Humic Acid Induces Expression of Tissue Factor by Cultured Endothelial Cells: Regulation by Cytosolic Calcium and Protein Kinase C

SummaryBlackfoot disease is a thrombotic peripheral vascular disease causally related to the fluorescent humic acid (HA) found in the drinking water of wells in endemic areas in Taiwan. In this study we examined the effect of HA on tissue factor (TF) expression by vascular endothelial cells.Incubation of cultured human umbilical vein endothelial cells (HUVEC) with HA isolated from endemic area drinking water or with a synthetic humic acid polymer (SHA), resulted in enhanced cell surface expression of TF activity by HUVEC. The intracellular calcium level ([Ca24]i) was measured using a calcium-specific fluorescent probe, fura 2. Changes in [Ca24]i level were followed and quantitatively analyzed by spectrofluorometric microscopy, after incubation of the fura 2-loaded HUVEC with HA or SHA in a medium containing 1.8 mM CaCl2. Both HA and SHA increased [Ca2+]i in the presence of extracellular calcium ions, but not in their absence, indicating that influx of extracellular Ca2+ occurred during incubation of HUVEC with HA or SHA. Verapamil, a potent calcium channel blocker, did not abolish the enhancement of [Ca24]i induced by HA or SHA, indicating that specific calcium channels may not be involved in the HA/SHA-induced elevation of [Ca24]i. The elevated [Ca24]i level induced by HA or SHA returned to basal level following removal of HA or SHA and incubation of the washed cells in medium containing 1.8 mM CaCl2. All these changes occurred in the absence of significant cytotoxic effects. The HA/SHA-induced enhancement of cell surface TF activity was inhibited by a specific inhibitor of protein kinase C, H7, suggesting that protein kinase C is involved in the process leading to the enhanced expression of TF activity induced by HA or SHA.In conclusion, this study demonstrates that HA and SHA enhance cell surface expression of TF activity by permeabilization of the cell membrane to extracellular Ca24 ions, leading to elevation of [Ca2+]i that functions as a second messenger to activate protein kinase C, leading finally to enhanced cell surface TF expression. Enhancement of vascular endothelial cell surface TF activity by HA may play a role in the HA-induced thrombotic vascular disorders of Blackfoot disease.