Reciprocal regulation of AMP-activated protein kinase (AMPK) activity and C/EBP binding protein (CHOP) expression in chondrocytes. (A) Normal bovine-knee chondrocytes (passage 1) were pre-treated with a pharmacological AMPK activator 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) (1 mM) or a pharmacological AMPK inhibitor compound C (10 μM) for 1 hour before stimulation with IL-1β (10 ng/ml) for 18 hours. AMPK activity (threonine phosphorylation of AMPKα) and total AMPKα were examined by SDS-PAGE/western blot. (B) CHOP siRNA knockdown in first-passage human knee chondrocytes suppressed the capacity of IL-1β (10 ng/ml) to promote de-phosphorylation of AMPKα. Data in panel A representative of three individual experiments with different bovine-chondrocyte donor knees, and in panel B representative of separate experiments on three different human knee chondrocyte donors.