Regulation of death receptor expression and TRAIL/Apo2L-induced apoptosis by NF-kappaB.

TRAIL (tumour-necrosis factor-related apoptosis ligand or Apo2L) triggers apoptosis through engagement of the death receptors TRAIL-R1 (also known as DR4) and TRAIL-R2 (DR5). Here we show that the c-Rel subunit of the transcription factor NF-kappaB induces expression of TRAIL-R1 and TRAIL-R2; conversely, a transdominant mutant of the inhibitory protein IkappaBalpha or a transactivation-deficient mutant of c-Rel reduces expression of either death receptor. Whereas NF-kappaB promotes death receptor expression, cytokine-mediated activation of the RelA subunit of NF-kappaB also increases expression of the apoptosis inhibitor, Bcl-xL, and protects cells from TRAIL. Inhibition of NF-kappaB by blocking activation of the IkappaB kinase complex reduces Bcl-x L expression and sensitizes tumour cells to TRAIL-induced apoptosis. The ability to induce death receptors or Bcl-xL may explain the dual roles of NF-kappaB as a mediator or inhibitor of cell death during immune and stress responses.

p53-mediated repression of nuclear factor-kappaB RelA via the transcriptional integrator p300.

The p53 tumor suppressor gene plays an instrumental role in transcriptional regulation of target genes involved in cellular stress responses. p53-dependent transactivation and transrepression require its interaction with p300/CBP, a coactivator that also interacts with the RelA subunit of nuclear factor-kappaB. We find that p53 inhibits RelA-dependent transactivation without altering RelA expression or inducible kappaB-DNA binding. p53-mediated repression of RelA is relieved by p300 overexpression and the increased RelA activity conferred by p53-deficiency is counteracted by either transactivation domain-deficient p300 fragments that bind RelA or a transdominant mutant of IkappaB alpha. Our results suggest that p53 can regulate diverse kappaB-dependent cellular responses.

Selective radiosensitization of p53-deficient cells by caffeine-mediated activation of p34cdc2 kinase.

The induction of tumor cell death by anticancer therapy results from a genetic program of autonomous cell death termed apoptosis. Because the p53 tumor suppressor gene is a critical component for induction of apoptosis in response to DNA damage, its inactivation in cancers may be responsible for their resistance to genotoxic anticancer agents. The cellular response to DNA damage involves a cell-cycle arrest at both the G1/S and G2/M transitions; these checkpoints maintain viability by preventing the replication or segregation of damaged DNA. The arrest at the G1 checkpoint is mediated by p53-dependent induction of p21WAF1/CIP1, whereas the G2 arrest involves inactivation of p34cdc2 kinase. Following DNA damage, p53-deficient cells fail to arrest at G1 and accumulate at the G2/M transition. We demonstrate that abrogation of G2 arrest by caffeine-mediated activation of p34cdc2 kinase results in the selective sensitization of p53-deficient primary and tumor cells to irradiation-induced apoptosis. These data suggest that pharmacologic activation of p34cdc2 kinase may be a useful therapeutic strategy for circumventing the resistance of p53-deficient cancers to genotoxic anticancer agents.

Multiple head and neck tumors: evidence for a common clonal origin.

Patients with head and neck cancers have a high (2-3%/year) incidence of second primary lesions. Clinically, these new lesions are identified either simultaneously with the primary lesion (synchronous) or after a period of time (metachronous). This observation has been attributed to the concept of "field carcinogenesis," which is based on the hypothesis that prolonged exposure to carcinogens leads to the independent transformation of multiple epithelial cells at several sites. An alternative theory is based on the premise that any transforming event is rare; following initial transformation, the progeny of the transformed clone spread through the mucosa and give rise to geographically distinct but genetically related tumors. We analyzed the pattern of X-chromosome inactivation in multiple primary tumors from eight female patients with head and neck cancer. In addition, we used microsatellite analysis to examine the pattern of allelic loss on chromosomes 9p and 3p, identified as early events in the progression of head and neck malignancies. In four of four cases, multiple tumors demonstrated the same pattern of X-chromosome inactivation. In the remaining four cases, X-chromosome deletions prevented interpretation (n = 3), or the androgen receptor locus was noninformative (n = 1). In three of nine patients, multiple tumors displayed the same pattern of loss of heterozygosity, two with identical breakpoints on chromosome 9p. In one patient, there was an identical microsatellite alteration at a 3p locus, definitive evidence that these tumors arose from the same clone. Our findings suggest that in at least a proportion of patients with head and neck cancers, multiple primary tumors arise from a single clone.

BCR-ABL-mediated inhibition of apoptosis with delay of G2/M transition after DNA damage: a mechanism of resistance to multiple anticancer agents.

A critical determinant of the efficacy of antineoplastic therapy is the response of malignant cells to DNA damage induced by anticancer agents. The p53 tumor-suppressor gene is a critical component of two distinct cellular responses to DNA damage, the induction of a reversible arrest at the G1/S cell cycle checkpoint, and the activation of apoptosis, a genetic program of autonomous cell death. Expression of the BCR-ABL chimeric gene produced by a balanced translocation in chronic myeloid leukemia, confers resistance to multiple genotoxic anticancer agents. BCR-ABL expression inhibits the apoptotic response to DNA damage without altering either the p53-dependent WAF1/CIP1-mediated G1 arrest or DNA repair. BCR-ABL-mediated inhibition of DNA damage-induced apoptosis is associated with a prolongation of cell cycle arrest at the G2/M restriction point; the delay of G2/M transition may allow time to repair and complete DNA replication and chromosomal segregation, thereby preventing a mitotic catastrophe. The inherent resistance of human cancers to genotoxic agents may result not only by the loss or inactivation of the wild-type p53 gene, but also by genetic alterations such as BCR-ABL that can delay G2/M transition after DNA damage.

Inhibition of apoptosis during development of colorectal cancer.

Colorectal tumorigenesis proceeds through an accumulation of specific genetic alterations. Studies of the mechanism by which these genetic changes effect malignant transformation have focused on the deregulation of cell proliferation. However, colorectal epithelial homeostasis is dependent not only on the rate of cell production but also on apoptosis, a genetically programmed process of autonomous cell death. We investigated whether colorectal tumorigenesis involved an altered susceptibility to apoptosis by examining colorectal epithelium from normal mucosa, adenomas from familial adenomatous polyposis, sporadic adenomas, and carcinomas. The transformation of colorectal epithelium to carcinomas was associated with a progressive inhibition of apoptosis. The inhibition of apoptosis in colorectal cancers may contribute to tumor growth, promote neoplastic progression, and confer resistance to cytotoxic anticancer agents.

Microsatellite instability in primary neoplasms from HIV + patients.

AIDS is associated with a high risk of certain malignancies, notably Kaposi's sarcoma (KS) and B-cell non-Hodgkin's lymphoma (NHL). The pathogenesis of these malignancies is not fully understood. One mechanism of malignant transformation recently described in colon tumorigenesis results from defects in DNA mismatch repair, manifest as widespread microsatellite instability. We demonstrate a high rate of microsatellite instability in KS and aggressive lymphomas obtained from HIV-infected patients, whereas there is no evidence of instability in similar lesions from HIV-negative patients. Further elucidation of the underlying mechanisms responsible for HIV-associated instability in primary tumours may provide insight into the pathogenesis of these AIDS-related neoplasms.

Investigation of modality-specific distractibility in children.

Seventy-three nonreferred children were administered a battery of cognitive, academic and behavioral measures along with distinct tests of aural and visual distractibility. The data suggest that distractibility is modality specific. Completely distinct groups of visually- and aurally-distractible children were found. In addition, visual distractibility was associated with both a continuous performance test (CPT) measure of inattention and teacher ratings of behavior, but not with cognitive nor academic achievement measures. In contrast, aural distractibility was associated with cognitive functioning and reading scores, but not with the CPT inattention measure nor teacher behavior ratings.