Structural constraints and the importance of lipophilicity for the mitochondrial uncoupling activity of naturally occurring caffeic acid esters with potential for the treatment of insulin resistance.

Caffeic acid phenethyl ester (CAPE) has recently been shown to potently stimulate glucose uptake in cultured skeletal muscle cells through the AMPK pathway and therefore to have anti-diabetic potential. We report here that CAPE increases glucose uptake in C2C12 muscle cells by 225+/-21% at 50 microM, and that activation of AMPK is a consequence of the metabolic stress resulting from an uncoupling-type disruption of mitochondrial function (complete uncoupling at 50 microM). We also observe that the therapeutic potential of CAPE is offset by its high potential for toxicity. The purpose of this study was therefore to identify other active caffeic acid derivatives, evaluate their ratio of activity to toxicity, and elucidate their structure-activity relationship. Twenty naturally occurring derivatives were tested for glucose-uptake stimulating activity in C2C12 cells following 18 h of treatment and for uncoupling activity in isolated rat liver mitochondria. Cytotoxicity was assessed in C2C12 cells by the release of lactate dehydrogenase over 18 h. In addition to CAPE, four compounds were identified to be active, both stimulating glucose uptake and uncoupling isolated mitochondria. Activity required that the caffeic acid moiety be intact and that the compound not contain a strongly ionized group. Both activity and toxicity were found to be well-correlated to predicted lipophilicity. However, two compounds exhibited little to no toxicity while still stimulating glucose uptake by 65-72%. These results support a therapeutic potential for this family of compounds and provide the framework for the design of alternatives to Metformin with an optimized balance of safety and activity.