We previously reported that human
carboxylesterase 1 (CES1), a
serine esterase containing a unique N-linked glycosyl
group at Asn79 (N79 CES1), is a candidate serological marker of hepatocellular
carcinoma (HCC). CES1 is normally present at low-to-undetectable levels
in normal human plasma, HCC tumors, and major liver cancer cell lines.
To investigate the potential mechanism underlying the suppression
of CES1 expression in liver cancer cells, we took advantage of the
low detectability of this marker in tumors by overexpressing CES1 in multiple HCC cell lines, including stable Hep3B
cells. We found that the population of CES1-overexpressing
(OE) cells decreased and that their doubling time was longer compared
with mock control liver cancer cells. Using interactive transcriptome,
proteome, and subsequent Gene Ontology enrichment analysis of CES1-OE cells, we found substantial decreases in the expression
levels of genes involved in cell cycle regulation and proliferation.
This antiproliferative function of the N79 glycan of CES1 was further
supported by quantitative real-time polymerase chain reaction, flow
cytometry, and an apoptosis protein array assay. An analysis of the
levels of key signaling target proteins via Western blotting suggested
that CES1 overexpression exerted an antiproliferative
effect via the PKD1/PKCμ signaling pathway. Similar results
were also seen in another HCC cell line (PLC/RFP/5) after transient
transfection with CES1 but not in similarly treated non-HCC cell lines
(e.g., HeLa and Tera-1 cells), suggesting that CES1 likely exerts
a liver cell-type-specific suppressive effect. Given that the N-linked glycosyl group at Asn79 (N79 glycan) of CES1 is
known to influence CES1 enzyme activity, we hypothesized that the
post-translational modification of CES1 at N79 may be linked to its
antiproliferative activity. To investigate the regulatory effect of
the N79 glycan on cellular growth, we mutated the single N-glycosylation site in CES1 from Asn to Gln (CES1-N79Q) via site-directed mutagenesis. Fluorescence 2-D difference
gel electrophoresis protein expression analysis of cell lysates revealed
an increase in cell growth and a decrease in doubling time in cells
carrying the N79Q mutation. Thus our results suggest that CES1 exerts
an antiproliferative effect in liver cancer cells and that the single N-linked glycosylation at Asn79 plays a potential regulatory
role. These functions may underlie the undetectability of CES1 in
human HCC tumors and liver cancer cell lines. Mass spectrometry data
are available via ProteomeXchange under the identifier PXD021573.