Akebia saponin E, as a novel PIKfyve inhibitor, induces lysosome-associated cytoplasmic vacuolation to inhibit proliferation of hepatocellular carcinoma cells

Ethnopharmacological relevance: Hepatocellular carcinoma (HCC) is an aggressive malignancy with increasing mortality in China. Screening and identifying effective anticancer compounds from active traditional Chinese herbs for HCC are in demand. Akebia trifoliata (Thunb) Koidz, with pharmacological anti-HCC activities in clinical, has been shown in previous research. In the present research, we elucidated a potential anticancer effect of Akebia saponin E (ASE), which is isolated from the immature seeds of Akebia trifoliata (Thunb.) Koidz, and revealed that ASE could induce severe expanded vacuoles in HCC cells. But the potential mechanism of vacuole-formation and the anti-HCC effects by ASE remain uncover.

Aim of this study: To elucidate the potential mechanism of vacuole-formation and the proliferation inhibition effects by ASE in HCC cell lines.

Materials and methods: MTT assay, colony formation assay and flow cytometry were performed to detect cell viability. Immunofluorescence analysis was used to examine the biomarkers of endomembrane. Cells were infected with tandem mRFP-GFP-LC3 lentivirus to assess autophagy flux. RNA-seq was conducted to analyze the genome-wide transcriptional between treatment cell groups. In vitro PIKfyve kinase assay is detected by the ADP-Glo^TM Kinase Assay Kit.

Results: ASE could inhibit the proliferation of HCC with severe expanded vacuoles in vitro, and could significantly reduce the size and weight of xenograft tumor in vivo. Further, the vacuoles induced by ASE were aberrant enlarged lysosomes instead of autophagosome or autolysosomes. With cytoplasmic vacuolation, ASE induced a mTOR-independent TFEB activation for lysosomal biogenesis and a decrement of cholesterol levels in HCC cells. Furthermore, ASE could reduce the activity of PIKfyve (phosphoinositide kinase containing a FYVE-type finger), causing aberrant lysosomal biogenesis and cholesterol dyshomeostasis which triggered the expanded vacuole formation.

Conclusion: ASE can prospectively inhibit the kinase activity of PIKfyve to induce lysosome-associated cytoplasmic vacuolation, and may be utilized as an alternative candidate to treat human HCC.

Keywords: Akebia saponin; Cholesterol; Cytoplasmic vacuolation; Lysosome; PIKfyve.