Tamoxifen alters the localization of F-actin and alpha 5/beta 1-integrin fibronectin receptors in human endometrial stromal cells and carcinoma cells

We have investigated F-actin and the integrin fibronectin receptor as possible targets of tamoxifen (TAM) signaling in a cell-based model of the endometrium. Normal human endometrial stromal cells and RL95-2 human endometrial adenocarcinoma cells were treated for 1 h with TAM, a known antagonist of protein kinase C (PKC), or with staurosporine or HA1004, two broad-spectrum protein kinase antagonists capable of inhibiting PKC and PKA, respectively. We utilized fluorescein-phalloidin and confocal microscopy to visualize the cellular distribution of F-actin. Normal stromal cells and RL95-2 cells differed in the arrangement of F-actin in control cells and in their response to TAM. In control stromal cells, actin stress fibers were well organized throughout the cell, but in RL95-2 cells, they were disorganized and present mainly at the cell periphery. F-actin in RL95-2 cells treated with TAM (0.1 and 1.0 mu M) or with staurosporine (0.7 and 7.0 nM) exhibited a reorganization into stress fibers consistent with a more stationary phenotype. In contrast, TAM-or staurosporine-treated normal stromal cells exhibited an increase in the amount of organized F-actin. Interestingly, in normal stromal cells treated with staurosporine but not TAM or HA 1004, these F-actin fibers appeared to terminate in dense plaques proximal to the plasma membrane. The alpha(5)/beta(1) integrin fibronectin receptor mediates between the extracellular matrix and the actin cytoskeleton. TAM induced clustering of the fibronectin receptor at the plasma membrane in normal stromal cells, but not in carcinoma cells. This study supports the importance of plasma membrane-cytoskeletal protein interactions in the response of normal and carcinoma cells to TAM.