The utility and limitations of glycosylated human CD133 epitopes in defining cancer stem cells

Abstract

Human CD133 (human prominin-1), a five transmembrane domain glycoprotein, was originally identified as a cell surface antigen present on CD34+ hematopoietic stem cells. Although the biological function of CD133 is not well understood, antibodies to CD133 epitopes have been widely used to purify hematopoietic stem and progenitor cells. The cancer stem cell (CSC) hypothesis postulates that a rare population of tumor cells possessing increased capacities for self-renewal and tumor initiation is responsible for maintaining the growth of neoplastic tissue. The expression of the CD133 epitopes, AC133 and AC141, has been shown to define a subpopulation of brain tumor cells with significantly increased capacity for tumor initiation in xenograft models. Following the discovery of the AC133/AC141+ population of brain tumor stem cells, the AC133 and AC141 epitopes have been extensively used as markers for purifying CSCs in other solid tumors. There are, however, several issues associated with the use of the AC133 and AC141 CD133 epitopes as markers for CSCs. The antibodies routinely used for purification of AC133 and AC141-positive cells target poorly characterized glycosylated epitopes of uncertain specificity. Discordant expression of the AC133 and AC141 epitopes has been observed, and the epitopes can be absent despite the presence of CD133 protein. In addition, CD133 expression has recently been shown to be modulated by oxygen levels. These factors, in combination with the uncertain biological role of CD133, suggest that the use of CD133 expression as a marker for CSCs should be critically evaluated in each new experimental system and highlight the need for additional CSC surface markers that are directly involved in maintaining CSC properties.

Fig. 1

Diagram of predicted topology of CD133 and issues concerning the use of AC133 and AC141 mAbs to monitor CD133 expression. a Predicted topology of the mature CD133 polypeptide is shown as well as binding of AC133 and AC141 mAbs to glycosylated epitopes on the extracellular region of CD133. The N-terminal signal peptide (aa 1–19) is not drawn. The epitope location shown in the diagram is hypothetical, as the exact location of the epitopes bound by AC133 and AC141 has not been described. However, the AC133 and AC141 mAbs are known to bind spatially distinct epitopes. Only one of the epitopes is shown. Cylinders in the diagram indicate transmembrane regions. b Potential for false negative data with AC133 and AC141 mAbs. If nonglycosylated CD133 is present, it will not be detected by the AC133 and AC141 mAbs. c Potential for false positive data with AC133 and AC141 mAbs. Given that the nature of the glycosylated AC133 and AC141 epitopes are poorly defined, it is formally possible that glycosylated epitopes on extracellular molecules other than CD133 could cross-react with the AC133 or AC141 mAbs