An activating transcription factor 5-mediated survival pathway as a target for cancer therapy?

Genes that are highly expressed in cancer cells and are essential for their viability are attractive targets for the development of novel cancer therapeutics. Activating transcription factor 5 (ATF5) is an anti-apoptotic protein that is highly expressed in malignant glioma but not normal brain tissues, and is essential for glioma cell survival. Recent work has revealed an essential survival pathway mediated by ATF5 in malignant glioma; pharmacological inhibition of this pathway leads to tumor regression in mice. ATF5 is also highly expressed in a variety of other cancers, and preliminary studies have shown that the ATF5-mediated survival pathway is active in diverse human cancer cell lines. Targeting this pathway may therefore have therapeutic implications for the treatment of a wide range of cancers. In this perspective, we summarize recent advances in ATF5 research, focusing on its role in promoting cancer and its potential as a target for cancer therapy.

Inhibition of tumor angiogenesis by p53: a new role for the guardian of the genome.

The p53 tumor suppressor protein has long been recognized as the central factor protecting humans from cancer. It has been famously dubbed "the guardian of the genome" due to its ability to respond to genotoxic stress, such as DNA damage and other stress signals, and to protect the genome by inducing a variety of biological responses including DNA repair, cell cycle arrest, and apoptosis. However, the tumor suppressive effects of p53 go far beyond its roles in mediating these three processes. There is growing evidence that p53 also exerts its effects on multiple aspects of tumor formation, including suppression of metastasis and, as summarized in this review, inhibition of new blood vessel development (angiogenesis). The p53 protein has been shown to limit angiogenesis by at least three mechanisms: (1) interfering with central regulators of hypoxia that mediate angiogenesis, (2) inhibiting production of proangiogenic factors, and (3) directly increasing the production of endogenous angiogenesis inhibitors. The combination of these effects allows p53 to efficiently shut down the angiogenic potential of cancer cells. Inactivation of p53, which occurs in approximately half of all tumors, reverses these effects; as a consequence, tumors carrying p53 mutations appear more vascularized and are often more aggressive and correlate with poor prognosis for treatment. Thus, the loss of functional p53 during tumorigenesis likely represents an essential step in the switch to an angiogenic phenotype that is displayed by aggressive tumors.

Transcriptional regulatory elements in the human genome.

The faithful execution of biological processes requires a precise and carefully orchestrated set of steps that depend on the proper spatial and temporal expression of genes. Here we review the various classes of transcriptional regulatory elements (core promoters, proximal promoters, distal enhancers, silencers, insulators/boundary elements, and locus control regions) and the molecular machinery (general transcription factors, activators, and coactivators) that interacts with the regulatory elements to mediate precisely controlled patterns of gene expression. The biological importance of transcriptional regulation is highlighted by examples of how alterations in these transcriptional components can lead to disease. Finally, we discuss the methods currently used to identify transcriptional regulatory elements, and the ability of these methods to be scaled up for the purpose of annotating the entire human genome.

Fluorescence resonance energy transfer as a method for dissecting in vivo mechanisms of transcriptional activation.

The first step in transcriptional activation of protein-coding genes involves the assembly on the promoter of a large PIC (pre-initiation complex) comprising RNA polymerase II and a suite of general transcription factors. Transcription is greatly enhanced by the action of promoter-specific activator proteins (activators) that function, at least in part, by increasing PIC formation. Activator-mediated stimulation of PIC assembly is thought to result from a direct interaction between the activator and one or more components of the transcription machinery, termed the 'target'. The unambiguous identification of direct, physiologically relevant in vivo targets of activators has been a considerable challenge in the transcription field. The major obstacle has been the lack appropriate experimental methods to measure direct interactions with activators in vivo. The development of spectral variants of green fluorescent protein has made it possible to perform FRET (fluorescence resonance energy transfer) analysis in living cells, thereby allowing the detection of direct protein-protein interactions in vivo. Here we discuss how FRET can be used to identify activator targets and to dissect in vivo mechanisms of transcriptional activation.

The Est1 subunit of Saccharomyces cerevisiae telomerase makes multiple contributions to telomere length maintenance.

The telomerase-associated Est1 protein of Saccharomyces cerevisiae mediates enzyme access by bridging the interaction between the catalytic core of telomerase and the telomere-binding protein Cdc13. In addition to recruiting telomerase, Est1 may act as a positive regulator of telomerase once the enzyme has been brought to the telomere, as previously suggested by the inability of a Cdc13-Est2 fusion protein to promote extensive telomere elongation in an est1-Delta strain. We report here three classes of mutant Est1 proteins that retain association with the telomerase enzyme but confer different in vivo consequences. Class 1 mutants display a telomere replication defect but are capable of promoting extensive telomere elongation in the presence of a Cdc13-Est2 fusion protein, consistent with a defect in telomerase recruitment. Class 2 mutants fail to elongate telomeres even in the presence of the Cdc13-Est2 fusion, which is the phenotype predicted for a defect in the proposed second regulatory function of EST1. A third class of mutants impairs an activity of Est1 that is potentially required for the Ku-mediated pathway of telomere length maintenance. The isolation of mutations that perturb separate functions of Est1 demonstrates that a telomerase holoenzyme subunit can contribute multiple regulatory roles to telomere length maintenance.