A new mechanism of methylprednisolone and other corticosteroids action demonstrated in vitro: inhibition of a proteinase (calpain) prevents myelin and cytoskeletal protein degradation.

The affect of methylprednisolone (MP), an anti-inflammatory drug upon purified calpain and the Ca2+-mediated degradation of endogenous proteins of spinal cord homogenate in vitro has been examined. Activity of calpain purified from rabbit muscle was greatly inhibited in a dose-dependent fashion by MP. A 50% inhibition was obtained with 3.2 mM MP concentration and the activity was inhibited further (80%) at 8.1 mM. More potent inhibition of the purified enzyme (70-80%) was produced by dexamethasone (3.9 mM) and prednisolone (4.1 mM). Calpain-mediated degradation of myelin basic protein (MBP) was also inhibited by MP as was cathepsin B-mediated MBP breakdown. The effect of MP and other steroids upon calcium-mediated degradation of spinal cord homogenate was also evaluated. SDS-PAGE analysis revealed significant inhibition of neurofilament protein breakdown by MP and other corticosteroids. This inhibitory effect was much less than that exerted by the calpain inhibitors calpeptin and/or E64-d. These results indicate that MP acts as a proteinase (calpain) inhibitor and define a new mechanism for its actions.