Campbell and Moorehead BMC Cancer 2011, 11:480 http://www.biomedcentral.com/1471-2407/11/480 Page 10 of 13 Figure 6 Knockdown of PDGFRs inhibited invasion in recurrent cell lines. Matrigel coated chambers were used to assess cell invasion after PDGFR siRNA. Cells were transfected with siRNA and 24 h later, cells were lifted, resuspended in serum free media and added to the upper chamber of the matrigel insert; full serum media was added to the bottom well. After 24 h, cells passing through the matrigel coated membrane were fixed and stained with toluidine blue. Representative images for each condition are shown (a). Scale bar = 50 μm. The average number of invading cells per field of view was calculated and related back to respective controls (b). Results represent the average of 3 replicates. * = p < 0.05. matrix (ECM) invasion in these recurrent cells (Fig- ure 6a). Downregulation of PDGFRa in both cell lines resulted in a statistically significant decrease in inva- sion of approximately 70%, while PDGFRb knock- down inhibited invasion by approximately 40% (not determined to be statistically significant) (Figure 6b). Dual knockdown of both PDGFR isoforms resulted in an even greater increase in invasion inhibition compared to downregulation of either isoform alone (92% and 88% inhibition for RJ348 and RJ423 respectively). Discussion While IGF-IR-targeting drugs hold promise for the treatment of breast cancer, based on what is known of other such targeted therapeutics, resistance is sure to be a major roadblock for the treatment of certain patients. Our work on the MTB-IGFIR transgenic mice supports the hypothesis that some tumors, although initially dependent on IGF-IR signaling for growth, can become independent of IGF-IR signaling. We found that down- regulation of the IGF-IR transgene (through removal of doxycycline) in established mammary tumors led to the