Summary
An unexplained paradox of malignant melanoma is the apparent failure of the blood
within the tumor to clot despite the presence of multiple factors that should promote
blood clotting. Here we present histochemical evidence that human and murine melanomas
are extensively infiltrated by abundant mast cells. Because mast cells contain the
natural anticoagulant heparin, the present studies were aimed at defining the role
of mast cell heparin in preventing the blood from clotting within B16 melanoma grafts
in C57BL/6 J mice. Mice bearing B16 melanoma grafts were treated with non-specific
or specific inhibitors of mast cell heparin (protamine or heparinase, respectively).
After the drug treatment there was histologic and functional evidence of selective
thrombosis of the blood vessels within the protamine and heparinase treated melanoma
grafts. A similar, high degree of thrombosis was also observed in B16 tumors grown
in transgenic NDST-2 knockout mice bearing a targeted disruption in the gene coding
for mast cell heparin synthesis. The tumors grown in the protamine-treated animals
were significantly smaller than the tumors from control (untreated mice). By contrast,
the tumors treated with heparinase or grown in the NDST-2 knockout mice were significantly
larger than the tumors from control (untreated) mice. We conclude that the intrinsic
procoagulant properties of malignant melanoma are neutralized in vivo by the anticoagulant
properties of endogenous heparin produced by mast cells that naturally infiltrate
the tumor. Our results also suggest that thrombosis and hemostasis within melanoma
may play a complex role in modulating the growth of the tumor.
Keywords
Mast cell - heparin - cancer - coagulation