Proteome analysis of human
hepatocellular carcinoma tissues was conducted using two-dimensional difference gel electrophoresis coupled with mass spectrometry. Paired samples from the normal and
tumor region of resected human liver were labeled with Cy3 and
Cy5, respectively while the pooled standard sample was labeled with Cy2. After analysis by the DeCyder software,
protein spots that exhibited at least a two-fold difference in intensity were excised for in-gel tryptic digestion and matrix-assisted
laser desorption/ionization-time of flight mass spectrometry. A total of 6 and 42
proteins were successfully identified from the well- and poorly-differentiated samples, respectively. The majority of these
proteins are related to detoxification/oxidative stress and metabolism. Three down-regulated metabolic
enzymes,
methionine adenosyltransferase,
glycine N-methyltransferase, and
betaine-homocysteine S-methyltransferase that are involved in the methylation cycle in the liver are of special interest. Their expression levels, especially,
methionine adenosyltransferase, seemed to have a major influence on the level of
S-adenosylmethionine (
AdoMet), a vital intermediate metabolite required for the proper functioning of the liver. Recent work has shown that chronic deficiency in
AdoMet in the liver results in spontaneous development of
steatohepatitis and
hepatocellular carcinoma, and hence the down-regulation of hepatic
methionine adenosyltransferase in our
hepatocellular carcinoma samples is in line with this observation. Moreover, when a comparison is made between the differentially expressed
proteins from our human
hepatocellular carcinoma samples and from the liver tissues of knockout mice deficient in
methionine adenosyltransferase, there is a fairly good correlation between them.