Interrelationship between protein phosphatase-2A and cytoskeletal architecture during the endothelial cell response to soluble products produced by human head and neck cancer

Tumor neovascularization is necessary for the progressive development of al l solid tumors, including head and neck squamous cell carcinomas (HNSCCs). The angiogenic process includes increased endothelial cell motility. Our pr ior studies have shown the importance of protein phosphatase-2A (PP-2A) in restricting endothelial cell motility. Because motility is regulated by the polymerization/depolymerization of the cellular cytoskeleton, the present study defined the interrelationship between PP-2A and the cytoskeleton duri ng endothelial cell responses to HNSCC-derived angiogenic factors. PP-2A wa s shown to colocalize with microtubules of unstimulated endothelial cells. However, exposure to HNSCC-derived products resulted in a more diffuse dist ribution of PP-2A staining and a loss of filamentous tubulin. The feasibili ty of pharmacologically preventing this cytoskeletal disorganization as a m eans of blocking tumor-induced angiogenesis was tested. This wets accomplis hed by use of l alpha,25-dihydroxyvitamin D-3 (1,25 (OH)(2)D-3) and all-tra ns-retinoic acid to indirectly stimulate PP-2A activity through their capac ity to elevated intracellular levels of the second messenger ceramide. Pret reatment of endothelial cells with either 1,25(OH)(2)D-3 or retinoic acid p revented the cytoskeletal disorganization that otherwise occurs in endothel ial cells on exposure to HNSCC-derived products. These studies support the feasibility of using elevation of PP-2A to prevent the morphogenic componen t of the angiogenic process that is stimulated by HNSCC-derived factors.