Protein kinase Cα activation by RET: evidence for a negative feedback mechanism controlling RET tyrosine kinase

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Abstract
We have studied the role of protein kinase C (PKC) in signaling of the RET tyrosine kinase receptor. By using a chimeric receptor (E/R) in which RET kinase can be tightly controlled by the addition of epidermal growth factor (EGF), we have found that RET triggering induces a strong increase of PKCα, PKCδ and PKCζ activity and that PKCα, not PKCδ and PKCζ, forms a ligand-dependent protein complex with E/R. We have identified tyrosine 1062 in the RET carboxyl-terminal tail as the docking site for PKCα. Block of PKC activity by bisindolylmaleimide or chronic phorbol esters treatment decreased EGF-induced serine/threonine phosphorylation of E/R, while it caused a similarly sized increase of EGF-induced E/R tyrosine kinase activity and mitogenic signaling. Conversely, acute phorbol esters treatment, which promotes PKC activity, increased the levels of E/R serine/threonine phosphorylation and significantly decreased its phosphotyrosine content. A threefold reduction of tyrosine phosphorylation levels of the constitutively active RET/MEN2A oncoprotein was observed upon coexpression with PKCα. We conclude that RET binds to and activates PKCα. PKCα, in turn, causes RET phosphorylation and downregulates RET tyrosine kinase and downstream signaling, thus functioning as a negative feedback loop to modulate RET activity.

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