Insulin receptor mediates the stimulatory effect of Sarcopoterium spinosum on insulin signaling cascade

Improving insulin sensitivity is an important therapeutic target in the treatment of type 2 diabetes and other related disorders. In previous studies the anti-diabetic and insulin sensitizing properties of Sarcopoterium spinosum extract (SSE) were established. In this study we investigated whether SSE stimulates insulin signaling through the activation of insulin receptor (IR).

Phospho-proteomics of serine/threonine residues predicted the activation of insulin signaling by SSE. This was confirmed by glucose transport assays and Western blot analyses performed on CHO-IR cells, L6 myotubes and 3T3-L1 adipocytes. SSE stimulated the two arms of insulin signaling cascade, the metabolic and the mitogenic arms, pointing on IR as a potential common upstream regulator. Inhibition of IR abolished the transmission of signaling by SSE, indicating that IR is required for SSE effect. SSE enhanced tyrosine phosphorylation of IR, as detected by ELISA and Western blot. Since tyrosine phosphatases were not inhibited by SSE, a direct stimulatory effect on IR phosphorylation was proposed. Fluorescence quenching analysis demonstrated a lower fluorescence intensity of IR under different concentrations of SSE, its composite trimeric catechin and additional yet unidentified component. This quenching was absent with denaturized receptor, indicating that certain composites of SSE directly bind to the native form of IR. This was also supported by cellular thermal shift assay, showing higher stability of IR in the presence of SSE and trimeric catechin. In summary, bio-active compounds in SSE might be considered as IR activators. This study emphasizes the therapeutic potential hidden in this botanical.