The c-RET proto-oncogene encodes a receptor-type
tyrosine kinase, and its mutations in the germ line are responsible for the inheritance of
multiple endocrine neoplasia type 2A (
MEN2A) and 2B (
MEN2B). Ret
kinases are constitutively activated as a result of
MEN2A mutations (Ret-MEN2A) or
MEN2B mutations (Ret-MEN2B). Here we demonstrate that UV light (UV) irradiation induces superactivation of the constitutively activated Ret-MEN2A and Ret-MEN2B as well as activation of c-Ret. Before UV irradiation, small percentages of c-Ret (3-4%) and Ret-MEN2B (1-2%) and large percentages of Ret-MEN2A (30-40%) were dimerized through
disulfide bonds. These dimerized Ret
proteins were preferentially autophosphorylated, suggesting a close relation between up-regulated
kinase activity and
disulfide bond-mediated dimerization of Ret
proteins. We found that UV irradiation promotes the
disulfide bond-mediated dimerization of the Ret
proteins, in close association with activation and superactivation of Ret
kinases. UV irradiation also induced dimerization and activation of the extracellular domain-deleted mutant Ret (Ret-PTC-1). Interestingly, the levels of basic
kinase activity and dimerization of Ret-PTC-1-C376A, in which
cysteine 376 in the
tyrosine kinase domain of Ret-PTC-1 was replaced by
alanine, were low and were not increased by UV irradiation. These results suggest that Ret-PTC-1
cysteine 376 is one of possibly multiple critical target
amino acids of UV for Ret
kinase activation. Overexpression of
Cu/Zn superoxide dismutase in cells as a result of gene transfection prevented both the UV-mediated promotion of dimerization and the superactivation of Ret-MEN2A
kinase. These results suggest that the UV-induced
free radicals in cells attack intracellular domains of Ret to dimerize the
kinase proteins for superactivation.