Calcium-dependent regulation of NEMO nuclear export in response to genotoxic stimuli.
The mechanisms involved in activation of the transcription factor NF-kappaB by genotoxic agents are not well understood. Previously, we provided evidence that a regulatory subunit of the IkappaB kinase (IKK) complex, NF-kappaB essential modulator (NEMO)/IKKgamma, is a component of a nuclear signal that is generated after DNA damage to mediate NF-kappaB activation. Here, we found that etoposide (VP16) and camptothecin induced increases in intracellular free calcium levels at 60 min after stimulation of CEM T leukemic cells. Inhibition of calcium increases by calcium chelators, BAPTA-AM and EGTA-AM, abrogated NF-kappaB activation by these agents in several cell types examined. Conversely, thapsigargin and ionomycin attenuated the BAPTA-AM effects and promoted NF-kappaB activation by the genotoxic stimuli. Analyses of nuclear NEMO levels in VP16-treated cells suggested that calcium was required for nuclear export of NEMO. Inhibition of the nuclear exporter CRM1 by leptomycin B did not interfere with NEMO nuclear export. Similarly, deficiency of a plausible calcium-dependent nuclear export receptor, calreticulin, failed to prevent NF-kappaB activation by VP16. However, temperature inactivation of the Ran guanine nucleotide exchange factor RCC1 in the tsBN2 cell line harboring a temperature-sensitive mutant of RCC1 blocked NF-kappaB activation induced by genotoxic stimuli. Overexpression of Ran in this cell model showed that DNA damage stimuli induced formation of a complex between Ran and NEMO, suggesting that RCC1 regulated NF-kappaB activation through the modulation of RanGTP. Indeed, evidence for VP16-inducible interaction between Ran-GTP and NEMO could be obtained by means of glutathione S-transferase (GST) pull-down assays using GST fused to the Ran binding domain of RanBP2, which specifically interacts with the GTP-bound form of Ran. BAPTA-AM did not alter these interactions, suggesting that calcium is a necessary step beyond the formation of a Ran-GTP-NEMO complex in the nucleus. These results suggest that calcium has a unique role in genotoxic stress-induced NF-kappaB signaling by regulating nuclear export of NEMO subsequent to the formation of a nuclear export complex composed of Ran-GTP, NEMO, and presumably, an undefined nuclear export receptor.
Berchtold
Craig M
CM
Department of Pharmacology, 301 SMI, 1300 University Avenue, University of Wisconsin, Madison, WI 53706, USA.
Wu
Zhao-Hui
ZH
Huang
Tony T
TT
Miyamoto
Shigeki
S
D021581:Active Transport, Cell Nucleus; D000818:Animals; D020013:Calcium Signaling; D002166:Camptothecin / Q000633:toxicity; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism*; D002614:Chelating Agents / Q000494:pharmacology; D004249:DNA Damage; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D004789:Enzyme Activation; D005047:Etoposide / Q000633:toxicity; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism; D006801:Humans; D051550:I-kappa B Kinase / Q000378:metabolism*; D015759:Ionomycin / Q000494:pharmacology; D007476:Ionophores / Q000494:pharmacology; D016328:NF-kappa B / Q000037:antagonists & inhibitors / Q000502:physiology*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D019284:Thapsigargin / Q000494:pharmacology
D002614:Chelating Agents; D020662:Guanine Nucleotide Exchange Factors; D007476:Ionophores; D016328:NF-kappa B; C070379:1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester; D004533:Egtazic Acid; D015759:Ionomycin; D019284:Thapsigargin; D005047:Etoposide; C046770:EGTA acetoxymethyl ester; D051550:I-kappa B Kinase; D002166:Camptothecin