Induced telomerase activity in primary aortic endothelial cells by low-LET gamma-radiation is mediated through NF-kappaB activation.

Our objective was to understand the mechanism through which cells that initially survive irradiation could acquire survival advantage. In this study, we show evidence that low-linear energy transfer gamma-radiation can induce telomerase enzyme activity in primary aortic endothelial cells, and that an upstream regulator, nuclear factor kappa B (NF-kappaB), controls this activation. Telomeric repeat amplification protocol (TRAP) assay showed that cells exposed to a dose of 2 Gy induce telomerase activity. Subsequent analysis revealed that radiation-induced telomeric activity is regulated at the transcriptional level by triggering activation of the promoter of the telomerase catalytic subunit, telomerase reverse transcriptase (TERT). A mechanistic study revealed that NF-kappaB becomes functionally activated upon radiation exposure and mediates the upregulation of telomerase activity by binding to the kappaB-binding region in the promoter region of the TERT gene. More significantly, elimination of the NF-small ka, CyrillicB recognition site on the telomerase promoter or inhibition of NF-small ka, CyrillicB by ectopically expressing the inhibitor protein IkappaBalpha mutant (Ismall ka, CyrillicBalpha(S32A/S36A))) compromises radiation-induced telomerase promoter activation. Consistent with the notion that NF-kappaB mediates gamma-ray-induced telomerase responses, TRAP assay revealed that ectopically expressed IkappaBalpha(S32A/S36A)) also attenuated telomerase enzyme activity. These findings indicate that NF-kappaB activation following ionizing radiation exposure may elicit a survival advantage by upregulating and maintaining telomerase activity.

NF-kappaB activation plays an antiapoptotic role in human leukemic K562 cells exposed to ionizing radiation.

Exposure of cells to ionizing radiation (IR) determines cellular lesions, such as DNA and membrane damage, which involve a coordinate network of signal transduction pathways responsible for resistance to or delay of apoptosis, depending on cell type and administered dose. Since, after IR exposure, the apoptotic profile appeared different in the two chosen cell lines K562 and Jurkat along with caspase-3 activation, we paid attention to the influence exerted by Protein kinase C delta on transcription factor NF-kappaB activation. Interestingly, K562 resist to IR carrying out a survival strategy which includes PKC delta/NF-kappaB pathway activation, probably mediated by novel IKKs and a role for PI-3-kinase in activating PKC delta at Thr 505 by PDK-1 phosphorylation is suggested. In addition, since caspase-3 is not activated in these cells upon ionizing radiation exposure, it could be supposed that NF-kappaB antagonizes apoptosis induction interfering with pathways which lead to caspase activation, may be by inducing expression of IAP, caspases 3, 7, 9, inhibitor. Thus NF-kappaB activation explains the resistance displayed by K562 to IR and drug potential interference directed to this protein could overcome apoptosis resistance in clinical settings.

Post-translational modification of I-kappa B alpha activates NF-kappa B in human monocytes exposed to 56Fe ions.

The objective of this study was to investigate whether heavy ion (56Fe) radiation exposure activates one of the key transcriptional regulators, nuclear factor-kappa B (NF-kappa B), in normal human monocytes (Mono Mac 6 cells: MM6). The study revealed that the exposure of MM6 cells to 56Fe ions resulted in increased NF-kappa B DNA-binding activity. The activation was both dose- and time-dependent, with a maximum response at the 2 h time point after a 0.7 Gy dose. Cells pre-incubated with inhibitors of the phosphorylation and proteasome signaling pathway, completely blocked heavy ion-induced activation of NF-kappa B. These results clearly indicate that 56Fe ions can induce NF-kappa B DNA-binding activity in normal human monocytes, that the activation is rapid and persistent, and that the heavy ion-induced activation of NF-kappa B is mediated through phosphorylation of I-kappa B alpha and the subsequent proteasome-dependent degradation pathway. Since activation of NF-kappa B by extracellular stimuli is implicated in inflammation, infection and cancer induction, as well as in protection of cells against insult, it will be important in subsequent studies to elucidate whether heavy ion-induced NF-kappa B activation is involved in downstream gene expression.