An increased incidence of breast cancer has been observed following hormone replacement therapy. However the precise link between treatment with 17β-estradiol (E2) and an increased cancer risk is unclear. The rapid responses to E2 have been well documented and these responses involve the activation of cell signalling cascades that regulate ion transport and cell proliferation. The MCF-7 human breast carcinoma cell line is E2 responsive and can serve as a model for the effects of cancer. It has been shown that phospholipase A₂ (PLA₂) and cyclooxygenase (COX) enzymes are signalling intermediates involved in a rapid response to E2 in rat colonic epithelial cells (Doolan & Harvey, 2003). In this study we have used spectrofluorescence microscopy to investigate the effects of E2 on intracellular Ca²⁺ activity in the MCF-7 cells loaded with the Ca²⁺-sensitive dye Fura-2.

A rapid, transient increase in intracellular calcium levels occurred in E2 (10 nM) treated MCF-7 cells. The calcium influx peaked 1 to 2 min after treatment and returned to basal levels within a further 5 min. Pre-treatment with quinacrine (5 mM) for 15 min prevents the E2 induced calcium response, suggesting that the E2 calcium response is PLA₂ dependent. A plasmid encoding GFP tagged cPLA₂ was transfected into MCF-7 cells and the effect of E2 on cPLA₂ localization was investigated by confocal microscopy. The GFP tagged cPLA₂ was diffusely located throughout the cytoplasm in untreated cells. Preliminary experiments have shown translocation from the cytoplasm to a perinuclear location following E2 treatment. Indomethacin, an inhibitor of COX enzymes partially blocks the E2 induced calcium response in MCF-7 cells. This implies a role for the conversion of arachidonic acid (AA) to prostaglandin E2 (PGE₂) in the regulation of the rapid calcium response.

The rapid response to E2 in MCF-7 cells appears to be linked to the PLA₂ /COX pathway. The rapid nature of the effects would imply that a change in expression levels does not occur and instead a change in the availability of substrate couples the COX pathway to the calcium events. The translocation of cPLA₂ from the cytoplasm to the ER and perinuclear membranes can be regulated through phosphorylation by protein kinases that are activated as a result of E2 treatment. E2 treatment and COX-2 activity have been implicated in breast cancer progression making their downstream signalling cascades important avenues for investigation.