Mitochondrial electron transport and glycolysis are coupled in articular cartilage

Abstract

Objective Although the majority of the ATP in chondrocytes is made by glycolysis rather than by oxidative phosphorylation in mitochondria there is evidence to suggest that reactive oxygen species produced by mitochondrial electron transport help to maintain cellular redox balance in favor of glycolysis. The objective of this study was to test this hypothesis by determining if rotenone, which inhibits electron transport and blocks oxidant production inhibits glycolytic ATP synthesis. Design Bovine osteochondral explants were treated with rotenone, an electron transport inhibitor; or oligomycin an ATP synthase inhibitor; or 2-fluoro-2-deoxy-D-glucose, a glycolysis inhibiter; or peroxide, an exogenous oxidant; or mitoquinone, a mitochondria-targeted anti-oxidant. Cartilage extracts were assayed for ATP, NAD+, and NADH, and culture medium was assayed for pyruvate and lactate after 24 hours of treatment. Imaging studies were used to measure superoxide production in cartilage. Results Rotenone and 2-fluoro-2-deoxy-D-glucose caused a significant decline in cartilage ATP (p < 0.001). In contrast, ATP levels were not affected by oligomycin. Peroxide treatment blocked rotenone effects on ATP, while treatment with MitoQ significantly suppressed ATP levels. Rotenone and 2-fluoro-2-deoxy-D-glucose caused a significant decline in pyruvate, but not in lactate production. NADH:NAD+ ratios decreased significantly in both rotenone and 2-fluoro-2-deoxy-D-glucose-treated explants (p < 0.05). Rotenone also significantly reduced superoxide production Conclusions These findings showing a link between glycolysis and electron transport are consistent with previous reports on the critical need for oxidants to support normal chondrocyte metabolism. They suggest a novel role for mitochondria in cartilage homeostasis that is independent of oxidative phosphorylation.

Combining Targeted Metabolomic Data with a Model of Glucose Metabolism: Toward Progress in Chondrocyte Mechanotransduction

“…Namely, a decrease in ADP levels coupled with a slight increase in NADH, as previously mentioned. Though high flux through the TCA cycle is unusual in chondrocytes, they are known to supplement ATP production through respiration under nutrient stress and mechanical loading [36–39]. Our analysis suggests compression may trigger the TCA cycle in an in vitro environment containing atmospheric oxygen levels, possibly for increased ATP synthesis, or for synthesis of pathway intermediates.…”

Targeting mitochondrial responses to intra-articular fracture to prevent posttraumatic osteoarthritis

“…To capture the breadth of this effect, we have used well-described, custom automated scoring algorithms (24, 25, 44–46) to provide large scale quantitation of histological changes associated with PTOA spanning the synovium, the weight-bearing areas of both articular surfaces, and the subchondral bone. Intra-articular administration of NAC or amobarbital significantly abrogated PTOA after an IAF as shown by classic PTOA indicators including PG content, cartilage thickness, and structural damage scores as well as intracellular phenomena linked to PTOA such as oxidative stress (4, 10, 11, 15, 19, 47, 48) and altered mitochondrial metabolism (10, 11, 13, 14, 17–19, 24, 27, 49). This occurred in the absence of any overt anti-inflammatory effect during the acute phase, suggesting either a distinct, oxidation-dependent pathway contributing to rapid PTOA progression or an uncoupling of inflammation from rapid progression via protection against oxidation.…”

Injurious Loading of Articular Cartilage Compromises Chondrocyte Respiratory Function

“…Studies suggest that manipulations of key factors including surface mechanics, inflammation, or oxidative injury can alleviate different aspects of cellular injury from overloading [5,6,7]. Among the outcomes, antioxidants appear to combat overactivity after overload of a mechanotransductive pathway whereby loading stimulates rotenone-inhibitable electron transport chain (ETC) activity, subsequent ROS generation as a byproduct of respiration, and, ultimately, increased anabolism by chondrocytes [8,9,10,11,12]. The hypothesis that ROS function as a metabolism-mediating signal in chondrocytes was put forth as early as 1997 by Lee and Urban in studies showing that without oxygen, chondrocyte anabolic function ceases and that this can be restored with addition of exogenous oxidants [13,14].…”