Radioprotectors are compounds that protect against radiation injury when given prior to radiation exposure. Mitigators can protect against radiation injury when given after exposure but before ...symptoms appear. Radioprotectors and mitigators can potentially improve the outcomes of radiotherapy for cancer treatment by allowing higher doses of radiation and/or reduced damage to normal tissues. Such compounds can also potentially counteract the effects of accidental exposure to radiation or deliberate exposure (e.g., nuclear reactor meltdown, dirty bomb, or nuclear bomb explosion); hence they are called radiation countermeasures. Here, we will review the general principles of radiation injury and protection and describe selected examples of radioprotectors/mitigators ranging from small-molecules to proteins to cell-based treatments. We will emphasize agents that are in more advanced stages of development.
The breast cancer susceptibility genes BRCA1 and BRCA2 are classic tumor suppressor genes that exhibit an autosomal dominant pattern of inheritance with high penetrance. BRCA carriers inherit one ...mutant BRCA allele and one wild-type allele; and the wild-type allele is invariably deleted or mutated within the tumor. These genes function as caretakers in the maintenance of genomic stability, in part, by participating in homology-directed DNA repair (HDR), an error- free mechanism for the repair of double-strand breaks (DSBs). PARP1 (poly (ADP-ribose) polymerase 1) is an enzyme that functions in the base excision repair (BER) pathway, where its ability to post-translationally modify histones and DNA damage response proteins is required for repair of single-strand breaks (SSBs). In 2005, it was observed that knockdown of PARP1 or treatment with a small molecule PARP inhibitor was far more toxic to cells with BRCA1 or BRCA2 mutations than BRCA1/2-competent cells. This observation is an example of "synthetic lethality", a concept whereby two gene mutations combine to cause cell death, when neither mutation alone is lethal. These results spawned the idea to use PARP inhibitors to treat BRCA1/2 mutant cancers. Here, we will review the basic science underlying the discoveries described above, the preclinical research, and the clinical trials designed to exploit the sensitivity of BRCA1/2 mutant tumor cells to PARP inhibitors. We will also describe problems associated with the use of these agents, including development and mechanisms of drug resistance; and we will provide a forward look at new agents and strategies currently under development.
DIM (3,3′-diindolylmethane), a small molecule compound, is a proposed cancer preventive agent that can be safely administered to humans in repeated doses. We report that administration of DIM in a ...multidose schedule protected rodents against lethal doses of total body irradiation up to 13 Gy, whether DIM dosing was initiated before or up to 24 h after radiation. Physiologic submicromolar concentrations of DIM protected cultured cells against radiation by a unique mechanism: DIM caused rapid activation of ataxia-telangiectasia mutated (ATM), a nuclear kinase that regulates responses to DNA damage (DDR) and oxidative stress. Subsequently, multiple ATM substrates were phosphorylated, suggesting that DIM induces an ATM-dependent DDR-like response, and DIM enhanced radiation-induced ATM signaling and NF-κB activation. DIM also caused activation of ATM in rodent tissues. Activation of ATM by DIM may be due, in part, to inhibition of protein phosphatase 2A, an upstream regulator of ATM. In contrast, DIM did not protect human breast cancer xenograft tumors against radiation under the conditions tested. In tumors, ATM was constitutively phosphorylated and was not further stimulated by radiation and/or DIM. Our findings suggest that DIM is a potent radioprotector and mitigator that functions by stimulating an ATM-driven DDR-like response and NF-κB survival signaling.
Mutations of the breast and ovarian cancer susceptibility gene 1 (BRCA1) account for about 40-45% of hereditary breast cancer cases. Moreover, a significant fraction of sporadic (non-hereditary) ...breast and ovarian cancers exhibit reduced or absent expression of the BRCA1 protein, suggesting an additional role for BRCA1 in sporadic cancers. BRCA1 follows the classic pattern of a highly penetrant Knudsen-type tumor suppressor gene in which one allele is inactivated through a germ-line mutation and the other is mutated or deleted within the tumor. BRCA1 is a multi-functional protein but it is not fully understood which function(s) is (are) most important for tumor suppression, nor is it clear why BRCA1-mutations confer a high risk for breast and ovarian cancers and not a broad spectrum of tumor types. Here, we will review BRCA1 functions in the DNA damage response (DDR), which are likely to contribute to tumor suppression. In the process, we will highlight some of the controversies and unresolved issues in the field. We will also describe a recently identified and under-investigated role for BRCA1 in the regulation of telomeres and the implications of this role in the DDR and cancer suppression.
Inactivation of the breast cancer susceptibility gene BRCA1 plays a significant role in the development of a subset of breast cancers, although the major tumor suppressor function of this gene ...remains unclear. Previously, we showed that BRCA1 induces antioxidant-response gene expression and protects cells against oxidative stress. We now report that BRCA1 stimulates the base excision repair pathway, a major mechanism for the repair of oxidized DNA, by stimulating the activity of key base excision repair (BER) enzymes, including 8-oxoguanine DNA glycosylase (OGG1), the DNA glycosylase NTH1, and the apurinic endonuclease redox factor 1/apurinic endonuclease 1 (REF1/APE1), in human breast carcinoma cells. The increase in BER enzyme activity appears to be due, primarily, to an increase in enzyme expression. The ability of BRCA1 to stimulate the expression of the three BER enzymes and to enhance NTH1 promoter activity was dependent upon the octamer-binding transcription factor OCT1. Finally, we found that OGG1, NTH1, and REF1/APE1 each contribute to the BRCA1 protection against oxidative stress due to hydrogen peroxide and that hydrogen peroxide stimulates the expression of BRCA1 and the three BER enzymes. These findings identify a novel mechanism through which BRCA1 may regulate the repair of oxidative DNA damage.
Previous studies have shown that the breast cancer suppressor BRCA1 stimulates antioxidant gene expression and protects cells against oxidative stress. To further examine this important function, we ...tested whether BRCA1 could modulate intracellular levels of reactive oxygen species (ROS). Wild-type BRCA1 (but not a cancer-associated mutant) significantly reduced ROS levels, determined by DCF fluorescence assays by flow cytometry and confocal microscopy. The BRCA1 and REF1 pathways for reduction of ROS levels appear to exhibit cross-talk. BRCA1 also reduced the levels of protein nitration and H
2O
2-induced oxidative damage to DNA. Thus, BRCA1 may protect cellular macromolecules by reducing intracellular ROS levels.
Studies have shown that x-rays delivered as arrays of parallel microplanar beams (microbeams), 25- to 90-μm thick and spaced 100-300 μm on-center, respectively, spare normal tissues including the ...central nervous system (CNS) and preferentially damage tumors. However, such thin microbeams can only be produced by synchrotron sources and have other practical limitations to clinical implementation. To approach this problem, we first studied CNS tolerance to much thicker beams. Three of four rats whose spinal cords were exposed transaxially to four 400-Gy, 0.68-mm microbeams, spaced 4 mm, and all four rats irradiated to their brains with large, 170-Gy arrays of such beams spaced 1.36 mm, all observed for 7 months, showed no paralysis or behavioral changes. We then used an interlacing geometry in which two such arrays at a 90° angle produced the equivalent of a contiguous beam in the target volume only. By using this approach, we produced 90-, 120-, and 150-Gy 3.4 x 3.4 x 3.4 mm³ exposures in the rat brain. MRIs performed 6 months later revealed focal damage within the target volume at the 120- and 150-Gy doses but no apparent damage elsewhere at 120 Gy. Monte Carlo calculations indicated a 30-μm dose falloff (80-20%) at the edge of the target, which is much less than the 2- to 5-mm value for conventional radiotherapy and radiosurgery. These findings strongly suggest potential application of interlaced microbeams to treat tumors or to ablate nontumorous abnormalities with minimal damage to surrounding normal tissue.
BRCA1 regulation of transcription Rosen, Eliot M.; Fan, Saijun; Ma, Yongxian
Cancer letters,
05/2006, Letnik:
236, Številka:
2
Journal Article
Recenzirano
BRCA1, a tumor suppressor gene on chromosome 17q21, was identified in 1994 based on its linkage to hereditary breast and ovarian cancer syndromes. The BRCA1 gene encodes a 220
kDa nuclear ...phosphoprotein. Studies aimed at elucidating the mechanisms of its tumor suppressor activity have revealed, in part, that BRCA1 participates in the DNA damage response and acts to maintain the integrity of the genome. This activity is generic and does not account for the propensity of
BRCA1 mutation carriers to develop specific tumor types rather than a broad spectrum of cancers. In addition to genome maintenance, BRCA1 has been found to broadly regulate gene transcription, even though it is not itself a sequence-specific DNA-binding transcription factor. The ability of BRCA1 to function as a coregulator of transcription may underlie some of its tumor suppressor activity and may explain the tissue-specific nature of this activity. This review will focus on how BRCA1 selectively regulates transcription and how this regulatory function may relate to tumor suppression.
Inherited mutations of the breast cancer susceptibility gene BRCA1 confer a high risk for breast cancer development. The 300RXKK and 266KXK motifs have been identified previously as sites for ...acetylation of the estrogen receptor-α (ER-α), and 302K was also found to be a site for BRCA1-mediated mono-ubiquitination of ER-α in vitro. Here we show that ER-α proteins with single or double lysine mutations of these motifs (including K303R, a cancer-associated mutant) are resistant to inhibition by BRCA1, even though the mutant ER-α proteins retain the ability to bind to BRCA1. We also found that BRCA1 overexpression reduced and knockdown increased the level of acetylated wild-type ER-α, without changing the total ER-α protein level. Increased acetylation of ER-α due to BRCA1 small interfering RNA was dependent upon phosphatidylinositol 3-kinase/Akt signaling and on up-regulation of the coactivator p300. In addition, using an in vitro acetylation assay, we found that in vitro-translated wild-type BRCA1 but not a cancer-associated point mutant (C61G) inhibited p300-mediated acetylation of ER-α. Furthermore, BRCA1 overexpression increased the levels of mono-ubiquitinated ER-α protein, and a BRCA1 mutant that is defective for ubiquitin ligase activity but retains other BRCA1 functions (I26A) did not ubiquitinate ER-α or repress its activity in vivo. Finally, ER-α proteins with mutations of the 300RXKK or 266KXK motifs showed modest or no BRCA1-induced ubiquitination. We propose a model in which BRCA1 represses ER-α activity, in part, by regulating the relative degree of acetylation vs. ubiquitination of ER-α.
In this report, we show that BRCA1 inhibits estrogen receptor activity, in part, by inhibiting its acetylation and increasing its ubiquitination.
The B-cell translocation gene 2, BTG2, a member of the BTG/TOB (B-cell translocation gene/transducers of ErbB2) gene family, has been implicated in cell cycle regulation, normal development, and ...possibly tumor suppression. Previously, it was shown that BTG2 expression is lost or down-regulated in human breast cancers. We now report that BTG2 protects human mammary epithelial cells from oxidative stress due to hydrogen peroxide and other oxidants. BTG2 protection against oxidative stress is BRCA1-independent but requires the antioxidant transcription factor NFE2L2 and is associated with up-regulation of the expression of antioxidant enzymes, including catalase and superoxide dismutases 1 and 2. BTG2 stimulation of antioxidant gene expression is also NFE2L2-dependent. We further demonstrate that BTG2 is a binding partner for NFE2L2 and increases its transcriptional activity. In addition, BTG2 is detectable at the antioxidant response element (ARE) of several NFE2L2-responsive genes. Finally, we show that the ability of BTG2 to associate with NFE2L2, to protect cells against oxidative stress, and to stimulate antioxidant gene expression requires box B, a short highly conserved amino acid motif characteristic of BTG2/TOB family proteins, but does not require boxes A or C. These findings suggest a novel role for BTG2 as a co-activator for NFE2L2 in up-regulating cellular antioxidant defenses.
Background: The putative tumor suppressor BTG2 is frequently down-regulated in human cancers.
Results: BTG2 protects cells against oxidative stress, stimulates the activity of the antioxidant transcription factor NFE2L2, and associates with NFE2L2 at the antioxidant response element (ARE).
Conclusion: BTG2 serves as a transcriptional coactivator for NFE2L2/ARE signaling.
Significance: These findings suggest a novel mechanism to explain BTG2 function as a tumor suppressor.
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