To clarify the role of stem cells in hepatocarcinogenesis, CD44 expression was investigated in mouse livers as well as embryonic cell lineages treated with diethylnitrosamine (DEN). Liver tumors induced by DEN were analyzed by immunohistochemisty for CD44. Liver tissues were sampled at 6, 24, and 48 hr after treatment with saline or DEN. Mouse embryonic stem cells (ESCs), hepatic progenitor cells (HPCs), and hepatocyte like cells (HCs), representing 0, 22, and 40 days of differentiation, respectively, were treated with DEN at four doses (0, 1, 5, and 15 mM, respectively) for 24 hr, after which CD44 expression levels were examined by relative quantitative real-time PCR. CD44 expression was weakly detected in tumor cells as well as in some hepatocytes surrounding the tumor cells. However, CD44 expression was not detected in liver tissue treated with DEN at early time points. The CD44 mRNA expression level was significantly different among cells treated with 5 mM DEN at day 22 (P<0.01) as well as 1, 5, and 15 mM DEN at day 40 (P<0.01) compared with control. Taken together, CD44 expression slightly increased in mouse DEN-induced tumors. Furthermore, expression of CD44 in embryonic cell lineages treated with various doses of DEN significantly differed among embryo stem cells and derived hepatic lineage cells. This suggests that CD44 expression may be modulated in the progeny of stem cells during their differentiation toward hepatocytes, and its expression may increase in the tumor stage but not during early carcinogenesis.

Cytokines are known to function as regulatory molecules that can be produced by virtually every nucleated cell type in the body, including lymphocytes, monocytes/macrophages, epithelial cells, fibroblasts, and many others. Cytokines include lymphocyte-derived factors (lymphokines), monocyte-derived factors (monokines), hematopoietic factors (colony-stimulating factors), connective tissue/ growth factors, and chemotactic chemokines. Cytokines released in response to infection can affect tumor development in different ways. When exposed to infectious agents, cytokines are secreted by sentinel cells, such as macrophages and dendritic cells. These cytokines include interleukin 1 (IL-1) and tumor necrosis factor-α, as well as others, such as IL-6, IL-12, and IL-18. When released in sufficient quantities, these molecules can cause inflammation. Chronic inflammation is highly associated with tumor initiation, promotion, and progression. In this article, we review the roles and mechanisms of cytokines in tumor development.