The transitional endoplasmic reticulum (tER) is composed of both rough and smooth ER membranes and thus participates in functions attributed to both these two subcellular compartments. In this paper we have compared the
protein composition of tER isolated from dissected liver
tumor nodules of
aflatoxin B1-treated rats with that of tER from control liver. Tandem mass spectrometry (MS),
peptide counts and immunoblot validation were used to identify and determine the relative expression level of
proteins. Inhibitors of apoptosis (i.e. PGRMC1,
tripeptidyl peptidase II),
proteins involved in ribosome biogenesis (i.e.
nucleophosmin,
nucleolin),
proteins involved in translation (i.e. eEF-2, and subunits of eIF-3),
proteins involved in
ubiquitin metabolism (i.e.
proteasome subunits, USP10) and
proteins involved in membrane traffic (i.e. SEC13-like 1, SEC23B,
dynactin 1) were found overexpressed in
tumor tER.
Transcription factors (i.e. Pur-beta, BTF3) and molecular targets for C-Myc and
NF-kappa B were observed overexpressed in tER from
tumor nodules. Down-regulated
proteins included
cytochrome P450 proteins and
enzymes involved in
fatty acid metabolism and in
steroid metabolism. Unexpectedly expression of the protein folding machinery (i.e.
calreticulin) and
proteins of the MHC class I
peptide-loading complex did not change.
Proteins of unknown function were detected in association with the tER and the novel
proteins showing differential expression are potential new
tumor markers. In many cases differential expression of
proteins in
tumor tER was comparable to that of corresponding genes reported in the Oncomine human database. Thus the molecular profile of
tumor tER is different and this may confer survival advantage to
tumor cells in
cancer.