Rac GTPases in prostate cancer extravasation across bone marrow endothelium
Date
2012
Authors
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Publisher
University of Delaware
Abstract
Prostate cancer metastasis to bone is the leading cause of morbidity and mortality in men with this disease. One of the crucial steps in the metastatic cascade is the extravasation of the cancer cell across bone marrow endothelium into the bone stroma which can be compared to leukocyte diapedesis across vasculature during inflammation. Rho GTPases are monomeric small GTPases that regulate cytoskeleton and have been shown to play a vital role in prostate cancer metastases by regulating cell morphology and motility. Over expression of RhoC has been reported in prostate cancer cell line PC-3 where it was shown to be important for invasion. Our lab has previously shown that down regulation of RhoC in PC-3 cells results in the morphological changes reminiscent of epithelial to mesenchymal transition with changes in lamellopodia formation attributed to increased activation of Rac GTPases. At the same time, down regulation of Rac GTPase using siRNA resulted in decreased ability of PC-3 to undergo diapedesis across bone marrow endothelium suggesting its role during this process. Rac GTPase branch of Rho subfamily of monomeric GTPases is comprised of four members i.e. Rac1, Rac2, Rac3 and RhoG. The specific roles played by these individual Rac isoforms in prostate cancer skeletal metastases has not been studied forming the basis of the current study and the aim was to identify specific roles played by these isoforms in PC-3 diapedesis across bone marrow endothelium . Rac1 GTPase was found to be the predominant Rac-isoform in PC-3 cells and is important in mediating binding interactions with BMECs. It was found that Rac 1 can be activated through direct activation of Rac GEFs or indirectly through hierarchical activation of RhoG. Chemokine CCL2 secreted by BMEC can also cause activation of Rac 1 through activation of PCNT1, a novel actin regulating protein, which sequentially interacts with Rho GEFs and DOCK-180 ELMO. Rac1 in BMECs presumably facilitates expression of ICAM-1 levels and interactions with ß1 integrin on PC-3. Reciprocal relationship between Rac 1 and Rac3 was noted in that down regulation of Rac3 increased Rac1 activation levels in PC-3 . Rac 1 was found to promote PC-3 diapedesis and its down regulation led to decrease in diapedesis while down regulation of Rac 3 resulted in an increase in diapedesis presumably through increase in Rac1 levels. Rac-1 maintains BMEC monolayer integrity and its down regulation results in increase in permeability while down regulation of Rac3 or RhoG does not affect permeability of BMEC. PC-3/endothelial interactions appear to be endothelium specific with decreased permeability observed on interactions with BMEC while no affect on permeability was noted on interaction with MDCK cells.