Aberrant expression of GTPase-activating protein ARAP1 triggers circular dorsal ruffles associated with malignancy in hepatocellular carcinoma Hep3B cells.

Abstract

Background: Circular dorsal ruffles (CDRs) are large and rounded membrane ruffles that function as precursors of macropinocytosis. We recently reported that CDRs form in Hep3B hepatocellular carcinoma (HCC) cells, but not in Huh7 and HepG2 HCC cells or LO2 cells, suggesting that an unknown molecular mechanism implicates CDRs in Hep3B malignancy through macropinocytosis uptake of excessive extracellular nutrients. In this study, we investigated the cellular role and the mechanism of CDRs in Hep3B cells by focusing on the GTPase-activating protein ARAP1.

Methods: ARAP1 knock-out (KO) cells were generated. Confocal microscopy and high-resolution scanning electron microscopy (SEM) were used for identification of the target proteins and structure analysis, respectively. Proteasome inhibitor MG132, mitochondrial function inhibitor CCCP, ARF1 inhibitor Golgicide A, and macropinocytosis inhibitor EIPA were used to investigate the molecular mechanism. Cell proliferation and Transwell migration/invasion assays were used to investigate the role of ARAP1 in cellular malignancy.

Results: ARAP1 was localized to CDRs, which had reduced size following ARAP1 KO. CDRs comprised small vertical lamellipodia, the expression pattern of which was disrupted in ARAP1 KO cells. Extracellular solute uptake, rate of cell growth, and malignant potential were attenuated in KO cells. ARAP1 was also localized to mitochondria in Hep3B cells but not in the control cell lines. Mitochondrial fission protein was increased in KO cells. CCCP treatment blocked CDRs in Hep3B cells but not in controls. Surprisingly, ARAP1 expression level in Hep3B cells was lower than in Huh7, HepG2, and LO2 cells. MG132 treatment increased the ARAP1 levels in Hep3B cells, but not in Huh7 cells, revealing that ARAP1 is actively degraded in Hep3B cells.

Conclusions: These results strongly suggest that the aberrant expression of ARAP1 in Hep3B cells modulates CDRs via mitochondrial function, thereby resulting in excess uptake of nutrients as an initial event in cancer development. Based on these findings, we propose that the molecular mechanisms underlying the formation of CDRs, focusing on ARAP1, may serve as an effective therapeutic target in some types of HCC and cancers.