Carcinoma-produced factors activate myeloid cells through TLR2 to stimulate metastasis

Abstract

Distant-site metastases are the leading cause of cancer-associated mortality. A study of the inflammatory microenvironment of cancer cells has identified versican, an extracellular matrix proteoglycan, as a possible factor in driving metastasis. It was isolated from the highly metastatic Lewis lung carcinoma cells as a potent inducer of macrophage activation. Versican, which is upregulated in many human tumours, stimulates bone marrow cells via the Toll-like receptor TLR2 to produce the inflammatory protein TNF-?, which enhances the spread of metastases. This points to versican and the other components of this signalling pathway as potential targets for antimetastatic intervention. Lung carcinoma cells were found to secrete the extracellular matrix proteoglycan versican. Versican directly activates the TLR2 receptor complex on macrophages, which in turn promotes tumour metastasis by producing TNF-?. Thus cancer cells utilize signalling pathways of the innate immune system to support metastatic spread. Metastatic progression depends on genetic alterations intrinsic to cancer cells as well as the inflammatory microenvironment of advanced tumours1,2. To understand how cancer cells affect the inflammatory microenvironment, we conducted a biochemical screen for macrophage-activating factors secreted by metastatic carcinomas. Here we show that, among the cell lines screened, Lewis lung carcinoma (LLC)3 were the most potent macrophage activators leading to production of interleukin-6 (IL-6) and tumour-necrosis factor-α (TNF-α) through activation of the Toll-like receptor (TLR) family members4 TLR2 and TLR6. Both TNF-α and TLR2 were found to be required for LLC metastasis. Biochemical purification of LLC-conditioned medium (LCM) led to identification of the extracellular matrix proteoglycan versican, which is upregulated in many human tumours including lung cancer5,6, as a macrophage activator that acts through TLR2 and its co-receptors TLR6 and CD14. By activating TLR2:TLR6 complexes and inducing TNF-α secretion by myeloid cells, versican strongly enhances LLC metastatic growth. These results explain how advanced cancer cells usurp components of the host innate immune system, including bone-marrow-derived myeloid progenitors7, to generate an inflammatory microenvironment hospitable for metastatic growth.