GTP-dependent membrane fusion during hepatocarcinogenesis and liver regeneration

Taking organelles apart, putting them back together and creating new ones: lessons from the endoplasmic reticulum

The endoplasmic reticulum (ER) is a highly dynamic organelle. It is composed of four subcompartments including nuclear envelope (NE), rough ER (rER), smooth ER (sER) and transitional ER (tER). The subcompartments are interconnected, can fragment and dissociate and are able to reassemble again. They coordinate with cell function by way of protein regulators in the surrounding cytosol. The activity of the many associated molecular machines of the ER as well as the fluid nature of the limiting membrane of the ER contribute extensively to the dynamics of the ER. This review examines the properties of the ER that permit its isolation and purification and the physiological conditions that permit reconstitution both in vitro and in vivo in normal and in disease conditions.

Proteomic analysis of the transitional endoplasmic reticulum in hepatocellular carcinoma: an organelle perspective on cancer

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.

Modulation of GTP-dependent fusion by linoleic and arachidonic acid in derivatives of rough endoplasmic reticulum from rat liver

The effect of modulation of the content of unsaturated free fatty acids on GTP-dependent fusion of stripped rough microsomes from rat liver was determined. Cytidine monophosphate, CDP and CTP were all observed to be able to stimulate free fatty acid accumulation and coincident membrane fusion. GTP was required for membrane fusion in the presence of cytidine nucleotide but was not required for free fatty acid accumulation. In the presence of GTP and cytidine nucleotide, the addition of ATP and CoA led to the synthesis of triacyglycerol and marked inhibition of both free fatty acid accumulation and membrane fusion. Delipidated bovine serum albumin also inhibited both free fatty acid accumulation and membrane fusion. Analysis by gas chromatography indicated that linoleic acid and arachidonic acid were the most actively fluctuating of the accumulated free fatty acids. Comparison by quantitation indicated a high correlation between GTP-dependent membrane fusion and changes in amount of unesterified linoleic acid and arachidonic acid. The results suggest that polyunsaturated free fatty acids may be required for GTP-dependent membrane fusion.

Freeze-fracture analysis of the effects of intermediates of the phosphatidylinositol cycle on fusion of rough endoplasmic reticulum membranes

While searching for the identity of the effector of the putative GTP-binding protein involved in fusion of rough endoplasmic reticulum (RER) cell-free incubation conditions were found permitting fusion in a GTP-independent manner. Membrane fusion was obtained using medium required to study synthesis of phosphatidylinositol (PI). We now report on the effects of various co-factors and intermediates of the PI cycle on the interaction of rough microsomes. By freeze-fracture, fusion of rough microsomes was defined as the appearance of fracture-planes of membrane larger than those of unincubated membrane. Cytosine triphosphate (CTP, 3 mM) in the presence of 2 mM MnCl2 was most effective in stimulating fusion. Guanosine triphosphate (GTP) at the same concentration, could substitute for CTP to stimulate fusion, ATP, ITP, UTP and guanosine 5'-[gamma-thio]triphosphate (GTP gamma S) could not. When combined together in the same medium CTP potentiated the effect of GTP. Arachidonic acid (20 micrograms/ml) also stimulated fusion in the presence of MnCl2. This led to the appearance of large fracture-planes of membrane with a heterogeneous distribution of intramembranous particles. Other saturated fatty acids at the same concentration did not stimulate fusion. Phosphatidylinositol (PI, 50 micrograms) and 2 mM MnCl2 had a similar effect as arachidonic acid and MnCl2 in stimulating fusion. The PI effect was largely augmented in the presence of CTP. Our results are consistent with the concept that metabolism of phospholipids may modulate GTP-dependent fusion of RER membranes.

Accumulation of polyunsaturated free fatty acids coincident with the fusion of rough endoplasmic reticulum membranes

The accumulation of polyunsaturated free fatty acids (PUFAs) was observed coincident with GTP-dependent fusion of liver rough microsomes. Whereas 0.5 mM NADPH led to a parallel reduction (greater than 50%) in membrane fusion and PUFA accumulation, indomethacin (50 microM) either had little effect or slightly augmented both processes. CTP was observed to stimulate accumulation of PUFAs and diacylglycerol (DAG). Therefore PUFAs may be relevant for GTP-dependent membrane fusion and together with DAG may play a role in fusion stimulated in the presence of CTP.

cAMP-dependent phosphorylation of RER proteins from rat liver: relationship with GTP-dependent membrane fusion

Incubation of stripped rough microsomes (SRM) with the catalytic subunit of protein kinase A (PKA) permitted specific phosphorylation of seven proteins having relative molecular mass values of 55, 35, 23, 22.5, 22, 18.5 and 16.5 kDa (P55, P35 etc.). By two dimensional gel analysis, we compared these phosphoproteins with low-molecular-weight GTP-binding proteins and revealed that P23 and P22.5 co-migrated with known GTP-binding proteins. Next we examined the effect of cAMP-dependent phosphorylation on a GTP-dependent membrane function, membrane fusion. Quantitative analysis indicated no difference in the amount of membrane fusion obtained whether SRM were incubated in the absence or in the presence of PKA. Thus several rough microsomal proteins underwent cAMP-dependent phosphorylation and this post-translational modification did not affect GTP-dependent membrane fusion in a cell free system.