Hierarchical organization of prostate cancer cells in xenograft tumors:: The CD44+α2β1+ cell population is enriched in tumor-initiating cells

Prostate cancer cells are heterogeneous in their tumorigemcity. For example, the side population cells isolated from LAPC9 xenografts are 100 to 1,000 times more tumorigenic than the corresponding non-side population cells. Highly purified CD44(+) prostate cancer cells from several xenografts are also enriched in prostate cancer stem/progenitor cells. Because the CD44+ prostate cancer cell population is still heterogeneous, we wonder whether we could further enrich for tumorigenic prostate cancer cells in this population using other markers. Integrin alpha(2)beta(1) has been proposed to mark a population of normal human prostate stem cells. Therefore, we first asked whether the alpha 2 beta 1(+/hi) cells in prostate tumors might also represent prostate cancer stem cells. Highly purified (>= 98%) alpha 2 beta 1(+/hi) cells from three human xenograft tumors, Du145, LAPC4, and LAPC9, show higher clonal and clonogenic potential than the alpha 2 beta 1(-/lo) cells in vitro. However, when injected into the nonobese diabetic/severe combined immunodeficient (NOD/SCID) mouse prostate or s.c., the alpha 2 beta 1(+/hi) prostate cancer cells are no more tumorigenic than the alpha 2 beta 1(-/lo) cells. Immunofluorescence studies reveal that CD44 and alpha 2 beta identify an overlapping and inclusive population of prostate cancer cells in that similar to 70% of alpha 2 beta 1(+/hi) cells are CD44(+) and 20% to 30% of CD44(+) cells are distributed in the alpha 2 beta 1(-lo) cell population. Subsequently, we sorted CD44+2 alpha beta(1+/hu), CD44(+) alpha 2 beta 1(-/lo), and CD44(-)alpha 2 beta 1(+hi) and tumorigenicity experiments. The results revealed a hierarchy CD44+alpha 2 beta 1(-/lo) > CD44(-)alpha 2 beta 1(+/hi) >> CD44(-) alpha 2 beta 1(-/lo). These observations together suggest that prostate cancer cells are organized as a hirearchy.