CD44 is associated with proliferation, rather than a specific cancer stem cell population, in cultured canine cancer cells

BACKGROUND: The cancer stem cell hypothesis proposes that tumours are maintained by a population of cancer stem cells (CSC), which must be eradicated to prevent disease relapse after treatment. Cells expressing high levels of CD44 have been identified as candidate CSC in a variety of human tumours. This study sought to investigate CD44 expression and its potential as a CSC marker in canine cancer.

METHODS: CD44 expression in several canine cancer cell lines was determined by flow cytometry. Cells with low and high levels of CD44 expression were examined for differences in growth characteristics, colony forming ability, drug sensitivity and cell cycle profile.

RESULTS: CD44(High) cells demonstrated enhanced growth and colony forming capacity, under both adherent and low-density serum free ("tumoursphere") conditions. However, no difference in sensitivity to doxorubicin was seen between the two populations. Moreover, whilst most CD44(Low) cells were in resting or G(1) growth phase, an increased proportion of CD44(High) cells were in G(2)M phase of the cell cycle. Upon proliferation in culture, both populations gave rise to progeny with a full spectrum of CD44 expression.

CONCLUSION: CD44 expression is associated with proliferation in cultured canine cancer cells, but transient and fluctuating expression may limit its utility as a CSC marker.