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Introduction The major histocompatibility complex (MHC) class II invariant chain (CD74) is a protein which functions as a chaperone for MHC class II in antigen-presenting cells. Furthermore, it acts as a receptor for the cytokine macrophage migration inhibitory factor (MIF), mediating downstream signalling. Many clinical studies have determined that CD74 is overexpressed at the protein level in a subset of human breast tumours. However, there are discrepant findings regarding its correlation to clinical parameters. In some reports, CD74 has been correlated to triple-negative (TN) status and increased presence of metastases, but in separate studies it correlated with better overall survival. In vitro studies of CD74’s molecular functions in cancer cells have generally supported that it has a cancer-promoting effect through stimulating proliferation, survival and invasion. Material and methods Human breast cancer cell lines MDA-MB-231 and M4A4 were transfected with siRNA targeting CD74 or a negative control. Cells were then subjected to in vitro analysis of proliferation or invasion through matrigel. Lysates of transfected cells were analysed by immunoblot for protein levels of cell survival and autophagy markers. A tumour microarray (TMA) containing 651 human breast tumour cores was stained by immunohistochemistry (IHC) for CD74 protein expression. Staining intensity was manually scored for each sample in a blinded fashion. Statistical analysis was performed in R. Results and discussions siRNA knockdown of CD74 in MDA-MB-231 and M4A4 resulted in reduced proliferation and invasion. Furthermore, survival signalling through Akt was decreased. These findings replicate previously published in vitro studies of CD74’s functions in cancer cells. Furthermore, a novel finding was that CD74 knockdown resulted in reduced levels of markers of autophagy. The correlations between CD74 IHC staining of TMA samples and TN status, lymph node status and overall survival were examined. In contrast to previous studies we observed no correlation between expression of CD74 and TN status or cancer spread to the lymph nodes. Instead, survival analysis revealed increased overall survival for cancers with moderate or high CD74 intensity. Conclusion Our results from cell lines support that CD74, when studied in vitro, has functions that stimulate cancer cells to proliferate and invade. However, results from clinical samples show a correlation of CD74 expression with increased survival. |
