Stimulation or endothelin-1 secretion by human breast cancer cells through protein kinase A activation: a possible novel paracrine loop involving breast fibroblast-derived prostaglandin E2.

Breast cancer cells secrete endothelin-1 (ET-1), which may act as a paracrine mitogen in breast tumours. The paracrine factors and signal transduction pathways responsible for regulating ET-1 production in breast cancer are unknown. In this study we have examined the involvement of the protein kinase A (PKA) signalling pathway in the control of ET-1 secretion in the human breast cancer cell line MCF-7. Treatment of MCF-7 cells with various agents that activate protein kinase A (PKA) through increases in intracellular cAMP levels including forskolin, cholera toxin (ChT), the cAMP analogue 8-Br-cAMP, or the cAMP phosphodiesterase inhibitor, 3-isobutyl-1-methyl-xanthine (IBMX) all markedly increased ET-1 release. Prostaglandin E2 (PGE2) while stimulating cAMP production, but not inositol lipid hydrolysis also significantly stimulated ET-1 release. Activation of PKC by 2-O-tetradecanoyl phorbol 13-acetate (TPA) also stimulated ET-1 secretion in MCF-7 cells. The PKA inhibitor H-89 attenuated the ET-1 response to PGE2, forskolin and ChT, but not that due to the PKC agonist TPA. The possibility that human breast fibroblasts (HBFs) are a target for ET-1 action with regard to PGE2 production was also investigated, and revealed that while HBFs were unresponsive to ET-1 alone, pretreatment with the cytokine IL-beta greatly potentiated PGE2 release in response to ET-1. In conclusion our results show that activation of either the PKA or PKC signalling pathways in human breast cancer cells increases ET-1 secretion. We also found that HBFs release PGE2 after treatment with ET-1 and that PGE2 itself stimulates ET-1 production in MCF-7 cells. The implication of this potential novel paracrine loop may be significant in view of the high levels of PGE2 and ET-1 found in malignant breast tissues.