Requirement of an IκB-β COOH terminal region protein for acidic-adaptation in CHO cells

We previously reported that an IκB-β COOH terminal region protein (designated CTIB) was essential for the proliferation of CHO cells under acidic stress (Lao et al., 2005. J Cell Physiol 203(1):186-192). In order to investigate the mechanisms underlying the requirement of CTIB for acidic adaptation, CTIB was silenced with an RNAi technique in CHO cells. CTIB silencing resulted in those cells completely failing to proliferate and maintain intracellular pH (pHi) homeostasis at an extracellular pH (pHe) of 6.3. An increased activation of p38 MAP kinase was induced by CTIB silencing at the low pH value. CTIB was only present in the cytoplasm and co-immunoprecipitation of the cytoplasmic fraction revealed that the loss of CTIB led to a loss of p65 in the immunoprecipitate complex. CTIB silencing reduced both the decrease in p65 and the increase in p50 in the nucleus when the cells were incubated at pHe 6.3. In cells with CTIB silenced, the transcriptions of p65, p105, and IL1-β were suppressed, and decreases in both the transcription and activity of MnSOD were observed at pHe 6.3. Suppression of these genes suggested a suppressed NF-κB activity since p105, IL1-β, and MnSOD were target genes of NF-κB. Our data demonstrated that CTIB functioned to prevent the overaccumulation of p65 in the nucleus, ensuring the appropriate composition of the NF-κB complex in the nucleus to respond to stimuli under acidic conditions.