Driving cancer cells to undergo cellular senescence has recently been proposed to be a novel mechanism to target for developing cancer therapeutics (1
). For example, pharmacological inhibitor of PTEN drives senescence and, consequently, inhibits tumorigenesis in vivo
in xenograft models of PTEN heterozygous prostate cancer cells (42
). Compared with apoptosis, therapeutics that drive cellular senescence are proposed to have less cytotoxic side effects (6
), which makes pro-senescence therapy attractive. Herein, we describe that restoration of Wnt5a signaling drives senescence of human EOC cells both in vitro
and in vivo
in an orthotopic mouse model of EOC ( and ). Restoring gene expression by gene therapy has had limited success. Therefore, restoring Wnt5a signaling via
exogenous ligand could prove to be an alternative approach. Interestingly, it has been previously reported that a Wnt5a-derived hexapeptide is sufficient to restore Wnt5a signaling both in vitro
and in vivo
in xenograft models of breast cancer (44
). It would be interesting to test whether the Wnt5a-derived hexapeptide will be sufficient to reconstitute Wnt5a signaling and drive senescence of EOC cells. Our data suggest that cellular senescence is a potential target for developing EOC therapeutics. In addition, these data imply that restoration of Wnt5a signaling represents a potential novel strategy to drive senescence of EOC cells.
This study is the first to demonstrate a role for Wnt5a in regulating senescence. We demonstrated that Wnt5a activated the senescence-promoting HIRA/PML pathway in human EOC cells ( and S4A
). In primary human cells, activation of HIRA/PML pathway is sufficient to drive senescence by facilitating epigenetic silencing of proliferation-promoting genes (such as E2F target genes) (19
). Herein, we reported for the first time that the key HIRA/PML senescence pathway can be reactivated to drive senescence of human cancer cells. Further studies are warranted to elucidate the molecular basis by which Wnt5a restoration and activation of HIRA/PML pathway drive cellular senescence in human EOC cells.
Interestingly, senescence induced by Wnt5a restoration in human EOC cells was independent of both the p53 and p16INK4a
tumor suppressors, which implies that EOC cells that lack p53 and p16INK4a
retain the capacity to undergo senescence via HIRA/PML pathway through suppressing the canonical Wnt signaling. This is consistent with previous reports showing that cancer cells that lack p53 and pRB retain the capacity to undergo senescence when treated with anticancer agents or ionizing radiation (6
). Notably, although the levels of total phosphorylated pRB were not decreased by Wnt5a, we observed a decrease in the levels of pRBpS780 that is mediated by cyclin D1/CDK4 (). Future studies will determine whether the decrease in pRBpS780 levels plays a role in regulating senescence of human EOC cells.
Expression of Wnt5a is altered in many types of cancers (45
). For example, in melanoma, Wnt5a overexpression correlates with cancer progression and a higher tumor stage (16
). However, in colorectal and esophageal squamous cell carcinomas, Wnt5a has been described to be a tumor suppressor and was frequently silenced by promoter hypermethylation (16
). Consistently, we also observed Wnt5a
promoter hypermethylation in a number of human EOC cell lines in which Wnt5a is downregulated ( and Table S1
). This result is consistent with the idea that Wnt5a
promoter hypermethylation contributes to Wnt5a downregulation in human EOC cells.
Wnt5a function is highly dependent on cellular context (45
). For example, the cellular Wnt receptor/co-receptor context dictates the downstream signaling pathways upon the binding of Wnt5a, which include activating non-canonical Wnt signaling or antagonizing canonical Wnt/β-catenin signaling (47
). These reports illustrate that Wnt5a expression and its resulting activity are cell type and context dependent. The Wnt receptor/co-receptor profile in EOC cells is currently unknown, and our future studies will elucidate the mechanism by which Wnt5a antagonizes Wnt/β-catenin signaling in human EOC cells. Regardless, our data show that Wnt5a downregulation is an independent predictor for overall survival in EOC patients. In contrast, two other studies showed that higher Wnt5a expression predicts poor survival in EOC patients (48
). The basis for this discrepancy remains to be elucidated. An explanation may be that our study included more cases than the other two studies (130 EOC cases in our study vs. 38 cases in the study by Badiglian et al
or 63 cases in the study by Peng et al
). It may also be due to the difference in the composition of Type I and Type II cases in this study compared to the other two studies. The vast majority of EOC cases in this study are of Type II high-grade serous subtypes. Consistently, our data showed that there is a difference in Wnt5a expression between Type I and Type II EOC (, p=0.005). Further, it has been demonstrated in microarray analysis that Wnt5a is expressed at lower levels in laser capture and microdissected high-grade serous EOC compared with normal primary HOSE cells (50
In summary, the data reported here show that Wnt5a is often expressed at lower levels in human EOCs compared with either normal human ovarian surface epithelium or fallopian tube epithelium. A lower level of Wnt5a expression correlates with tumor stage and predicts shorter overall survival in EOC patients. Reconstitution of Wnt5a signaling inhibits the growth of human EOC cells both in vitro and in vivo. In addition, Wnt5a reconstitution suppresses the proliferation-promoting canonical Wnt/β-catenin signaling in human EOC cells. Significantly, Wnt5a reconstitution drives cellular senescence in human EOC cells and this correlates with activation of the senescence-promoting HIRA/PML pathway. Together, our data imply that reconstitution of Wnt5a signaling to drive senescence of human EOC cells is a potential novel strategy for developing EOC therapeutics.