Abstract
Loss of estrogen‐responsiveness and impaired E‐cadherin expression/function has been linked to increased metastatic potential of breast cancer cells. In this study, we report that proliferation of breast cancer cells can resume following removal of a toxic stimulus causing severe impairment of cell adhesion and estrogen responsiveness. This type of response was induced by okadaic acid (OA) in MCF‐7 cells, and was accompanied by an almost complete block of DNA synthesis, loss of cell–cell contact and cell detachment from culture dishes, loss of estrogen receptor (ER), progesterone receptor (PR) and E‐cadherin, whereas only a weak, if any, inhibition of protein synthesis could be observed. These responses were detected in MCF‐7 cells after a 1‐day treatment with 50 nM OA, and could be reversed if OA‐treated cells were recovered in a culture medium devoid of the toxin, so that rescued cells resumed growth 8–12 days after replating. By pulse‐chase experiments, we found that protein synthesis was not significantly affected in rescued cells, whose DNA synthesis, instead, was almost completely blocked during the first days of MCF‐7 cell rescue from OA treatment. We also analyzed E‐cadherin, mitogen activated protein kinase isoforms ERK1 and ERK2, Bcl‐2 and BAX proteins during the rescue of MCF‐7 cells from OA‐induced cell death, and found that their expression followed temporally defined patterns. Cellular levels of E‐cadherin returned to control levels within the first days of the rescue, followed by ER, ERK1, and ERK2, and finally by Bcl‐2 and BAX proteins. Under our experimental conditions, restoration of cell adhesion did not require a functional ER system, but recovery of a normal ER pool accompanied resumption of estrogen‐dependent proliferation of OA‐treated MCF‐7 cells. © 2002 Wiley‐Liss, Inc.