Expression of YKL-40 by peritumoral macrophages in human small cell lung cancer

YKL-40 is a 40 kDa protein with possible involvement in tissue remodeling, cell proliferation and angiogenesis. Elevated serum YKL-40 levels in patients with metastatic cancers (including small cell lung cancer (SCLC)) are associated with poor prognosis. The aim of this study was to identify the cellular source of YKL-40 in SCLC patient biopsies and in a panel of 20 human SCLC lines cultured in vitro and in vivo in nude mice. In general, the SCLC cell lines had no or very limited (human) YKL-40 expression, whereas, by RT-PCR a pronounced murine (i.e., stromal) YKL-40 expression was present in all tumors. YKL-40 mRNA transcripts were detected by in situ hybridization in 9 of 10 biopsies from SCLC patients, and in each case the signal was localized in the peritumoral stroma in cells of typical macrophage morphology (confirmed by a CD68 macrophage specific stain). No YKL-40 mRNA expression was found in the cancer cells, in macrophages infiltrating the solid tumor areas, or in non-malignant tissue. In conclusion, the predominant source of elevated serum YKL-40 in SCLC is peritumoral macrophages.