Down-regulation of NF-κB activity and NF-κB p65 subunit expression by ras and polyoma middle T oncogenes in human colonic Caco-2 cells

Abstract

The products of ras and src proto-oncogenes are frequently activated in a constitutive state in human colorectal cancer. In this study we attempted to establish whether the tumorigenic progression induced by oncogenic activation of p21^ras or pp60^c-src in human colonic cells is associated with alterations of the activity and expression of nuclear factor κB (NF-κB), a transcription factor suspected to participate in the development of cancer. To this end, we used Caco-2 cells made highly tumorigenic by transfection with an activated Val-12 human Ha-ras gene or with the polyoma middle T (PyMT) oncogene, a constitutive activator of pp60^c-src tyrosine kinase activity. Compared with control vector-transfected Caco-2 cells, both oncogene-transfected cell lines exhibited: (i) decreased constitutive NF-κB DNA-binding activity and NF-κB-mediated reporter gene expression, without alteration of their response to TNF-α for activation of these parameters; (ii) reduced NF-κB cytosolic stores along with a decreased p65 expression due, at least in part, to destabilization of p65 mRNA; (iii) a decrease in adhesion to extracellular matrix component-coated substrata which was partially corrected when stimulating NF-κB transcriptional activity with TNF-α. These results indicate that the tumorigenic progression induced by oncogenic p21^ras or PyMT/pp60^c-src in human colonic Caco-2 cells is associated with a down-regulation of p65 expression and NF-κB activity which could be responsible for the reduced adhesive properties of these cells after oncogene transfection.