The in vitro spheroid melanoma cell culture assay: cues on tumor initiation?

Abstract

Cancer stem cells (CSCs) represent malignant subpopulations that initiate and maintain tumorigenic growth in hierarchically organized tumors via their considerable capacity for self-renewal and differentiation. CSCs have been identified in several human malignancies, including human malignant melanoma. Perego and colleagues' report in this issue indicates that CSCs capable of melanoma initiation in serial human-to-mouse xenotransplantation assays may be contained both among spheroid melanoma cell cultures (melanospheres) and among adherent melanoma cultures upon in vitro expansion. These results challenge the utility of the melanosphere assay as a surrogate tool for CSC identification in human melanomas and underline the importance of molecularly defined malignant melanoma initiating cells for CSC-focused diagnostic and therapeutic investigations.

Figure 1. The gold standard assay for the study of cancer stem cells (CSCs)

A CSC is a cell within a tumor that possesses the capacity to undergo both self-renewing cell divisions and cell divisions that result in more differentiated cancer cell progeny. CSCs can be defined experimentally only by their ability to recapitulate the generation of a continuously growing tumor. The gold standard assay that fulfills this criterion is serial xenotransplantation at limiting dilution of marker-defined clinical cancer subpopulations into an orthotopic site of immunocompromised (typically nonobese diabetic/severe combined immunodeficient (NOD/SCID)) mice. In this assay, long-term self-renewal of CSCs is confirmed by serial xenotransplantation of prospectively reisolated CSC populations into secondary recipients. Differentiation of CSCs is typically demonstrated by their ability to generate tumor xenografts that reflect the cellular heterogeneity of the original patient tumor. Modifications to this assay system, including the exogenous addition of growth and extracellular matrix factors, may complicate biological readouts because they could enable non-CSCs to contribute to tumor growth.