Blockade of PDGFR family together with SRC leads to diminished proliferation of CRC cells

Aims: Among the family of receptor tyrosine kinases (RTKs) the platelet-derived growth factor receptor (PDGFR) has attracted increasing attention as potential target of anti-tumor therapy in colorectal cancer (CRC).

Methods/Results: Varying expression of the PDGFRβ was demonstrated in CRC cell lines. SW480 cells showing the highest PDGFRβ expression were used for receptor down-regulation by siRNA and by the pharmacological inhibitor of PDGFRβ Ki11502. Blockade of PDGFRβ using both approaches led to a moderate inhibition of cell proliferation and diminished the PDGF-BB-induced phosphorylation of the downstream signaling pathway AKT. Surprisingly, incubation with Ki11502 resulted in an arrest of SW480 cells in the G2 phase of the cell cycle, whereas the siRNA approach did not. To address this difference, we analyzed proliferation and cell cycle distribution in DLD-1 and Caco-2 cells either with down-regulated PDGFRβ expression or after Ki11502 treatment. Whereas DLD-1 cells (high c-KIT expression) showed a strong decrease of proliferation after treatment with Ki11502, this effect was not observed in the siRNA approach. Caco-2 cells (no c-KIT expression) did not respond at all. Hence, we reduced expression of c-KIT in SW480 and DLD-1 cells, but proliferation studies could not prove the involvement of c-KIT inactivation during Ki11502 treatment.

To identify the underlying cause of effectiveness of Ki11502 in CRC cell lines we applied a RTK activation antibody array on SW480 cells treated with siRNAs against c-KIT, PDGFRβ or both and two concentrations of Ki11502. This analysis revealed that SRC kinase is inactivated after Ki11502 treatment, but not after the siRNA approach. Further studies using the SRC-specific inhibitor PP2 showed that SRC inhibition upon treatment with the inhibitor Ki11502 is responsible for the observed effects of Ki11502 in SW480 CRC cells.

Conclusion: In summary our results demonstrate, that inhibition of PDGFRβ alone using siRNA has only moderate cellular effects in CRC cell lines, instead, multi-target inhibition of PDGFRβ, c-KIT and SRC, e.g. using Ki11502, represent a promising therapeutic intervention for the treatment of CRC.