Mitochondrial stress-induced p53 attenuates HIF-1α activity by physical association and enhanced ubiquitination

Retrograde signaling is a mechanism by which mitochondrial dysfunction is communicated to the nucleus for inducing a metabolic shift essential for cell survival. Previously we showed that partial mtDNA depletion in different cell types induced mitochondrial retrograde signaling pathway (MtRS) involving Ca⁺² sensitive Calcineurin (Cn) activation as an immediate upstream event of stress response. In multiple cell types, this stress signaling was shown to induce tumorigenic phenotypes in immortalized cells. In this study we show that MtRS also induces p53 expression which was abrogated by Ca²⁺ chelators and shRNA mediated knock down of CnAβ mRNA. Mitochondrial dysfunction induced by mitochondrial ionophore, carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and other respiratory inhibitors, which perturb the transmembrane potential, were equally efficient in inducing the expression of p53 and downregulation of MDM2. Stress-induced p53 physically interacted with HIF-1α and attenuated the latter’s binding to promoter DNA motifs. Additionally, p53 promoted ubiquitination and degradation of HIF-1α in partial mtDNA depleted cells. The mtDNA depleted cells, with inhibited HIF-1α, showed upregulation of glycolytic pathway genes, glucose transporter 1–4 (Glut1–4), phosphoglycerate kinase 1 (PGK1) and Glucokinase (GSK) but not of prolyl hydroxylase (PHD) isoforms. For the first time we show that p53 is induced as part of MtRS and it renders HIF-1α inactive by physical interaction. In this respect our results show that MtRS induces tumor growth independent of HIF-1α pathway.

Evaluation of physician knowledge and referral practices for colorectal cancer (CRC) genetic risk assessment: The experience at the Hospital of University of Pennsylvania (HUP)

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Background: 3-5% of CRCs are due to inherited genetic mutations. We surveyed knowledge and practices of academic physicians for identification and care of individuals at risk for inherited CRC.

Methods: 264 physicians (oncologists (ON), gastroenterologists (GA), surgeons, internists, gynecologists, and radiation oncologists) at HUP were invited to participate in a web-based 9-min survey. The ability to obtain appropriate medical history and to make referral to genetic services was evaluated. Knowledge of hereditary CRC syndromes was examined both pre and post viewing of an educational web-page on inherited CRCs. Mantel-Haenszel, Fisher exact, and McNemar statistical tests were applied.

Results: Response rate was 33.3%; and of those, 97.4% accessed the educational webpage. In the cohort, 98.9 % obtained a medical history very frequently (VF), 88.6% obtained cancer history in 1st and 45.5% in the 2nd degree relatives VF, and 63.9% asked about the relatives' age at time of cancer diagnosis VF. Of those most likely to care for patients with CRC, the GA more frequently asked about relatives' age at cancer diagnosis (p=0.014) and family history of polyps (p< 0.001) than ON. GA were more likely than ON to refer patients for genetic counseling (73.9% vs. 36.8%, p=0.008). GA had superior knowledge of the availability of genetic testing for Lynch syndrome (LS) (95.6% GA vs. 63.2% ON, p=0.005) and for familial adenomatous polyposis (FAP) (100.0% GA vs. 65.8% ON, p<0.001). For the entire cohort, the educational intervention raised awareness of genetic testing for LS (64.5% pre vs. 94.7% post, p<0.001), FAP (69.7% pre vs. 97.4% post, p<0.001), and Peutz-Jeghers Syndrome (31.6% pre vs. 84.2% post, p<0.001); and it significantly improved recognition of LS family pedigrees and selection of appropriate surveillance.

Conclusions: Of the respondents, GA are more likely to obtain a detailed family history, utilize genetic services, and have a greater awareness of the availability of genetic testing, than ON. A simple educational intervention improves physician knowledge on inherited CRC risk recognition and surveillance recommendations.

No significant financial relationships to disclose.

Identification of mutations that disrupt phosphorylation-dependent nuclear export of cyclin D1

Although cyclin D1 is overexpressed in a significant number of human cancers, overexpression alone is insufficient to promote tumorigenesis. In vitro studies have revealed that inhibition of cyclin D1 nuclear export unmasks its neoplastic potential. Cyclin D1 nuclear export depends upon phosphorylation of a C-terminal residue, threonine 286, (Thr-286) which in turn promotes association with the nuclear exportin, CRM1. Mutation of Thr-286 to a non-phosphorylatable residue results in a constitutively nuclear cyclin D1 protein with significantly increased oncogenic potential. To determine whether cyclin D1 is subject to mutations that inhibit its nuclear export in human cancer, we have sequenced exon 5 of cyclin D1 in primary esophageal carcinoma samples and in cell lines derived from esophageal cancer. Our work reveals that cyclin D1 is subject to mutations in primary human cancer. The mutations identified specifically disrupt phosphorylation of cyclin D1 at Thr-286, thereby enforcing nuclear accumulation of cyclin D1. Through characterization of these mutants, we also define an acidic residue within the C-terminus of cyclin D1 that is necessary for recognition and phosphorylation of cyclin D1 by glycogen synthase kinase-3 beta. Finally, through construction of compound mutants, we demonstrate that cell transformation by the cancer-derived cyclin D1 alleles correlates with their ability to associate with and activate CDK4. Our data reveal that cyclin D1 is subject to mutations in primary human cancer that specifically disrupt phosphorylation-dependent nuclear export of cyclin D1 and suggest that such mutations contribute to the genesis and progression of neoplastic growth.

An essential E box in the promoter of the gene encoding the mRNA cap-binding protein (eukaryotic initiation factor 4E) is a target for activation by c-myc

The mRNA cap-binding protein (eukaryotic initiation factor 4E [eIF4E]) binds the m7 GpppN cap on mRNA, thereby initiating translation. eIF4E is essential and rate limiting for protein synthesis. Overexpression of eIF4E transforms cells, and mutations in eIF4E arrest cells in G, in cdc33 mutants. In this work, we identified the promoter region of the gene encoding eIF4E, because we previously identified eIF4E as a potential myc-regulated gene. In support of our previous data, a minimal, functional, 403-nucleotide promoter region of eIF4E was found to contain CACGTG E box repeats, and this core eIF4E promoter was myc responsive in cotransfections with c-myc. A direct role for myc in activating the eIF4E promoter was demonstrated by cotransfections with two dominant negative mutants of c-myc (MycdeltaTAD and MycdeltaBR) which equally suppressed promoter function. Furthermore, electrophoretic mobility shift assays demonstrated quantitative binding to the E box motifs that correlated with myc levels in the electrophoretic mobility shift assay extracts; supershift assays demonstrated max and USF binding to the same motif. cis mutations in the core or flank of the eIF4E E box simultaneously altered myc-max and USF binding and inactivated the promoter. Indeed, mutations of this E box inactivated the promoter in all cells tested, suggesting it is essential for expression of eIF4E. Furthermore, the GGCCACGTG(A/T)C(C/G) sequence is shared with other in vivo targets for c-myc, but unlike other targets, it is located in the immediate promoter region. Its critical function in the eIF4E promoter coupled with the known functional significance of eIF4E in growth regulation makes it a particularly interesting target for c-myc regulation.

An increased risk of Crohn's disease in individuals who inherit the HLA class II DRB3*0301 allele

The role of inflammatory T cells in Crohn's disease suggests that inherited variations in major histocompatibility complex (MHC) class II genes may be of pathogenetic importance in inflammatory bowel disease. The absence of consistent and strong associations with MHC class II genes in Caucasian patients with inflammatory bowel disease probably reflects the use of less precise typing approaches and the failure to type certain loci by any means. A PCR-sequence-specific oligonucleotide-based approach was used to type individual alleles of the HLA class II DRB1, DRB3, DRB4, and DRB5 loci in 40 patients with ulcerative colitis, 42 Crohn's disease patients, and 93 ethnically matched healthy controls. Detailed molecular typing of the above alleles has previously not been reported in patients with inflammatory bowel disease. A highly significant positive association with the HLA-DRB3*0301 allele was observed in patients with Crohn's disease (P = 0.0004) but not in patients with ulcerative colitis. The relative risk for this association was 7.04. Other less significant HLA class II associations were also noted in patients with Crohn's disease. One of these associations involved the HLA-DRB1*1302 allele, which is known to be in linkage disequilibrium with HLA-DRB3*0301. These data suggest that a single allele of an infrequently typed HLA class II locus is strongly associated with Crohn's disease and that MHC class II molecules may be important in its pathogenesis.