Abstract
The emerging role of heparanase in tumor initiation, growth, metastasis, and chemoresistance is well recognized and is encouraging the development of heparanase inhibitors as anticancer drugs. Unlike the function of heparanase in cancer cells, very little attention has been given to heparanase contributed by cells composing the tumor microenvironment. Here we used a genetic approach and examined the behavior and function of macrophages isolated from wild-type (WT) and heparanase-knockout (Hpa-KO) mice. Hpa-KO macrophages express lower levels of cytokines (e.g., TNFα, IL1-β) and exhibit lower motility and phagocytic capacities. Intriguingly, inoculation of control monocytes together with Lewis lung carcinoma (LLC) cells into Hpa-KO mice resulted in nearly complete inhibition of tumor growth. In striking contrast, inoculating LLC cells together with monocytes isolated from Hpa-KO mice did not affect tumor growth, indicating that heparanase is critically required for activation and function of macrophages. Mechanistically, we describe a linear cascade by which heparanase activates Erk, p38, and JNK signaling in macrophages, leading to increased c-Fos levels and induction of cytokine expression in a manner that apparently does not require heparanase enzymatic activity. These results identify heparanase as a key mediator of macrophage activation and function in tumorigenesis and cross-talk with the tumor microenvironment.