Nuclear factor-kappa B is constitutively active in C-cell carcinoma and required for RET-induced transformation

Specific paint mutations of the RET proto-oncogene have been demonstrated t o be responsible for multiple endocrine neoplasia (MEN) types 2A and 2B, fo r familial medullary thyroid carcinoma (MTC) syndromes, as well as for spor adic MTC, sere we show that nuclear factor (NF)-kappaB is activated in RET- associated C-cell carcinoma specimens. TT cells, a human MTC cell line expr essing MEN 2A type RET, display transcriptionally active RelA(p65) in the n ucleus. NF-kappaB activity in these cells is attributable to constitutive I kappaB kinase (IKK) activity and high turn over of I kappaB alpha. RET har boring the mutations C634R (MEN 2A) or M918T (MEN 2B), in contrast to wild- type RET, activates a NF-KB-dependent reporter construct upon transient tra nsfection in HeLa cells. We show that the prototype RET mutation C634R enha nces phosphorylation of I kappaB alpha by IKK beta but not by IKK alpha. RE T-induced NF-kappaB and IKK beta activity requires Ras function but does ne ither involve the classical mitogen-activated protein kinase kinase/extrace llular signal-regulated kinase nor the phosphoinositide 3-kinase/Akt pathwa ys. In contrast, RET-induced NF-kappaB activity is dependent on Raf and MEK K1, Inhibition of constitutive NF-kappaB activity results in cell death of TT cells and blocks focus formation induced by oncogenic forms of RET in NM 3T3 cells. These results suggest that RET-mediated carcinogenesis critical ly depends on IKK activity and subsequent NF-kappaB activation.