In HeLa cells, TNFa-induced apoptosis can be blocked in a NF-kB-dependent manner

be a requirement specific for SDHD, as tumor development in SDHB, SDHC and SDHA MedChemExpress MEK162 appear to follow a two-hit kinetics. To date, the nature of this hypothetical third hit is completely unknown. In the light of our results, and given the implication of p21WAF1/Cip1 in the cell cycle, proliferation, and senescence, we propose that the SdhD-mutation-induced p21WAF1/Cip1 up-regulation could represent a checkpoint mechanism activated upon MCII failure. Thus, any subsequent molecular event that causes the cell division machinery to by-pass this checkpoint would drive the cells to replicative catastrophe, accumulating mutations and p21WAF1/Cip1 Overexpression in a SdhD Mouse Mutant eventually resulting in tumor transformation. Our findings, therefore, open new avenues 10224109 for the 20615965 identification of candidate genes involved in the cell cycle and growth, genome integrity, and other processes, whose mutations could result in the oncogenic transformation of Sdh-deficient tissues. Previous reports in humans, based on gene expression profiling and unsupervised hierarchical clustering, demonstrate a tight association between pheochromocytomas with VHL and SDH mutations, which distinguishes them from pheochromocytomas with MEN2, RET, and NF1 mutations. Among the genes determining these clustering phenomena, the p21 encoding Cdkn1a gene was found in the study by Dahia et al, whereas the closely related Cdkn1c, encoding the cyclin-dependent kinase inhibitor 1C, was found differentially expressed in a study by Lopez-Jimenez et al. In addition, in a work by Merlo et al where paraganglioma tissue was compared with normal paraganglia, Cdkn1a was present among a total of 1296 differentially expressed genes. Together, these data support a role of p21 in Sdh-related tumorigenesis in humans. The fact that a third hit event does not take place in our SDHDESR mouse model could be attributable to many circumstances. It could be that the shorter life-span resulting from administration of the minimal effective dose of tamoxifen, i. e., the minimal amount tested to cause reliable deletion of the SdhD gene, does not allow subsequent tumorigenic events to occur. Nevertheless, a tissuespecific mutant with a longer life-span than that of the SDHDESR model does not show any tendency to form tumors. Therefore, other genetic factors, such as gene redundancy, different chromosomal arrangement, or different susceptibility to the lack of one SdhD allele, could account for these differences between humans and mice. An alternative role for p21WAF1/Cip1 as an onco-protein in Sdhdeficient tissues could also be discussed. Although the well described role of p21WAF1/Cip1 as a tumor suppressor opposes a mechanism in which its induction is associated with tumorigenesis , it has been demonstrated that under certain conditions, p21WAF1/Cip1 can promote cellular proliferation and oncogenicity. Indeed, its overexpression or cytoplasmic localization correlates with poor prognosis in certain malignant tumors. The transcriptional activation of the Cdkn1a gene seems to be independent of p53 at least in the SDHD-ESR derived MEFs and BMK cells, as these cells were immortalized by stable expression of a p53-dominant negative protein or SV40 large T antigen, which eventually also inactivates p53. Although we do not rule out that in mouse tissues p53 plays a role in SdhDmutation-induced p21WAF1/Cip1 induction, such p53-independent expression of p21WAF1/Cip1 has been reported in other situations. It is al be a requirement specific for SDHD, as tumor development in SDHB, SDHC and SDHA appear to follow a two-hit kinetics. To date, the nature of this hypothetical third hit is completely unknown. In the light of our results, and given the implication of p21WAF1/Cip1 in the cell cycle, proliferation, and senescence, we propose that the SdhD-mutation-induced p21WAF1/Cip1 up-regulation could represent a checkpoint mechanism activated upon MCII failure. Thus, any subsequent molecular event that causes the cell division machinery to by-pass this checkpoint would drive the cells to replicative catastrophe, accumulating mutations and p21WAF1/Cip1 Overexpression in a SdhD Mouse Mutant eventually resulting in tumor transformation. Our findings, therefore, open new avenues for the identification of candidate genes involved in the cell cycle and growth, genome integrity, and other processes, whose mutations could result in the oncogenic transformation of Sdh-deficient tissues. Previous reports in humans, based on gene expression profiling and unsupervised hierarchical clustering, demonstrate a tight association between pheochromocytomas with VHL and SDH mutations, which distinguishes them from pheochromocytomas with MEN2, RET, and NF1 mutations. Among the genes determining these clustering phenomena, the p21 encoding Cdkn1a gene was found in the study by Dahia et al, whereas the closely related Cdkn1c, encoding the cyclin-dependent kinase inhibitor 1C, was found differentially expressed in a study by Lopez-Jimenez et al. In addition, in a work by Merlo et al where paraganglioma tissue was compared with normal paraganglia, Cdkn1a was present among a total of 1296 differentially expressed genes. Together, these data support a role of p21 in Sdh-related tumorigenesis in humans. The fact that a third hit event does not take place in our SDHDESR mouse model could be attributable to many circumstances. It could be that the shorter life-span resulting from administration of the minimal effective dose of tamoxifen, i. e., the minimal amount tested to cause reliable deletion of the SdhD gene, does not allow subsequent tumorigenic events to occur. Nevertheless, a tissuespecific mutant with a longer life-span than that of the SDHDESR model does not show any tendency to form tumors. Therefore, other genetic factors, such as gene 12969760 redundancy, different chromosomal arrangement, or different susceptibility to the lack of one SdhD allele, could account for these differences between humans and mice. An alternative role for p21WAF1/Cip1 as an onco-protein in Sdhdeficient tissues could also be discussed. Although the well described role of p21WAF1/Cip1 as a tumor suppressor opposes a mechanism in which its induction is associated with tumorigenesis , it has been demonstrated that under certain conditions, p21WAF1/Cip1 can promote cellular proliferation and oncogenicity. Indeed, its overexpression or cytoplasmic localization correlates with poor prognosis in certain malignant tumors. The transcriptional activation of the Cdkn1a gene seems to be independent of p53 at least in the SDHD-ESR derived MEFs and BMK cells, as these cells were immortalized by stable expression of 18753409 a p53-dominant negative protein or SV40 large T antigen, which eventually also inactivates p53. Although we do not rule out that in mouse tissues p53 plays a role in SdhDmutation-induced p21WAF1/Cip1 induction, such p53-independent expression of p21WAF1/Cip1 has been reported in other situations. It is al