Platelet Glycoprotein VI Contributes to Murine Experimental Metastasis

A key receptor supporting the normal role of platelets in hemostasis and thrombosis is the collagen receptor, glycoprotein (GP)VI. GPVI is a member of the immunoglobulin super family and is uniquely expressed on the surface of platelets where it is assembled with the ITAM-bearing activation subunit, FcR-g. We have previously reported the generation of a murine model of GP VI deficiency that revealed profound defects in collagen-induced platelet aggregation and major defects in platelet activation following adhesion under flow to fibrillar collagen. More recently, we have generated congenic GPVI deficient animals through an extensive breeding scheme to the inbred C57BL/6J strain. The relevance of specific platelet receptors in experimental metastasis is emerging with work from our laboratory establishing a key role for the platelet glycoprotein Ib- IX complex. We have now performed similar studies using congenic GPVI deficient animals. In this experimental model, murine tumor cells are placed in the tail vein of mice and establishment of lung tumor burden or lung tumor foci is examined two weeks following injection. This model represents a syngeneic model where animals are fully immunocompetent and the injected tumor cells were originally derived from animals of the C57BL/6J strain. Experiments comparing B16F10.1 cells (murine melanoma) and D121 cells (murine Lewis lung carcinoma) have been performed revealing a consistent and statistically significant reduction in tumor foci as a consequence of GPVI absence. In the case of melanoma cell injections into wild type C57BL/6J mice, a mean of 270 foci (SEM 44.0, n=6) is reduced to a mean of 134 foci (SEM 11.6, n=7) in the absence of platelet GPVI (p = 0.013). Similar reductions were observed using Lewis lung carcinoma cells with wild type animals revealing a mean of 132 surface foci (SEM 13.4, n=15) and GPVI deficient animals having a mean of 69 foci (SEM 7.9, n=12; p = 0.0003). Using either cell line an approximate reduction of 50% in the number of visible tumor foci was observed. Additional studies have been performed to compare the size and growth rate of subcutaneously implanted tumor cells, i.e., primary tumor growth. Here, we observed no noticeable size difference in primary tumors harvested 17 days following subcutaneous injection of D121 cells comparing the presence or absence of platelet GPVI. These results demonstrate in the platelet GPVI facilitates experimental tumor metastasis but does not contribute in the growth of primary tumors. These studies underscore the well-established paradigm of platelet adhesion and activation in hemostasis and thrombosis being applicable to mechanisms participating in the spread of cancer. The importance of metastasis in the prognosis for recovery from cancer can not be under emphasized. Indeed, the spread of metastatic disease represents a fundamental change in significantly shortening the life span of the cancer patient. Thus, understanding the molecules that regulate metastasis identifies potential targets for therapeutic intervention that could significantly improve the patient’s prognosis.