Hypoxic Activation of Nuclear Factor-kB Is Mediated by a Ras and Raf Signaling Pathway and Does Not Involve MAP Kinase (ERK1 or ERK2)

We have previously shown that hypoxia causes the activation of nuclear factor-kB (NF-kB), and the phosphorylation of its inhibitory subunit, licita, on tyrosine residues. With the use of dominant negative mutants of Ha-laas and Raf-1, we investigated some of the early signaling events leading to the activation of NF-kB by hypoxia. Both dominant negative alleles of Ha -Ras and Raf-1 inhibited NF-kB induction by hypoxia, suggesting that the hypoxia-induced pathway of NF-kB induction is dependent on Ras and Raf-1 kinase activity. Furthermore, although conditions of low oxygen can also activate mitogen-activated protein kinases (ERK1 and ERK2), these kinases do not appear to be involved in regulating NF-kB by low oxygen conditions, as dominant negative mutants of mitogen-activated protein kinase do not inhibit NF-kB activation by hypoxia. Since Ras and Raf-1 have been previously shown to work downstream from membrane-associated tyrosine kinases such as Src, we determined if the Src membrane-associated kinase was also activated by low oxygen conditions. We detected an increase in Src proto-oncogene activity within 15-30 min of cellular exposure to hypoxia. We postulate that Src activation by hypoxia may be one of the earliest events that precedes Ras activation in the signaling cascade which ultimately leads to the phosphorylation and dissociation of the inhibitory subunit of NF-kB, IkBα.