Abstract Nm23-H1 transcriptionally down-regulates expression of the lysophosphatidic acid receptor EDG2 and this down-regulation is critical for Nm23-H1–mediated motility suppression in vitro. We investigated the effect of altered EDG2 expression on Nm23-H1–mediated metastasis suppression in vivo. Clonal MDA-MB-435–derived tumor cell lines transfected with Nm23-H1 together with either a vector control or EDG2 had similar anchorage-dependent and anchorage-independent growth rates in vitro. However, a 45- and 300-fold inhibition of motility and invasion (P < 0.0001), respectively, was observed in Nm23-H1/vector lines, whereas coexpression of EDG2 restored activity to levels observed in the parental line. Using fluorescently labeled cells and ex vivo microscopy, the capacity of these cells to adhere, arrest, extravasate, and survive in the murine lung over a 24-h time course was measured. Only 5% of Nm23-H1/vector–transfected cells were retained in the murine lung 6 h following tail vein injection; coexpression of EDG2 enhanced retention 8- to 13-fold (P < 0.01). In a spontaneous metastasis assay, the primary tumor size of Nm23-H1/vector and Nm23-H1/EDG2 clones was not significantly different. However, restoration of EDG2 expression augmented the incidence of pulmonary metastasis from 51.9% to 90.4% (P = 2.4 × 10−5), comparable with parental MDA-MB-435 cells. To determine the relevance of this model system to human breast cancer, a cohort of breast carcinomas was stained for Nm23-H1 and EDG2 and a statistically significant inverse correlation between these two proteins was revealed (r = −0.73; P = 0.004). The data indicate that Nm23-H1 down-regulation of EDG2 is functionally important to suppression of tumor metastasis. [Cancer Res 2007;67(24):11751–9]