SYK promotes TGF-beta-induced P-body clearance in breast cancer cells through the enhancement of autophagy
SYK is a protein tyrosine kinase that plays an essential role in the development and activation of immune cells. Its expression, however, is not limited to immune cells. SYK is expressed in a variety of epithelial cell types and epithelial-derived tumors. Reports regarding the role of SYK expression in these diverse cell types and tumors have been opposing. In breast cancer, SYK expression has been overwhelmingly associated with tumor suppression. The loss of Syk expression is observed in invasive breast carcinoma tissue and cell lines and the reintroduction of Syk into metastatic breast cancer cells suppresses tumor growth and metastasis. A progressive decrease in the expression of SYK has also been observed in other tumors that progress from moderately to poorly differentiated phenotypes such as bladder cancer, hepatocellular carcinoma, and pancreatic carcinoma. SYK appears to repress epithelial to mesenchymal transition (EMT), a dedifferentiation process that contributes to the invasive and metastatic potential of cancer cells. The mechanism of how SYK inhibits EMT is not clear. Comprehensive proteomic screenings conducted in our lab identified a variety of messenger ribonucleoproteins (mRNPs) as SYK substrates. A number of these mRNPs are associated with processing bodies (P-bodies), mRNP granules that are associated with mRNA degradation. These interactions led me to hypothesize that Syk co-localizes with P-bodies. I found that Syk co-localizes with P-bodies in HEK293T cells. Moreover, I show tyrosine phosphorylation occurring in P-bodies in a Syk-dependent manner, suggesting that SYK is active when in these complexes. A previous study demonstrated SYK co-localization and promotion of stress granule clearance in MCF7 cells through autophagy. Because of the close relationship between stress granules and P-bodies, I suspected that Syk would have a similar role in P-bodies. I report that SYK promotes P-body clearance through autophagy, which could possibly influence the integrity of P-body functions in the cell. These findings prompted me to investigate the physiological role of P-bodies in breast cancer cells. I discovered that TGF-β, a known driver of EMT, promotes P-body formation in breast cancer cells and that this increase correlates with the induction of EMT. Also, when mRNA-decapping enzyme 1A, DCP1A, is over-expressed, which results in the formation of aberrant P-bodies, EMT is blocked. In my report, I show that SYK expression and inhibition of mechanistic target of rapamycin, mTOR, by rapamycin both promote P-body clearance. The inhibition of either SYK or autophagy results in the accumulation of P-bodies in cells by preventing their removal. These results are consistent with the report of SYK promoting autophagy. Moreover, the induction of EMT is accompanied by SYK loss and autophagy deficiency. However, when mesenchymal cells are allowed to recover in the absence of TGF-β and revert back to an epithelial state (mesenchymal to epithelial transition, MET), SYK is re-expressed and autophagy is restored. The inhibition of either SYK activity or autophagy prevents MET. These findings suggest that TGF-β induces P-body formation in breast cancer cells to promote EMT and that, for MET to occur, P-bodies must then be removed. During EMT, the expression of SYK is downregulated and, consequently, autophagic activity is attenuated. For cells to revert back to an epithelial morphology, SYK is re-expressed, autophagy is induced, and P-bodies are cleared.^
Robert L. Geahlen, Purdue University.