WASPs, WAVEs, and cancer

Following investigation of WASP/WAVE family proteins in control of cellular motility and formation of cellular projec­tions in cancer cells, there has been a large focus on the role of WASP/WAVE family proteins in cancer cells and tissues. Several studies mentioned here have explored WASP/WAVE proteins in various types of cancers.

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WAVEs and melanoma

A study focused on WAVE subfamily activity in invasion and metastasis found higher levels of WAVE1 and WAVE2 protein and Rac activity in highly invasive B16F10 mouse melanoma cells compared with non-metastatic B16 parental cells.55

RNA interference techniques were used to silence WAVE1 and WAVE2 in these malignant cells, and membrane ruffling, Rac-induced invasion, motility, and metastasis were found to be suppressed in response to WAVE2 knockdown, whereas WAVE1 knockdown appeared to have little effect on these cellular behaviors.

Further, WAVE2 appeared to be concentrated at the edges of the protrusions in migrating cells. Also, this study undertook in vivo studies to examine the effect of WAVE inhibition on metastasis.

Cells with sup­pressed WAVE2 activity, through RNA interference again, suppressed metastasis of B16F10 cells to the lungs when injected intravenously into female C57BL/6 mice, whereas WAVE1 RNA interference did not prevent colonization of the lungs by B16F10 cells.

This study suggested that WAVE2 is necessary for motility of B16F10 cells and that WAVE2 suppression may be an effective means to block invasion and metastasis.55

WAVE2 as a prognostic indicator

The value of using WAVE2 as a prognostic indicator for various cancers has been explored by many studies. WAVE2 expression has been examined in 112 hepatocellular car­cinoma tissues through immunohistochemistry, real-time polymerase chain reaction (RT-PCR), and Western blotting experiments to determine if any correlation exists between expression and prognosis.

WAVE2 expression was found to be elevated in hepatocellular carcinoma tissues and, impor­tantly, this correlated significantly with a poor prognosis of hepatocellular carcinoma.65

Further, coexpression of Arp2 and WAVE2 was studied by immunohistochemical staining on sections of 115 adenocarcinomas of the lung from differ­ent classes. Arp2 and WAVE2 were coexpressed in a signifi­cantly higher proportion of cancer cells from patients with lymph node metastasis but at a significantly lower level in bronchioloalveolar carcinomas, which have a less malignant character.

Arp2 and WAVE2 coexpression was also correlated with a poorer patient outcome, leading to a suggestion that coexpression of Arp2 and WAVE2 may be a phenotype of tumor cells that acquire invasive potential and that they have a role in the mechanism of cancer metastasis.66

This finding was supported by another study that examined the spatial expression of Arp2 and WAVE2 using immunohistochemistry in 154 colorectal cancer samples. Both Arp2 and WAVE2 expression was detected in 35.7% of the cancer cells in the cohort but no colocalization was detected in normal colonic epithelial cells.

Their comparison of expression levels and patient outcome determined that colocalization of Arp2 and WAVE2 was significantly predictive of liver metastasis in colorectal carcinoma.67

WAVE1 and WAVE3 in prostate cancer

Fernando et al investigated the expression of WAVE 1 and WAVE3 in prostate cancer cells and their role in metastasis and invasion.68,69 Immunohistochemical staining of prostate cancer tissue showed that there was higher staining intensity of WAVE1 and WAVE3 compared with normal prostate epi­thelial cells.

Knockdown of WAVE1 and WAVE3 was carried out using hammerhead ribozyme transgenes. These were transfected by electroporation into metastatic prostate cancer cell lines, DU-145 and PC-3, both of which showed high expression levels of WAVE1 and WAVE3 through RT-PCR prior to transfection. In vitro invasion and growth assays were used to study the impact of WAVE1 and WAVE3 knockdown on cell behavior.

Loss of endogenous WAVE1 and WAVE3 resulted in a significant reduction in the invasive ability of both prostate cancer cell types. Additionally, in both DU-145 and PC-3 cells, knockdown of WAVE1 and WAVE3 achieved a significant decrease in the growth rate after 120 hours when compared with wild-type cells.68,69

An independent group also carried out a study to deter­mine the effect of WAVE3 (labeled WAS protein family member 3 in that study) in prostate cancer metastasis,70 which supports many of the findings reported by Fernando et al.68

High expression levels of WAVE3 were observed in metastatic PC-3 and DU-145 prostate cancer lines through quantitative RT-PCR, and immunohistochemical analysis of high-grade human prostate cancer demonstrated increased levels of WAVE3 when compared with normal prostate epi­thelium.

Knockdown of WAVE3 was achieved using shRNA methods and a significant decrease in cell motility and inva­sion in vitro was seen through scratch wounding assays, which were only carried out using the DU-145 cell model, and Matrigel invasion assays.

Further, knockdown of WAVE3 expression reduced the soft agar colony-forming ability of prostate cancer cells, suggesting that WAVE3 might also have a role in establishment of tumors. The effect of WAVE3 knockdown on in vivo tumor growth and metastasis of pros­tate cancer cells was observed by injecting the knockdown models into the flanks of severe combined immunodeficiency disease (SCID) mice.

Tumor growth rate and tumor weight following a 3-week period were significantly decreased with less or no evidence of metastasis to the lungs when WAVE3 knockdown DU-145 and PC-3 cells, respectively, were implanted into SCID mice when compared with the control group.70

These findings demonstrate that WAVE3 may have a role in promoting motility, invasion and metastasis in prostate cancer cells and that a loss of WAVE3 function affects tumor development, so highlighting the importance of WAVE3 as a research target.