Retinoic acid signaling through PI 3-kinase induces differentiation of human endometrial adenocarcinoma cells

The specific signals required for actin polymerization in response to extracellular factors remain unknown. However, in many cell types, there is a correlation between actin polymerization, activation of phosphatidylinositol 3-kinase (PI 3-kinase), and the production of the second messenger phosphatidylinositol-3,4,5-triphosphate. Increased levels of PI 3-kinase have been detected during cell growth and transformation. However, PI 3-kinase is also activated during differentiation, suggesting that PI 3-kinase and its lipid products also play a role in the regulation of cellular differentiation. The newly characterized CAC-1 cell line established from a poorly differentiated human endometrial adenocarcinoma (Exp. Mol. Pathol. 69 (2000), 175) was used as a model to investigate the role of PI 3-kinase in differentiation induction. CAC-1 cells differentiated upon treatment with pharmacological doses of retinoids (1 micro M of 13-cis or all-trans), evidenced by actin filament reorganization, and cell enlargement. PI 3-kinase staining is primarily localized to perinuclear regions in untreated cells. However, retinoic acid treatment induced PI 3-kinase to relocalize throughout the cytoplasm. Subcellular fractionation and Western blotting confirmed that PI 3-kinase decreased in the particulate fraction, concurrent with retinoid-induced differentiation. Interestingly, pretreatment with the PI 3-kinase inhibitor wortmannin (100 nM) prior to retinoic acid treatment prevented retinoic acid-induced actin reorganization and cell enlargement. To distinuish whether retinoid regulation of PI 3-kinase is mediated through traditional nuclear retinoic acid receptors, the levels of retinoic acid receptor-beta (RAR-beta) protein were evaluated. Retinoid treatment did not alter RAR-beta protein levels compared to controls. These data suggest that PI 3-kinase activity and cytoplasmic relocalization are required for retinoid-induced differentiation of poorly differentiated human endometrial adenocarcinoma cells.