Platelet Glycoprotein Ibα Supports Primary Tumor Growth and Experimental Metastasis.

Successful tumor formation and metastatic spread depend on the ability of tumor cells to interact with their host microenvironment. Studies from our groups and others indicate that platelets contribute to tumor host interactions through adhesive and stimulatory functions that may promote tumor growth and dissemination. Here we show that platelet receptor GPIbα plays a critical role in the ability of platelets to support primary tumor development and hematogenous metastasis. We previously developed knockout mice lacking platelet GP Ibα (GPIbα^Null, a model of the human Bernard-Soulier syndrome) and mice missing GPIbα, but expressing a fusion protein of cytoplasmic GP Ibα and the extracytoplasmic domain of the interleukin-4 receptor (Ibα/IL-4R). Expression of the Ibα/IL-4R ameliorates macrothrombocytopenia, a characteristic of the Bernard-Soulier syndrome. However, both models result in a severe hemorrhagic state owing to the absence of GPIbα. Congenic mouse strains of both GP Ibα models were generated by 10-generation backcrosses with wild-type (WT) C57BL/6J mice to study the tumorgenic and metastatic activity of syngeneic tumor cells. To analyze primary tumor growth, Lewis lung carcinoma cells (D121) were injected intradermally in the dorsal flank of the hind limb (10⁵ cells/injection) and tumor development analyzed in its initial and advanced stages. Seventeen days after injection, tumors in GPIbα^Null and Ibα/IL-4R mice were several-fold smaller than those in WT C57BL/6J mice. Histological examination of early and end-stage tumors revealed three major differences: a severe and immediate onset of tumor necrosis, a lack of new blood vessels large enough to connect to the circulation, and a significant reduction in tumor-associated macrophages. While the proliferation rate of tumor cells in non-necrotic areas of the tumors was unaffected by the lack of GPIbα, the percentage of necrosis in these tumors was 6-fold increased in GPIbα^Null mice. Each necrotic area showed increased hemorrhage in the absence of GPIbα. The overall density of microvessels in early primary tumors appeared unchanged, but the number of larger vessels associated with tumors was reduced ~2-fold and the density of tumor-associated macrophages was reduced ~6-fold compared to tumors developing in WT mice. To analyze a contribution of platelet GPIbα to hematogenous tumor metastasis, another syngeneic tumor cell model was used and B16F10.1 melanoma cells were injected into the tail vein (10⁵ cells/mouse) of GPIbα^Null, Ibα/IL-4R, or WT mice, to determine the ability of tumor cells to colonize the lungs 14 days later. In the absence of functional GPIbα, the number of metastatic foci at the surface of the lungs was significantly reduced (15-fold, p = 0.0002). These results indicate that successful target organ colonization by tumor cells from the blood stream is severely affected by the lack of extracytoplasmic GPIbα in Ibα/IL-4R animals, and not necessarily by the reduced number of platelets in GPIbα^Null animals. The results demonstrate a significant involvement of platelet GP Ibα in primary tumor growth by affecting tumor angiogenesis and the recruitment of tumor-associated macrophages while providing evidence GPIbα directly contributes to target organ colonization by circulating tumor cells. These studies identify platelet GP Ibα as a potential new target for anti cancer therapy.