Salicylate-induced Growth Arrest Is Associated with Inhibition of p70^s6k and Down-regulation of c-Myc, Cyclin D1, Cyclin A, and Proliferating Cell Nuclear Antigen*

Salicylate and its pro-drug form aspirin are widely used medicinally for their analgesic and anti-inflammatory properties, and more recently for their ability to protect against colon cancer and cardiovascular disease. Despite the wide use of salicylate, the mechanisms underlying its biological activities are largely unknown. Recent reports suggest that salicylate may produce some of its effects by modulating the activities of protein kinases. Since we have previously shown that the farnesyltransferase inhibitorl-744,832 inhibits cell proliferation and p70^s6k activity, and salicylate inhibits cell proliferation, we examined whether salicylate affects p70^s6k activity. We find that salicylate potently inhibits p70^s6k activation and phosphorylation in a p38 MAPK-independent manner. Interestingly, low salicylate concentrations (≤250 μm) inhibit p70^s6k activation by phorbol myristate acetate, while higher salicylate concentrations (≥5 mm) are required to block p70^s6k activation by epidermal growth factor + insulin-like growth factor-1. These data suggest that salicylate may selectively inhibit p70^s6kactivation in response to specific stimuli. Inhibition of p70^s6k by salicylate occurs within 5 min, is independent of the phosphatidylinositol 3-kinase pathway, and is associated with dephosphorylation of p70^s6k on its major rapamycin-sensitive site, Thr³⁸⁹. A rapamycin-resistant mutant of p70^s6k is resistant to salicylate-induced Thr³⁸⁹ dephosphorylation.