Effects of metformin on human monocytic THP-1 cells - Implications for type 2 diabetes mellitus

Abstract

Metformin is an anti-hyperglycaemic agent widely prescribed for type 2 diabetes (T2D). Despite decades of clinical use, its exact pharmacological mechanism(s) is yet to be definitively determined. Accumulative data have shown its mild inhibitory effect on the complex 1 of the mitochondrial respiratory chain, which is thought to contribute to its anti-hyperglycaemic effect. Metformin is also thought to have a protective effect on cardiovascular disease (CVD), the most common complication in T2D. It is widely accepted that T2D is an inflammatory state thus it is not surprising that T2D patients have 2-4 times higher risk in developing CVD. Furthermore, the pathology of T2D has been proposed to involve dysfunctioning mitochondria, the organelle of energy sensing and production. The use of metformin, a mitochondria inhibitor, in cells under mitochondrial stress led us to speculate the physiological responses that ensue. To understand metformin‟s mode of action and its involvement in inflammation processes the human monocytic leukemia THP-1 cells were used in this study. Since studies of metformin on this cell type is scarce if not non-existence the first task was to determine its cytoxicity in this cell line. Next the mRNA and protein expressions of heat shock protein 60 (Hsp60), a mitochondrial stress marker, was determined in metformin-treated THP-1 cells. Further, various cell types have shown surface expressed Hsp60 under stress conditions. Thus the study here attempted to optimise the detection of surface Hsp60 in THP-1 cells, as human Hsp60 has been shown to induce innate immune response. Localisation of Hsp60 on the cell surface in metformin-treated cells may exacerbate the already-inflammed state of T2D. Lastly, metformin‟s effect on PMA-stimulated monocyte differentiation was determined by the mRNA expression of CD14, a surface receptor on macrophages. The results here agreed with published data that metformin at therapeutic concentrations are not cytotoxic and is a mild inhibitor of mitochondrial respiration. We also provide evidence that metformin up-regulated the mitochondria stress protein, Hsp60, in a time- and dose-dependent manner. In addition, live cell Hsp60 immunocytochemistry proved to be a promising method for surface Hsp60 detection in THP-1 cells. Finally, at the therapeutic concentration metformin caused a significant up-regulation in cd14 expression, which contradicts with much acclaimed protective effect of metformin in the literature.