The cancer stem cell (CSC) hypothesis, despite the limitations of the currently available models and assays, has ushered in a new era of excitement in cancer research. The development of novel strategies for anti-tumour therapy relies on the use of biomarkers to identify, enrich, and/or isolate the cell population(s) of interest. In this context, various cell characteristics and antigen expression profiles are discussed as surrogate markers. The cell surface expression of the human prominin-1 (CD133) antigen, in particular of the AC133 epitope, is among those that have been most frequently studied in solid cancers, although no mechanism has yet been proposed to link CD133 expression with the CSC phenotype. Some inconsistencies between published data can be ascribed to different analytical tools as well as methodological limitations and pitfalls, highlighted in the present review. Therefore, a comprehensive overview on the current state of knowledge in this growing and exciting field with an emphasis on the most recent studies is presented. We highlight the link between the tumour microenvironment, tumour cell plasticity, and CD133 expression, and evaluate the utility of CD133 expression as a prognostic marker.

Single amino acid arginine deprivation is a promising strategy in modern metabolic anticancer therapy. Its potency to inhibit tumor growth warrants the search for rational chemo-and radio-therapeutic approaches to be co-applied. In this report, we evaluated, for the first time, the efficacy of arginine deprivation as anticancer therapy in three-dimensional (3D) cultures of human tumor cells, and propose a new combinatorial metabolic-chemo-radio-treatment regime based on arginine starvation, low doses of arginine natural analog canavanine and irradiation. A sophisticated experimental setup was designed to evaluate the impact of arginine starvation on four human epithelial cancer cell lines in 2D monolayer and 3D spheroid culture. Radioresponse was assessed in colony formation assays and by monitoring spheroid regrowth probability following single dose irradiation using a standardized spheroid-based test platform. Surviving fraction at 2 Gy (SF 2Gy ) and spheroid control dose 50 (SCD 50 ) were calculated as analytical endpoints. Cancer cells in spheroids are much more resistant to arginine starvation than in 2D culture. Spheroid volume stagnated during arginine deprivation, but even after 10 days of starvation, 100% of the spheroids regrew. Combination treatment, however, was remarkably efficient. In particular, pretreatment of cancer cells with the arginine-degrading enzyme arginase combined with or without low concentration of canavanine substantially enhanced cell radioresponse reflected by a loss in spheroid regrowth probability and SCD 50 values reduced by a factor of 1.5-3. Our data strongly suggest that arginine withdrawal alone or in combination with canavanine is a promising antitumor strategy with potential to enhance cancer cure by irradiation.Single amino acid arginine starvation is a new promising approach in the metabolic anticancer therapy. Phase I/II clinical studies showed that pharmacological deprivation of arginine with the recombinant arginine-degrading enzyme, arginine deiminase, was well tolerated by the organism and had antiproliferative effect against some types of tumors, e.g., human hepatocellular carcinomas 1-3 and metastatic melanomas. 4 Another arginine-degrading enzyme, recombinant human arginase I, is currently undergoing Phase I/II clinical trials (http://cme.cancer.gov/drugdictionary/?CdrID¼657226). However, the in vivo approbation and clinical studies of the proposed enzymotherapies revealed that this antitumor approach is not free of drawbacks. The limited spectrum of sensitive tumors seems one of them. It was shown that only tumors deficient in argininosuccinate synthetase (ASS), a rate-limiting enzyme of citrulline to arginine conversion in urea cycle, were sensitive to the treatment with recombinant arginine-degrading enzymes. [5][6][7][8][9] Another obstacle in the arginine deprivation-based anticancer approach is reappearance of therapy-resistant tumor clones due to the derepression of ASS. 10,11 It is assumed that these drawbacks can be overcome by the development of more e...