The transactivation of EGFR by GPCR ligands is recognized as an important signaling mechanism in the regulation of complex biological processes such as cancer development. Estrogen (E2), a steroid hormone that functions primarily through nuclear estrogen receptors (ER), has also been suggested to transactivate the EGFR through membrane-associated ER (mER) in a G protein-dependent manner. However, the true nature of the mER remains undefined, especially with regards to the mechanism underlying E2 induced EGFR transactivation. In the present study, we demonstrate that E2 stimulated activation of sphingosine kinase (SphK) and Edg-3, a specific GPCR for sphingosine 1-phosphate (S1P), play a prominent role in mediating the transactivation of EGFR. This was supported by the following findings derived from experiments on human MCF-7 breast cancer cells: (i) E2 was a strong stimulator of SphK activation and S1P secretion; (ii) S1P mimicked E2’s effect on the transactivation of EGFR in a variety of respects; (iii) E2 rapidly induced Edg-3 activation as demonstrated by the receptor internalization; (iv) Inhibition of SphK activity by expression of a dominant-negative mutant of SphK or siRNA for SphK1, but not SphK2, abrogated the E2-induced EGFR activation; and (v) E2-induced transactivation of EGFR was suppressed by Edg-3 antisense oligonucleotides that specifically knocked down Edg-3 expression. Collectively, these data demonstrate that E2-induced EGFR transactivation is mediated by Edg-3 via activation of the SphK/S1P pathway. This indicates that a given GPCR ligand (i.e., S1P) dependent transactivation of EGFR can be activated by another independent ligand (i.e., estrogen), suggesting a new model of signal transactivation between three individual ligand-receptor systems.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]