Transforming growth factor beta receptor type III is a tumor promoter in mesenchymal-stem like triple negative breast cancer

Introduction: There is a major need to better understand the molecular basis of triple negative breast cancer (TNBC) in order to develop effective therapeutic strategies. Using gene expression data from 587 TNBC patients we previously identified six subtypes of the disease, among which a mesenchymal-stem like (MSL) subtype. The MSL subtype has significantly higher expression of the transforming growth factor beta (TGF-beta) pathway-associated genes relative to other subtypes, including the TGF-beta receptor type III (T beta RIII). We hypothesize that T beta RIII is tumor promoter in mesenchymal-stem like TNBC cells. Methods: Representative MSL cell lines SUM159, MDA-MB-231 and MDA-MB-157 were used to study the roles of T beta RIII in the MSL subtype. We stably expressed short hairpin RNAs specific to T beta RIII (T beta RIII-KD). These cells were then used for xenograft tumor studies in vivo; and migration, invasion, proliferation and three dimensional culture studies in vitro. Furthermore, we utilized human gene expression datasets to examine T beta RIII expression patterns across all TNBC subtypes. Results: T beta RIII was the most differentially expressed TGF-beta signaling gene in the MSL subtype. Silencing T beta RIII expression in MSL cell lines significantly decreased cell motility and invasion. In addition, when T beta RIII-KD cells were grown in a three dimensional (3D) culture system or nude mice, there was a loss of invasive protrusions and a significant decrease in xenograft tumor growth, respectively. In pursuit of the mechanistic underpinnings for the observed T beta RIII-dependent phenotypes, we discovered that integrin-a2 was expressed at higher level in MSL cells after T beta RIII-KD. Stable knockdown of integrin-a2 in T beta RIII-KD MSL cells rescued the ability of the MSL cells to migrate and invade at the same level as MSL control cells. Conclusions: We have found that T beta RIII is required for migration and invasion in vitro and xenograft growth in vivo. We also show that T beta RIII-KD elevates expression of integrin-a2, which is required for the reduced migration and invasion, as determined by siRNA knockdown studies of both T beta RIII and integrin-a2. Overall, our results indicate a potential mechanism in which T beta RIII modulates integrin-a2 expression to effect MSL cell migration, invasion, and tumorigenicity.