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 hu man endometrial adenocarcinoma cells were treated for 1 h with TAM, a known antagonist of protein kinase C (PKC), or with staurosporine or H A1004, two broad-spectrum protein kinase antagonists capable of inhibi ting 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 str omal cells, actin stress fibers were well organized throughout the cel l, but in RL95-2 cells, they were disorganized and present mainly at t he 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 reorganiza tion into stress fibers consistent with a more stationary phenotype. I n contrast, TAM-or staurosporine-treated normal stromal cells exhibite d an increase in the amount of organized F-actin. Interestingly, in no rmal stromal cells treated with staurosporine but not TAM or HA 1004, these F-actin fibers appeared to terminate in dense plaques proximal t o the plasma membrane. The alpha(5)/beta(1) integrin fibronectin recep tor mediates between the extracellular matrix and the actin cytoskelet on. TAM induced clustering of the fibronectin receptor at the plasma m embrane in normal stromal cells, but not in carcinoma cells. This stud y supports the importance of plasma membrane-cytoskeletal protein inte ractions in the response of normal and carcinoma cells to TAM.