Lumican is a member of the small leucine-rich proteoglycan (SLRP) family and participates in the maintenance of tissue structures and tumor growth. Neuroendocrine cell tumors (NETs) including carcinoid tumors and NE cell carcinomas (NECs) possess numerous neuroendocrine (NE) granules, and differences between these tumors are in terms of their biological and metastatic aggressiveness during tumor progression. The purpose of this study was to examine the expression of lumican in NETs, and to determine whether the presence of lumican may be associated with the growth of NETs by comparing its expression between carcinoid tumors and NECs. Immunohistochemically, the positivity rates of lumican expression in the cytoplasm of the tumor cells were 87.5% in carcinoid tumors and 37.5% in NECs. Those of lumican expression in the stroma adjacent to the tumors were 90.1% in carcinoid tumors and 79.2% in NECs. In situ hybridization analysis revealed the lumican mRNA expression in the cytoplasm of carcinoid tumor and NEC cells. Ultrastructurally, the lumican protein was observed in the rough endoplasmic reticulum and NE granules of NETs and interspaces of collagen fibers. Furthermore, RT-PCR analysis revealed the presence of lumican mRNA in NEC cell lines. These results indicate that the higher expression level of cytoplasmic lumican in carcinoid tumors than in NECs may play a role in the slow growth of these tumors.

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