Abstract
Phenotypic heterogeneity of cancer cells is caused not only by genetic and epigenetic alterations but also by stochastic variation of intracellular signaling molecules. Using cells that stably express Förster resonance energy transfer (FRET) biosensors, we show here a correlation between a temporal fluctuation in the activity of Rac1 and the invasive properties of C6 glioma cells. By using long-term time-lapse imaging, we found that Rac1 activity in C6 glioma cells fluctuated over a timescale that was substantially longer than that of the replication cycle. Because the relative level of Rac1 activity in each cell was unaffected by a suspension-adhesion procedure, we were able to sort C6 glioma cells according to the levels of Rac1 activity, yielding Rac1(high) and Rac1(low) cells. The Rac1(high) cells invaded more efficiently than did Rac1(low) cells in a Matrigel invasion assay. We assessed the transcriptional profiles of Rac1(high) and Rac1(low) cells and performed gene ontology analysis. Among the 14 genes that were most associated with the term 'membrane' (membrane-related genes) in Rac1(high) cells, we identified four genes that were associated with glioma invasion and Rac1 activity by using siRNA knockdown experiments. Among the transcription factors upregulated in Rac1(high) cells, Egr2 was found to positively regulate expression of the four membrane-related invasion-associated genes. The identified signaling network might cause the fluctuations in Rac1 activity and the heterogeneity in the invasive capacity of glioma cells.
Highlights
Cancer cells that originate from a single cell acquire phenotypic heterogeneity because of genomic instability or heritable epigenetic changes (Lengauer et al, 1998; Shackleton et al, 2009)
We speculated that such a distribution of Rac1 activity among C6 glioma cells might be autonomously generated during cell growth and spreading
Even after the substantial change during cell division, Rac1 activity returned to a level as before cell division, suggesting that the level of Rac1 activity is maintained by a mechanism that is unaffected by cell division (Fig. 2D)
Summary
Cancer cells that originate from a single cell acquire phenotypic heterogeneity because of genomic instability or heritable epigenetic changes (Lengauer et al, 1998; Shackleton et al, 2009). This heterogeneity is advantageous for the progression of cancer and promotes its highly invasive nature in tissues (Heppner, 1984; Rubin, 1990; Shackleton et al, 2009). Heterogenous signaling patterns in monoclonal cancer cells can generate cells with diverse phenotypes, which have different drug sensitivities (Singh et al, 2010). Glioblastomas formed from C6 glioma cells implanted into syngeneic Wistar rats share many histological hallmarks with human glioblastoma and C6 glioblastoma cells preferentially migrate along neuronal fibers and through the perivascular space, a pattern which resembles the spread of human glioblastoma
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