Significance of local and circulating cancer stem cells in human liver cancer

Abstract

Background: Cancer stem cells (CSCs) have been shown to be the source of many solid tumor types, including cell-line induced hepatocellular carcinoma (HCC). However, few studies characterize liver CSCs in the tumor specimens and blood samples from liver cancer patients. Here, we design the present study to investigate whether CSCs are present in tumor specimens and blood samples from liver cancer patients, and the characteristics of those cells, using CD90 as a potential marker. Materials and Methods: Fresh tumor specimens and blood samples were collected from normal control, patients with cirrhosis, dysplatic nodules and liver cancers, respectively. CD45-CD90+ and CD45-CD90- cells were isolated from tissue and blood samples from different sources, and injected into the liver of severe-combined immunodeficient (SCID)/Beige mice to determine tumorigenicity. CD90+ and CD90- cells were also sorted from two HCC cell lines, PLC and MHCC97L, respectively, and injected into the liver of BALB/c nude mice to determine tumorigenicity. Flow cytometry was used to evaluate the distribution of CD90+ cells in the primary tumor tissues and generated tumor xenografts. Immunohistochemical staining was used to assess the localization of CD90 expression in the tumor tissues. Results: CD45-CD90hi cells were only detectable in the tumor tissues from liver cancer patients, but not in the control tissues. In addition, CD45-CD90+ cells were detected in more than 90% of blood samples from liver cancer patients, but not from the controls. CD45-CD90+ cells from the tumor specimens and blood samples of liver cancer patients generated tumor nodules in the liver of immunodeficient mice, whereas CD45-CD90+ cells from the normal and cirrhotic livers, and CD45-CD90- cells from the tumor tissues did not. Immunohistochemical staining revealed scattered and clustered distribution of CD90+ cells in the tumor tissues, with both membrane and cytoplasmic localization. Transplantation of CD90+ cells isolated from the generated tumor xenografts into a second or subsequent a third batch of immunodeficient mice also induced tumor formation, and the proportion of CD90+ cells in the serially generated tumor xenografts remained steady. In HCC cell lines, only CD90+ cells generated tumor nodules in nude mice. Administration of anti-CD44 antibody induced CD90+ cells undergoing apoptosis in a dose dependent manner. Conclusion: The present study reveals that CD90 is a potential marker for liver CSCs. The presence of circulating CSCs reflects an aggressive characteristic of human liver cancer. The evidence that CD44 inhibition induced CSCs undergoing apoptosis suggests that CD44 could be a therapeutic target for liver CSCs.