A NOVEL MECHANISM OF ACTION OF TETRACYCLINES - EFFECTS ON NITRIC-OXIDE SYNTHASES

Tetracyclines have recently been shown to have ''chondroprotective'' e ffects in inflammatory arthritides in animal models, Since nitric oxid e (NO) is spontaneously released from human cartilage affected by oste oarthritis (OA) or rheumatoid arthritis In quantities sufficient to ca use cartilage damage, we evaluated the effect of tetracyclines on the expression and function of human OA-affected nitric oxide synthase (OA -NOS) and rodent inducible NOS (iNOS), Among the tetracycline group of compounds, doxycycline > minocycline blocked and reversed both sponta neous and interleukin 1 beta-induced OA-NOS activity in ex vivo condit ions. Similarly, minocycline greater than or equal to doxycycline inhi bited both lipopolysaccharide- and interferon-gamma-stimulated iNOS in RAW 264.7 cells in vitro, as assessed by nitrite accumulation. Althou gh both these enzyme isoforms could be inhibited by doxycycline and mi nocycline, their susceptibility to each of these drugs was distinct. U nlike acetylating agents or competitive inhibitors of L-arginine that directly inhibit the specific activity of NOS, doxycycline or minocycl ine has no significant effect on the specific activity of iNOS in cell -free extracts, The mechanism of action of these drugs on murine iNOS expression was found to he, at least in part, at the level of RNA expr ession and translation of the enzyme, which would account for the decr eased iNOS protein and activity of the enzyme. Tetracyclines had no si gnificant effect on the levels of mRNA for beta-actin and glyceraldehy de-3-phosphate dehydrogenase nor on levels of protein of beta-actin an d cyclooxygenase 2 expression. These studies indicate that a novel mec hanism of action of tetracyclines is to inhibit the expression of NOS, Since the overproduction of NO has been implicated in the pathogenesi s of arthritis, as well as other inflammatory diseases, these observat ions suggest that tetracyclines should be evaluated as potential thera peutic modulators of NO For various pathological conditions.