Retinoic acid receptor β2 inhibition of metastasis in mouse mammary gland xenografts

The retinoic acid receptor beta(2) (RARbeta(2)) protein is a putative tumor suppressor that inhibits proliferation and can induce apoptosis when introduced into breast, cervical, lung, and pancreatic cancer cell lines. To determine if RARbeta(2) suppresses proliferation of mammary-derived cancer cells in vivo, we transduced MDA-MB-435 breast cancer cells with the LXSN retroviral vector containing RARbeta(2) and implanted LXSN vector- or RARbeta(2)-transduced cells into the mammary fat pads of nude and severe combined immune deficiency (SCID) mice. We analyzed the xenografts for several tumor parameters, including tumor size, inflammation, vascularity, mitoses, tumor recurrence at the primary site following resection, and metastases. We found that 19 of 52 mice inoculated with vector-transduced cells developed metastases in multiple organs while only one of 55 mice receiving RARbeta(2)-transduced cells displayed evidence of metastases (p < 0.000001, combined experiments, two-tailed Fisher's exact test). Moreover, RARbeta(2)-tumor cell recipient mice had a lower incidence of post-resection tumor recurrence (8/55 vs. 25/52, p = 0.0004), 34% less necrosis (in three of four experiments, p = 0.001), and 39% fewer mitoses in tumor tissue (p < 0.000001). Our findings suggest that RARbeta(2) may play a role in inhibiting the metastatic cascade in a mouse mammary gland xenograft tumor model and is a potential candidate for therapeutic intervention in human breast cancer.