There are two particularly interesting aspects of the SUM-190 cell line beyond the interesting cell culture medium that we used to develop the cell line. The first is that this is the second cell line we developed derived from a patient with inflammatory breast cancer (the other being SUM-149). As a result, there are many papers in the literature with data from SUM-190 cells from investigators interested in inflammatory breast cancer. But the second thing, and the one that I find most interesting, is that the SUM-190 cell line has an amplification and overexpression of the HER2 oncogene, and yet, HER2 was not a hit in our shRNA screen. Now, one may think that this was just a false negative result, but I think it actually highlights two other important things. First, it demonstrates how we powered our shRNA screen to minimize false positives at the risk of having some false negatives, which at first glance HER2 could be. But, if one looks at the data (See the KEGG pathway engine for this) you’ll see that HER2 was not close to being called a hit in the screen. And, if you think that might have something to do with the library, note that HER-2 was a strong hit in the SUM-225 screen, as this is the other line we developed with an amplification and overexpression of HER2. Finally, the screen data accurately predicts that SUM-190 cells would be more resistant to a HER2 targeted drug than the SUM-225 cell line. As we showed in our recent OncoTarget paper, SUM-190 cells are 10-times more resistant to the HER2 specific kinase inhibitor CP-724,714 than are SUM-225 cells in which HER2 was a hit in the screen. The figure below is from the OncoTarget paper: (https://www.ncbi.nlm.nih.gov/pubmed/27153554)
The figure shows that whereas the SUM-52 and SUM-185 cell lines do not respond to this drug at any concentration, the SUM-190 cells were growth inhibited but at concentrations 10-fold higher than for the SUM-225 cells (note the log scale). Thus, HER2 is indeed active in these cells but they are naturally more resistant to HER2 targeted drugs than other HER2 amplified cell lines, and their resistance is enough to cause the gene to be negative in our shRNA screen. This is a good indication that the genes that do hit in our screen do so when their knock-down has a profound effect on the growth and survival of the cells. It is noteworthy that despite the fact that these HER2 amplified cells are relatively resistant to a HER2 kinase inhibitor, they still do exhibit the expression profile of a HER2-enriched breast cancer.
What is the mechanism for their resistance to the HER2 targeted drug? While we have not studied this in great detail ourselves yet, it is important to note that SUM-190 cells have a PIK3CA point mutation, and indeed, PIK3CA was a strong hit in our functional screen. These results suggest that PIK3CA activity is more important than HER2 activity in these cells, and indeed the PIK3CA specific inhibitor A66 was more effective at reducing AKT phosphorylation in these cells than the HER2 specific drug. And, not surprisingly, the PI3’K/AKT KEGG pathway maps with the SUM-190 hit genes represented shows the importance of genes in this pathway to control growth and survival of these cells.
Other noteworthy observations from the screen data can be seen in the ECM-Receptor interaction pathway and the Hippo Pathway. The ECM-Receptor Interaction pathway shows the importance of integrins in these cells, and the Hippo Pathway shows the importance of E-cadherin. SUM-190 cells are like several other SUM-lines in that they form tight colonies. Thus, homotypic cell-cell interactions mediated by E-cadherin are important to the growth and survival of these cells. This is very important because it highlights one of the few common cellular characteristics of inflammatory breast cancer; overexpression of E-Cadherin and a requirement of this protein for cell growth/survival. Inflammatory breast cancer cells typically invade lymphatics as chords or sheets of cells with tight cell-cell connections, which are mediated by E-Cadherin. With so much literature ascribing an importance of E-Cadherin loss to the malignant and metastatic potential of breast and other cancer cells, it is important for researchers to know that E-Cadherin overexpression is a common and important feature of the most malignant and metastatic form of breast cancer; inflammatory breast cancer.
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