Expression and Signaling Activity of Wnt-5a/Discoidin Domain Receptor-1 and Syk Plays Distinct but Decisive Roles in Breast Cancer Patient Survival

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Abstract
Purpose: The loss of Wnt-5a, a G-protein-coupled receptor ligand, or Syk, an intracellular kinase, has in separate studies been associated with poor prognosis of breast cancer patients. Both proteins are involved in cell adhesion, a key event in epithelial cancer metastasis. Here, we have investigated whether Syk is part of the Wnt-5a/discoidin domain receptor-1 (DDR1) signaling pathway and if a signaling interaction of these proteins is important for breast cancer–specific survival.Experimental Design: The signaling interactions between Wnt-5a/DDR1 and Syk were addressed in mammary cell lines. Their mRNA and protein levels and the respective clinical correlates were investigated in 94 cases of primary breast cancer.Results: The expression of Wnt-5a and Syk correlated in four of five tumor cell lines. However, despite a constitutive association between Syk and the Wnt-5a-dependent adhesion receptor DDR1, we found no evidence of a Wnt-5a/DDR1-mediated activation of Syk. Instead, β1 integrins initiate the adhesion-induced activation of Syk. In tumors from breast cancer patients, the protein expression of Wnt-5a and Syk were differently regulated at the translational and transcriptional level, respectively. Analysis of breast cancer–specific survival revealed that the presence of Wnt-5a and Syk in primary tumors has good predictive value for a favorable outcome. Intriguingly, a simultaneous loss of both proteins did not reduce survival more than loss of either.Conclusions: Despite the difference in regulation of Wnt-5a and Syk protein expression and their lack of signaling interaction, our clinical data indicate that a favorable prognosis in breast cancer requires the expression and signaling activity of both.