The regulatory domain of protein kinase C delta positively regulates insulin receptor signaling

Protein kinase C delta (PKC delta) is induced by insulin to rapidly associate with insulin receptor (IR) and upregulates insulin signaling. We utilized specific JM and CT receptor domains and chimeras of PKC alpha and PKC delta regulatory and catalytic domains to elucidate which components of PKC delta are responsible for positive regulatory effects of PKC delta on IR signaling. Studies were performed on L6 and L8 skeletal muscle myoblasts and myotubes. PKC delta was preferentially bound to the JM domain of IR, and insulin stimulation increased this binding. Both PKC delta/alpha and PKC alpha/delta chimeras (regulatory/catalytic) were bound preferentially to the JM but not to the CT domain of IR. Although IR-PKC delta binding was higher in cells expressing either the PKC delta/alpha or PKC alpha/delta chimera than in control cells, upregulation of IR signaling was observed only in PKC delta/alpha cells. Thus, in response to insulin increases in tyrosine phosphorylation of IR and insulin receptor substrate-1, downstream signaling to protein kinase B and glycogen synthase kinase 3 (GSK3) and glucose uptake were greater in cells overexpressing PKC delta/alpha and the PKC delta/delta domains than in cells expressing the PKC alpha/delta domains. Basal binding of Src to PKC delta was higher in both PKC delta/alpha- and PKC alpha/delta-expressing cells compared to control. Binding of Src to IR was decreased in PKC alpha/delta cells but remained elevated in the PKC delta/alpha cells in response to insulin. Finally, insulin increased Src activity in PKC delta/alpha-expressing cells but decreased it in PKC alpha/delta-expressing cells. Thus, the regulatory domain of PKC delta via interaction with Src appears to determine the role of PKC delta as a positive regulator of IR signaling in skeletal muscle.