Syngeneic murine ovarian cancer model reveals that ascites enriches for ovarian cancer stem-like cells expressing membrane GRP78

Patients with ovarian cancer are generally diagnosed at FIGO (International Federation of Gynecology and Obstetrics) stage III/IV, when ascites is common. The volume of ascites correlates positively with the extent of metastasis and negatively with prognosis. Membrane GRP78, a stress-inducible endoplasmic reticulum chaperone that is also expressed on the plasma membrane (^memGRP78) of aggressive cancer cells, plays a crucial role in the embryonic stem cell maintenance. We studied the effects of ascites on ovarian cancer stem-like cells using a syngeneic mouse model. Our study demonstrates that ascitesderived tumor cells from mice injected intraperitoneally with murine ovarian cancer cells (ID8) express increased ^memGRP78 levels compared with ID8 cells from normal culture. We hypothesized that these ascites-associated ^memGRP78⁺ cells are cancer stem-like cells (CSC). Supporting this hypothesis, we show that ^memGRP78⁺ cells isolated from murine ascites exhibit increased sphere forming and tumor initiating abilities compared with ^memGRP78^- cells. When the tumor microenvironment is recapitulated by adding ascites fluid to cell culture, ID8 cells express more ^memGRP78 and increased self-renewing ability compared with those cultured in medium alone. Moreover, compared with their counterparts cultured in normal medium, ID8 cells cultured in ascites, or isolated from ascites, show increased stem cell marker expression. Antibodies directed against the carboxy-terminal domain of GRP78: (i) reduce self-renewing ability of murine and human ovarian cancer cells preincubated with ascites and (ii) suppress a GSK3α-AKT/SNAI1 signaling axis in these cells. Based on these data, we suggest that ^memGRP78 is a logical therapeutic target for latestage ovarian cancer.