Inhibition of macrophage metabolism by oxLDL
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Intracellular oxidative stress is induced by oxidised low density lipoprotein (oxLDL) in macrophages. In the atherosclerotic lesions, this oxLDL dependent oxidative stress appears to cause macrophage cell death, a key process in the development of the necrotic core within the complex plaque. Macrophages are activated by γ-interferon to synthesise and release a potent antioxidant, 7,8-dihydroneopterin (7,8-NP), which has been previously shown to protect human monocyte-like U937 cells and human monocyte-derived macrophage (HMDM) cells from oxLDL cytotoxicity. This study examined whether oxLDL causes the loss of cellular metabolic function and whether 7,8-dihydroneopterin can prevent this loss of metabolic activity in U937 cells and HMDM cells. OxLDL prepared by copper oxidation caused cell death in both U937 and HMDM cells at concentrations of 0.5 and 2.0 mg/ml, respectively. Cell morphology showed the oxLDL caused a necrotic like death in both cells as indicated by cell swelling and lysis. The decrease in cell viability was only observed after the loss of intracellular glutathione (GSH) which occurred in the first 3 hours in U937 cells following oxLDL addition. The loss of GSH appeared to be due to the production of intracellular oxidants generated in response to the presence of the oxLDL. Within 3 hours of oxLDL addition to both cell types, there was a rapid and progressive shutdown of cell metabolism indicated by a significant decrease in the enzymatic activity of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and a fall in lactate production and intracellular ATP levels. GAPDH activity was found to be inactivated rather than being lost from the cell. Gel electrophoresis with specific staining for oxidised proteins showed that the GAPDH had been oxidatively inactivated in the cells when oxLDL was present. Unlike GAPDH, lactate dehydrogenase (LDH) was not inactivated by the oxidation but was lost from the cells due to cell lysis. The observed rate of glycolysis failure was similar in both cell types except the HMDM cells did not lose lactate, LDH activity and cell viability until 6 hours compared to 3 hours with the U937 cells. The rate of oxygen consumption (VO2) was measured in U937 cells by taking cells at set time points and placing them in the respirometers to measure the VO2. U937 cells were found to increase their VO2 with incubation but this increase was inhibited in the presence of oxLDL within 3 hours. The addition of the 7,8 dihydroneopterin above 100 μM to both the U937 and HMDM cells significantly inhibited the oxLDL-induced loss of cell viability. GAPDH activity loss was also inhibited while lactate production was maintained. The 7,8-dihydroneopterin also prevented the decrease in the VO2 in oxLDL-treated U937 cells. OxLDL was labelled with fluorescent DiI to measure the uptake of oxLDL by HMDM cells. The incorporation of DiI into oxLDL was found to make it non-cytotoxic, possibly due to DiI’s antioxidant properties. Studies were therefore conducted using either a mixture of oxLDL and DiI labelled oxLDL (DiI-oxLDL) at non-protective concentrations or low concentration of DiI-oxLDL alone. These studies showed that 7,8-dihydroneopterin downregulated the oxLDL uptake in oxLDL-treated HMDM cells. Surprisingly the uptake rates also suggested that there was no relationship between oxLDL uptake and cell death assuming oxLDL and DiI-oxLDL are taken up by the same mechanism. This research showed that oxLDL-induced oxidative stress in macrophage cells causes a rapid oxidative loss of GAPDH activity which leads to the loss of glycolytic activity and a fall in ATP levels. The failure of cell metabolism appears to be a key event in the death mechanism triggered by the oxLDL. The radical scavenging activity of 7,8-dihydroneopterin appears to prevent the oxidative stress as indicated by the protection of the GSH pool. Without the oxidative stress, GAPDH remains functioning, glycolytic activity is maintained and both the U937 cells and HMDM cells did not die. This suggests that within the atherosclerotic plaque, 7,8-dihydroneopterin may act to stabilise the metabolism of macrophage cells in the presence of oxLDL and downregulate the oxLDL uptake.