Neuroblastoma cells negative for CD44 possess tumor-initiating properties
CD44 has been linked to favorable prognosis in neuroblastoma and in the present study we investigate if it can be used to prospectively isolate neuroblastoma-initiating cells. To define the cancer-initiating properties of CD44 positive and negative cells, we FACS-sorted the SK-N-SH neuroblastoma cell line on the basis of CD44 expression and proceeded to phenotypically and molecularly characterize the two cell subpopulations. We found that CD44 defines two morphologically distinctive cell populations with different adhesion molecule profiles, and that CD44 negative cells expressed higher levels of the neuroblastoma-initiating cell marker CD24. When inoculated subcutaneously into NOD/SCID animals, the CD44 negative cells were capable of tumor formation and organ infiltration, clearly demonstrating an inverse correlation of CD44 expression and neuroblastoma metastases formation. Gene expression analysis revealed that CD44 defines molecularly discrete cell types with the CD44 negative cells expressing proteins associated with uncontrolled cell cycle progression, immune evasion and a reduced capacity to undergo apoptosis. Collectively, our findings show that CD44 negative neuroblastoma cells possess all the phenotypic and molecular features required for a cancer-initiating cell.