Corosolic acid enhances the antitumor effects of chemotherapy on epithelial ovarian cancer by inhibiting signal transducer and activator of transcription 3 signaling

Abstract

Resistance to chemotherapy poses a serious problem for the treatment of advanced epithelial ovarian cancer patients. The mechanisms of chemoresistance are complex and studies have implicated signal transducer and activator of transcription 3 (STAT3) signaling in the chemoresistance of cancer cells. The present study investigated whether corosolic acid (CA), which has been previously reported to be a STAT3 inhibitor, was able to increase the sensitivity to chemotherapeutic drugs in epithelial ovarian cancer cells. CA also markedly enhanced the anticancer effect of paclitaxel, cisplatin and doxorubicin. In addition, CA abrogated the cell-cell interactions between macrophages and epithelial ovarian cancer cells and inhibited the macrophage-induced activation of epithelial ovarian cancer cells. These data indicated that CA was able to reverse the chemoresistance of epithelial ovarian cancer cells and suppress the cell-cell interaction with tumorigenic macrophages. Thus, CA may be useful as an adjuvant treatment to patients with advanced ovarian and other types of cancer due to the multiple anticancer effects.

Keywords: corosolic acid; macrophage; ovarian cancer; signal transducer and activator of transcription 3.

Figure 1

Effect of CA on the proliferation of ovarian carcinoma cells. The ovarian carcinoma cells (SKOV3, RMG-1, and ES-2) were incubated with the indicated concentrations of CA for 48 h, followed by (A) determination of cell viability and (B) cell cytotoxicity, by WST-8 assay and LDH assay, respectively (as described in Materials and methods). (C) The ES-2 cells were incubated with the indicated concentrations of CA for 5 h, followed by the determination of cell apoptosis by TUNEL staining (as described in Materials and methods). Data are presented as the mean ± SD. ^*P<0.01 and ^**P<0.001 vs. the control. CA, corosolic acid; DAPI, 4′,6-diamidino-2-phenylindole; TUNEL, terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling; LDH, lactate dehydrogenase.

Figure 2

Effect of CA on STAT3 activation of ovarian carcinoma cells. Ovarian carcinoma cells, (A) SKOV3, (B) RMG-1 and (C) ES-2, were incubated with the indicated concentrations of CA for 24 h, followed by determination of pSTAT3, STAT3 and β-actin expression by western blot analysis (as described in Materials and methods). CA, corosolic acid; STAT3, signal transducer and activator of transcription 3; pSTAT3, phosphorylated signal transducer and activator of transcription 3.

Figure 3

Combined effect of CA and anticancer drugs on the proliferation of ovarian carcinoma cells. Ovarian carcinoma cells, (A) SKOV3, (B) RMG-1 and (C) ES-2, were incubated with 10 μM anticancer drugs, PTX, CDDP and DOX, concurrently with or without 20 μM CA for 24 h. Cell viability was then determined using a WST-8 assay (as described in Materials and methods). Data are presented as the mean ± SD. ^*P<0.01 vs. the control. CA, corosolic acid; PTX, paclitaxel; CDDP, cisplatin; DOX: doxorubicin.

Figure 4

Combinational effect of CA and anticancer drugs on STAT3 activation of ovarian carcinoma cells. The ovarian carcinoma cells, (A) SKOV3, (B) RMG-1 and (C) ES-2, were incubated with 20 μM CA and/or 10 μM paclitaxel for 24 h, followed by determination of pSTAT3, STAT3 and β-actin expression by western blot analysis (as described in Materials and methods). CA, corosolic acid; STAT3, signal transducer and activator of transcription 3; pSTAT3, phosphorylated signal transducer and activator of transcription 3.

Figure 5

Effect of CA-treated macrophages on STAT3 activation and cell proliferation in epithelial ovarian carcinoma. (A) HMDM were treated with 20 μM CA for 24 h and the SKOV3 cells were incubated with CA-treated macrophages for 24 h, followed by determination of BrdU-positive cells by immunohistochemistry (as described in Materials and methods). (B) HMDM were treated with 20 μM CA for 24 h, while the SKOV3 cells were incubated with CA-treated macrophages for 24 h, followed by determination of STAT3 activation by immunohistochemistry (left) and western blot analysis (right) (as described in Materials and methods). CA, corosolic acid; STAT3, signal transducer and activator of transcription 3; HMDM, human monocyte-derived macrophages; Mø, macrophage.