Biology and clinical implications of CD133+ liver cancer stem cells

Hepatocellular carcinoma (HCC) is the most common primary malignant tumor of the liver, accounting for 80%–90% of all liver cancers. The disease ranks as the fifth most common cancer worldwide and is the third leading cause of all cancer-associated deaths. Although advances in HCC detection and treatment have increased the likelihood of a cure at early stages of the disease, HCC remains largely incurable because of late presentation and tumor recurrence. Only 25% of HCC patients are deemed suitable for curative treatment, with the overall survival at just a few months for inoperable patients. Apart from surgical resection, loco-regional ablation and liver transplantation, current treatment protocols include conventional cytotoxic chemotherapy. But due to the highly resistant nature of the disease, the efficacy of the latter regimen is limited. The recent emergence of the cancer stem cell (CSC) concept lends insight into the explanation of why treatment with chemotherapy often may seem to be initially successful but results in not only a failure to eradicate the tumor but also possibly tumor relapse. Commonly used anti-cancer drugs in HCC work by targeting the rapidly proliferating and differentiated liver cancer cells that constitute the bulk of the tumor. However, a subset of CSCs exists within the tumor, which are more resistant and are able to survive and maintain residence after treatment, thus, growing and self-renewing to generate the development and spread of recurrent tumors in HCC. In the past few years, compelling evidence has emerged in support of the hierarchic CSC model for solid tumors, including HCC. And in particular, CD133 has drawn significant attention as a critical liver CSC marker. Understanding the characteristics and function of CD133+ liver CSCs has also shed light on HCC management and treatment, including the implications for prognosis, prediction and treatment resistance. In this review, a detailed summary of the recent progress in CD133+ liver CSC research with regard to identification, regulation and clinical implications will be discussed.