LPA is present at high levels in ovarian cancer patients’ ascites (22
) and can potently stimulate ovarian cancer cell migration (8
). In this study, we found that only those ovarian cancer cell lines with LPA migratory response undergo peritoneal metastatic colonization (), suggesting a potential link between LPA-stimulated cell migration and ovarian cancer metastasis. Although recent studies indicated that multiple LPA receptors may contribute to LPA-stimulated cell migration (21
), we showed that silencing LPAR1
largely abrogated both LPA-stimulated cell migration and metastatic colonization (). The discrepancy may be caused by the means to silence LPA receptors as synthetic siRNAs were used in these early studies while we used shRNAs for LPAR1
knockdown. A recent study reports that direct administration of LPA in vivo
led to enhanced ovarian cancer metastasis and reduced animal survival (36
). Enforced LPA receptor expression has also been shown to increase ovarian cancer aggressiveness in xenograft model (35
). These findings all support the notion that LPA migratory response is a prerequisite for ovarian cancer metastasis.
Cell migration requires reorganization of actin cytoskeleton that is regulated by the members of Rho GTPases including Rac (37
). We show that LPA only induces cytoskeleton reorganization in metastatic ovarian cancer cells (), indicating that members of Rho GTPase family may not be properly activated by LPA in non-metastatic cells. This possibility is substantiated since LPA only significantly activates Rac in metastatic cells (). Inhibiting Rac activity abolished LPA-stimulated cell migration and peritoneal metastatic colonization of metastatic cells while constitutively active Rac1 confers non-metastatic cells with LPA migratory response and the ability to metastasize (). Rac activation has been shown to correlate with hepatocellular carcinoma and breast cancer metastasis (38
). Our findings extend the importance of Rac activation into ovarian cancer metastasis.
Tiam1, βPIX and SOS1/EPS8/ABI1 tri-complex can mediate Ras-induced Rac activation (9
). We show that silencing any member of SOS1/EPS8/ABI1 tri-complex, but not Tiam1 or βPIX, abrogates LPA or Ras-induced Rac activation, cell migration and peritoneal metastatic colonization (). SOS1/EPS8/ABI1 tri-complex has been shown to mediate EGF or PDGF-induced Rac activation and actin remodeling (40
). This complex is also involved in PI3K-induced Rac activation (41
). Our studies present another example of the usage of SOS1/EPS8/ABI1 tri-complex for Rac activation. Our observation with ovarian cancer cells is different from an early reports where LPA is shown to activate Rac through Tiam1 in murine fibroblasts (32
). This difference may be attributed to species and cell lineage difference.
As silencing any member of SOS1/EPS8/ABI1 tri-complex is sufficient to diminish ovarian cancer cell migration and metastatic colonization, it indicates that the integrity of SOS1/EPS8/ABI1 tri-complex may determine ovarian cancer metastatic potentials. Indeed, metastatic lines express all three members of this tri-complex while at least one member is absent in non-metastatic lines (). Introducing the missing member in the respective line renders it capable of undergoing metastatic colonization (). EPS8 has been reported to promote fibrosarcoma and oral squamous carcinoma cell migration (42
). ABI1 can positively regulate breast cancer cell migration and invasion (44
). Moreover, depletion of SOS1 blocks prostate cancer cell migration and invasion (45
). Although only a single member was focused in these studies, they support an essential role of SOS1/EPS8/ABI1 tri-complex in cell migration and metastasis.
Recent studies indicate the clinical relevance of SOS1 and EPS8 expression in cancer progression. The level of SOS1 is elevated in prostate cancer specimens with high Gleason’s score (45
). EPS8 overexpression is often detected in advanced stage of thyroid cancer (46
), pancreatic cancer (47
), oral squamous carcinoma (43
) and pituitary tumors (48
). To our knowledge, no study has been reported on the status of ABI1 in clinical cancer specimens. In this study, we detected SOS1, EPS8 and ABI1 expression in clinical ovarian cancer specimens (). However, the expression of any individual member of SOS1/EPS8/ABI1 tri-complex alone is not statistically associated with any clinicopathological parameters of patients (Table S2
). Instead, we find that SOS1/EPS8/ABI1 co-expression is significantly correlated with advanced stage and shorter survival (, ). This observation is in excellent agreement with the findings generated from the established cell lines that intact SOS1/EPS8/ABI1 tri-complex is required for ovarian cancer metastasis.
Our study demonstrates the importance of SOS1/EPS8/ABI1 tri-complex in ovarian cancer metastasis. Although we show it with the established ovarian cancer cell lines that may not fully simulate clinical setting, the consistency seen in multiple cell lines, the convergence of loss-and gain-of-function findings, and especially the excellent correlation observed between SOS1/EPS8/ABI1 co-expression and advanced disease stage/shorter patient survival strongly argue against any confounding influence derived from our experimental studies. Our studies suggest that the integrity of SOS1/EPS8/ABI1 tri-complex determines ovarian cancer metastasis and that therapeutic approach may be developed by targeting this tri-complex.