Oleanolic acid (OA) is a natural compound that can be found in a number of edible and medicinal plants and confers diverse biological actions. However, the direct target of OA in human
tumor cells remains poorly understood, preventing its application in clinical and health settings. A previous study revealed that overexpression of
caveolin-1 in human
leukemia HL-60 cells can increase its sensitivity to OA. The present study aimed to investigate the effects of OA on the
doxorubicin-resistant human
breast cancer MCF-7 cell line (MCF-7/DOX),
harringtonine-resistant human
leukemia HL-60 cells (HL-60/HAR) and their corresponding parental cell lines. Western blotting was performed to measure
protein expression levels, whilst Cell Counting Kit-8 (CCK-8) assays, cell cycle analysis (by flow cytometry) and apoptosis assays (with
Annexin V/PI staining) were used to assess drug sensitivity.
CCK-8 assay results suggested that MCF-7/DOX cells, which overexpress the
caveolin-1 protein, have similar OA susceptibility to their parent line. In addition, sensitivity of MCF-7/DOX cells to OA was not augmented by knocking down
caveolin-1 using RNA interference. HL-60/HAR cells exhibited a four-fold increased sensitivity to OA compared with that in their parental HL-60 cells according to
CCK-8 assay. Both of the resistant cell lines exhibited higher numbers of cells at G1 phase arrest compared with those in their parent lines, as measured via flow cytometry. Treatment of both MCF-7 cell lines with 100 µM OA for 48 h induced apoptosis, with increased effects observed in resistant cells. However, no PARP-1 or
caspase-3 cleavage was observed, with some positive
Annexin V staining found after HL-60/HAR cells were treated with OA, suggesting that cell death occurred via non-classical apoptosis or through other cell death pathways. It was found that OA was not a substrate of
ATP-binding cassette subfamily B member 1 (ABCB1) in drug-resistant cells, as indicated by the accumulation of
rhodamine 123 assessed using flow cytometry. However,
protein expression of ABCB1 in both of the resistant cell lines was significantly decreased
after treatment with OA in a concentration-dependent manner. Collectively, these results suggest that OA could reduce
ABCB1 protein expression and induce G1 phase arrest in multidrug-resistant
cancer cells. These findings highlight the potential of OA for
cancer therapy.