Possible hypoglycemic action of SX-fraction targeting insulin signal transduction pathway

Abstract

BACKGROUND: SX-fraction (SXF) is a bioactive glycoprotein with hypoglycemic activity that has been demonstrated in our pilot clinical study. However, how it would actually work in diabetic patients remains unclear. To explore such a mechanism, the effects of SXF on the insulin signal transduction pathway were investigated using skeletal muscle L6 cells in vitro. METHODS: L6 cells were first differentiated to myotubes expressing several biochemical parameters that were examined in this study. Myotubes were exposed to a high concentration (35 mM) of glucose (Glc) alone or in combination with SXF or insulin for 24 hours. Possible effects of these agents on activities of insulin receptor (IR), IR substrate 1 (IRS-1), and Akt, which are key elements involved in the signal pathway, were assessed using enzyme-linked immunosorbent assay (ELISA). Any changes in Glc uptake were also determined. RESULTS: High Glc indeed led to inactivation of IR, IRS-1, and subsequent Akt in myotubes, indicating an interruption of the signal pathway. However, such inactivation was reversed or reactivated by SXF, presumably aiding the occurrence of successive signaling events. Measurement of Glc uptake to assess the outcome of this signaling cascade showed that high Glc decreased Glc uptake (interfering with the signal pathway), but SXF was capable of overcoming such a suppressive effect, resulting in the increased Glc uptake. Insulin was used as a positive control in this study and all results were nearly compatible to those obtained from SXF. CONCLUSION: The present study suggests that SXF may specifically target the insulin signal pathway, and, in particular, the IR and IRS-1 therein that trigger the subsequent signaling events. As a result, SXF could activate such an impaired signal pathway through high Glc or under a hyperglycemic milieu, thereby ultimately facilitating Glc uptake. This may then account for possible hypoglycemic action of SXF.