Protective Roles of NF-κB for Chromium(VI)-induced Cytotoxicity Is Revealed by Expression of IκB Kinase-β Mutant

Abstract

A wide range of signals, many of which are thought to be related to cellular stress, induce expression of early response genes through the NF-B family of transcription factors (1-4). In resting cells, NF-B is retained in cytoplasm in its inactive form by interaction with one of a number of inhibitory molecules including IB␣, IB␤, IB⑀, p105, and p100. Activation of the NF-B signaling cascade results in a complete degradation of IB or carboxyl-terminal partial degradation of the p105 and p100 precursors, allowing nuclear translocation of the NF-B complexes. Activated NF-B binds to specific DNA sequences in target genes, designated as B elements, and regulates transcription of genes mediating inflammation, carcinogenesis, and pro-or antiapoptotic reactions. IB␣ is the most abundant inhibitory protein for NF-B (5). The mechanisms of signalinduced IB␣ degradation involve phosphorylation of two serine residues, Ser 32 and Ser 36 . This phosphorylation leads to polyubiquitination of two specific lysines on IB␣ (Lys 21 and Lys 22 ) by an SCF-␤-TrCP complex and its degradation by the 26 S proteasome (6). The phosphorylation is accomplished by a specific IB kinase (IKK) 1 complex containing two catalytic subunits, IKK␣ and IKK␤, and a structural component named NEMO/IKK␥/IKKAP (3, 5). IKK␣ and IKK␤ share 50% sequence homology. Both proteins contain an amino-terminal kinase domain, a carboxyl-terminal region with a leucine zipper, and a helix-loop-helix domain. In vitro and in vivo studies indicate that both IKK␣ and IKK␤ are capable of phosphorylating IB␣ on Ser 32 and Ser 36 , but IKK␤ is more potent in IB␣ phosphorylation induced by proinflammatory stimuli. Recent studies by several groups indicate the existence of an additional IKK-like kinase complex in T cells, named IKKi/⑀, which shares 27% homology with IKK␣ and IKK␤ and possibly mediates NF-B-activating kinase signaling and phorbol 12-myristate 13-acetate/protein kinase C⑀-induced Ser 36 phosphorylation of IB␣ and thus NF-B activation (7-11).

Resistance to apoptosis, increased growth potential, and altered gene expression in cells that survived genotoxic hexavalent chromium [Cr(VI)] exposure

“…Cr(VI) has been shown to both inhibit and activate the transcriptional activity of NF-(B [71,72], which may be related to differences in Cr concentration and cell type [73]. In the present study, Cr-induced upregulation of these oncogenes was observed in the normal BJ-hTERT cells, but abrogated in the B-5Cr cells.…”

Signaling from toxic metals to NF-kappaB and beyond: not just a matter of reactive oxygen species.

“…Such Cr(VI)-induced cell death could be partially inhibited by expression of exogenous cIAP1, an inhibitor of caspases, indicating noncaspase cytotoxic mechanisms may be involved in Cr(VI)-induced cell death. Indeed, combination of cIAP1 and the antioxidant N-acetylcysteine resulted in a significant inhibition of Cr(VI)-induced cell death of NF-κB-inhibited cells (53). These results suggest that NF-κB is essential for inhibiting ROS-dependent cytotoxicity.…”

“…Cr(VI) is able to activate NF-κB at lower concentrations (<50 µM) in T cells (51), macrophages (52), bronchial epithelial cells (53), and human breast cancer cells (29). The final concentration of Cr(VI) is critical for this metal to induce or inhibit NF-κB.…”

Cyclin D1 Induction through IκB Kinase β/Nuclear Factor-κB Pathway Is Responsible for Arsenite-Induced Increased Cell Cycle G1-S Phase Transition in Human Keratinocytes

“…To elucidate the signaling pathways leading to cyclin D1 induction by arsenite in human HaCat cells, we investigated the potential role of NF-nB in the cyclin D1 induction by arsenite, because it is believed that NF-nB transactivation is associated with arsenite-induced tumor promotion (47). NF-nB is bound to an inhibitory protein, InBa in an inactive form in most cells, and composed of NF-nB p50 and Rel A p65 subunits (48). In response to stress, cytokine, or growth factor, InBa is phosphorylated by IKKa/ h and then ubiquitinated and degraded by the proteasome, enabling nuclear translocation and binding of p50/p65 to the promoter region of target genes (49).…”