant), but not with transcriptionally inactive (TATA and TATA/initiator mutant), versions of the Drosophila Adh promoter. Thus, it appears that RNA polymerase II is able to initiate transcription subsequent to assembly of the DBPolF30 complex, which is a minitranscription complex that represents the central core of the RNA polymerase II transcriptional machinery.

Human general transcription factor IIH phosphorylates the C-terminal domain of RNA polymerase II.

Phosphorylation of the carboxy-terminal domain of the largest subunit of RNA polymerase II is believed to control the transition from transcription initiation to elongation. The general transcription factor IIH (TFIIH) contains a kinase activity capable of phosphorylating this domain. Factors that promote the association of RNA polymerase II with the preinitiation complex stimulate this activity. The transcription factor IIE, which is required for the stable association of TFIIH with the preinitiation complex, affects the processivity of TFIIH kinase.

Advances in RNA polymerase II transcription.

Multiple protein factors are necessary to mediate transcription by RNA polymerase II. Recently, a number of advances have been made in our understanding of how general transcription factors collectively modulate basal transcription in the context of different promoter environments and how this process is activated and repressed by accessory components.