WASPs, WAVEs, and breast cancer
As described, WASP/WAVE protein activity is not just important in the motility of healthy cells but has also been suggested to have a role in various types of cancer. A large number of studies have focused on breast cancer.
Numerous reports have identified WASP/WAVE family members in the control of the motile behavior of breast cancer cells and also as a prognostic indicator of breast cancer progression in patients. Some of the relevant findings are detailed below.
WASP and breast cancer
A recent study has identified that WASP has a role in regulating leukocyte-dependent breast cancer metastasis.71
This research group built on previous studies showing that the interaction between epidermal growth factor-secreting macrophages and colony-stimulating factor 1-secreting breast cancer carcinoma cells promotes invasion.72,73
They hypothesized that WASP deficiency may block the interaction between tumor-associated macrophages and breast carcinoma cells, and hence tumor progression toward invasion since chemotaxis of macrophages toward colony-stimulating factor 1 in patients with WAS, who have defective WASP activity, is abolished.74
The study showed that in order for macrophages derived from bone marrow and MTLn3 breast cancer carcinoma cells to adopt migratory phenotypes and coinvade a three-dimensional collagen matrix, WASP activity in the macrophages is required. Further, injection of fluorescent MTLn3 cells into the mammary glands of WASP-/- and WASP+/- mice found no difference in initial tumor growth.
However, metastasis to the lung was significantly reduced in mice that lacked WASP activity, a finding that was supported by studies in highly metastatic human breast cancer MDA-MB-231 cells and a transgenic model of mammary carcinogenesis.
This suggested that leukocyte-restricted WASP expression is required for metastasis of breast tumor cells to the lungs.
Overall, this study indicates a role of WASP signaling in motility, invasion, and intravasation, and hence metastasis, of breast cancer tumor cells possibly by regulation of epidermal growth factor shedding from tumor-associated macrophages.71
nWASP and breast cancer
nWASP has been implicated in the progression of mammary tumors by one group of researchers who interfered with endogenous nWASP levels in MTLn3 cells using shRNA techniques.
When these cells were injected into the mammary glands of SCID mice and rats, a significant reduction in the number of circulating tumor cells and lung metastases was observed when compared with the controls.
Further, in vivo time-lapse images of control and nWASP-deficient GFP-labeled cells in tumors demonstrated that there was a dramatic decrease in motile cells and formation of invadopodium-like protrusions in nWASP-deficient tumors compared with control tumors.
This study demonstrates that nWASP activity is involved in proteolytic protrusion formation, and consequently the invasion and metastasis to the lungs of MTLn3 cell tumors.63
A further study treated MDA-MB-231 breast cancer cells with the nWASP inhibitor,75 wiskostatin, which acts to stabilize the autoinhibited state of nWASP.76
A resulting decrease in motility of cells treated with the inhibitor was detected using electric cell-substrate impedance sensing, a method that detects the movement of cells over an electrode in real time. This study suggested that interactions between nWASP and the tight junction component, claudin-5, shown by immunoprecipitation and immunoblotting, may be involved in the motility of cancer cells.75
Interestingly, another study using immunohistochemical and quantitative RT-PCR analysis of human breast tissues has indicated that expression of nWASP in breast tumors is significantly lower than in normal background mammary tissues, and reduced expression of nWASP seems to be associated with a poor prognosis for patients.
Significantly lower levels (P,0.05) were seen in patients who developed metastatic disease or died from breast cancer. Further, induced overexpression of nWASP in MDA-MB-231 cells significantly reduced their invasiveness, motility, and growth, but showed a more adhesive phenotype.77
Although these studies suggest an opposing role of nWASP in the control of breast cancer cell motility, they both recognize that nWASP may be related to the behavior of breast cancer cells and progression of tumor formation.
The molecular mechanisms responsible for the changes in breast cancer cell motility, invasion, and migration related to nWASP activity are still being explored, but several interaction partners and activators of nWASP have already been implicated, such as Cdc42-interacting protein 4 and focal adhesion kinase, although it should be noted that these can work independently of the nWASP pathway.78,79
WAVE2 and breast cancer
The association of WAVEs with breast cancer has been examined in a study of the expression of each family member at the mRNA and protein levels in 122 human breast cancer samples and 32 normal breast tissue samples.
The levels of expression were also analyzed for any correlation with pathological and clinical outcome. Differential expression of the WAVE subfamily members in breast cancer tissues was shown in this study, and the main significant finding was that of high levels of WAVE2 in patients who subsequently died of breast cancer within a 10-year follow-up period.80
Several studies have highlighted the potential therapeutic value of targeting the WASP/WAVE family or molecules up/downstream of them in their respective signaling pathways in the inhibition of cancer.
Two research groups have recently investigated rhapontigenin and pterostilbene as potential inhibitors of breast cancer cell metastasis that may act through the WAVE2 pathway.81,82
Both studies showed that application of each treatment can suppress the migratory and invasive activity of MDA-MB-231 cells, which are highly metastatic, though scratch wound healing and Matrigel invasion assays. It was suggested that this effect was due to impairments in the Rac1-WAVE2 pathway, as a reduction in Rac1 activity and also decreased expression of downstream effectors of Rac, such as WAVE2, Arp2, and Arp3, are seen.
Further, Kim et al imply that treatment with rhapontigenin acts to inhibit invasive and migratory behavior in breast cancer cells through inhibition of formation of lamellipodia seen by observing morphological changes using immunofluorescence.81