The JNKs belong to the mitogen-activated protein kinase (MAPK) family. Cellular activation of MAPKs, including JNK, involves a distinct protein kinase cascade, which is organized in signaling modules by scaffold proteins for precise regulation in response to various stress stimuli.
58 A common feature of MAPK cascades is the organization of 3 kinases in which a MAPK kinase kinase (MAP3K) activates a MAP2K, which subsequently activates a MAPK.
59 In the mammalian JNK module, the cytosolic JIP group of scaffold proteins selectively enhance JNK signaling by interacting with and linking the upstream kinases to JNK activation.
60,61 Specifically, the JIP1 isoform serves as a docking site for the binding of JNK, MAP3Ks, and MAP2K7,
60 to facilitate sequential kinase activation of the JNK signaling pathway.
62 The physiologic role for JIP1 in JNK activation was supported by evidence that JIP1 knockout mice lack the ability to elicit anoxic and excitotoxic stress-induced activation of JNK in hippocampal neurons.
61 At the vascular level, in vitro studies showed that cultured rat retinal endothelial cells containing overexpression of glucose transporter-1 and elevated intracellular glucose concentrations exhibited an increased JIP1 protein expression and JNK phosphorylation.
63 Our current study extended these earlier findings
63 to determine whether JIP1 influences retinal arteriolar vasodilator function during acute and chronic hyperglycemia. In 2-week diabetic pigs, we observed reversal of impaired retinal arteriolar dilation to bradykinin with the JIP1 inhibitor BI-78D3,
41,42 which is a small molecule mimic of JIP1 that competitively blocks the JIP1 binding domain of JNK from interacting with its cognate substrates and endogenous JIP1. This JNK signaling inhibitor does not influence the ATP-binding region of JNK but instead blocks protein–protein interaction at the JNK-binding site of JIP1. Similarly, bradykinin-induced dilation of retinal arterioles during exposure to acute hyperglycemia was preserved in the presence of JIP1 blockade. Taken together, our findings with 2 structurally distinct JNK pathway inhibitors, SP600125 and BI-78D3, suggest that the interaction of JIP1 and JNK is important in initiating endothelial damage during hyperglycemia, and support the idea that this molecular event contributes to development of endothelial vasodilator dysfunction in retinal arterioles during the early stage of diabetes.