Potential Role of PDGFRβ-Associated THBS4 in Colorectal Cancer Development

Colorectal cancer is a significant cause of death since it frequently metastasizes to several organs such as the lung or liver. Tumor development is affected by various factors, including a tumor microenvironment, which may be an essential factor that leads to tumor growth, proliferation, invasion, and metastasis. In the tumor microenvironment, abnormal changes in various growth factors, enzymes, and cytokines can wield a strong influence on cancer. Thrombospondin-4 (THBS4), which is an extracellular matrix protein, also plays essential roles in the tumor microenvironment and mediates angiogenesis by transforming growth factor-β (TGFβ) signaling. Platelet-derived growth factor receptor β (PDGFRβ), which is a receptor tyrosine kinase and is also a downstream signal of TGFβ, is associated with invasion and metastasis in colorectal cancer. We identified that PDGFRβ and THBS4 are overexpressed in tumor tissues of colorectal cancer patients, and that PDGF-D expression increased after TGFβ treatment in the colon cancer cell line DLD-1. TGFβ and PDGF-D increased cellular THBS4 protein levels and secretion but did not increase THBS4 mRNA levels. This response was further confirmed by the inositol 1,4,5-triphosphate receptor (IP3R) and stromal interaction molecule 1 (STIM1) blockade as well as the PDGFRβ blockade. We propose that the PDGFRβ signal leads to a modification of the incomplete form of THBS4 to its complete form through IP3R, STIM1, and Ca²⁺-signal proteins, which further induces THBS4 secretion. Additionally, we identified that DLD-1 cell-conditioned medium stimulated with PDGF-D promotes adhesion, migration, and proliferation of colon myofibroblast CCD-18co cells, and this effect was intensified in the presence of thrombin. These findings suggest that excessive PDGFRβ signaling due to increased TGFβ and PDGF-D in colorectal tumors leads to over-secretion of THBS4 and proliferative tumor development.