Biological membranes form an essential barrier between living cells and their external environments, as well as serve to compartmentalize intracellular organelles within eukaryotes. The latter includes membranes that envelope the nucleus, the outer and inner membranes of the mitochondria, membrane cisternae complex of the ER, Golgi apparatus, as well as lysosomes and secretory vesicles. Depending on their localizations in the whole organism and also within the cell, these membranes have different, highly specialized functions. Although 30% of naturally occurring proteins are predicted to be embedded in biological membranes, membrane proteomics is traditionally understudied due to difficulties in solubilizing, separating, and identifying membrane proteins. Given the importance of membrane proteins in the various cellular processes listed in this review, as well as the roles they play in diseases and their potential as drug targets, it is imperative that this class of proteins be better studied. With the recent advancement in technology, it is expected that some of the difficulties in membrane proteomics will be overcome, yielding new data on membrane proteins.

The activation of signal transducer and activator of transcription 3 (STAT3) has been linked with the proliferation, survival, invasion, and angiogenesis of a variety of human cancer cells, including hepatocellular carcinoma (HCC). Agents that can suppress STAT3 activation have potential for the prevention and treatment of HCC. In this study, we tested an agent, ␤-escin, for its ability to suppress STAT3 activation. We found that ␤-escin, a pentacyclic triterpenoid, inhibited both constitutive and interleukin-6-inducible STAT3 activation in a doseand time-dependent manner in HCC cells. The suppression was mediated through the inhibition of activation of upstream kinases c-Src, Janus-activated kinase 1, and Janus-activated kinase 2. Vanadate treatment reversed the ␤-escin-induced down-regulation of STAT3, suggesting the involvement of a tyrosine phosphatase. Indeed, we found that ␤-escin induced the expression of tyrosine phosphatase Src homology phosphatase 1 that correlated with the down-regulation of constitutive STAT3 activation. ␤-Escin also down-regulated the expression of STAT3-regulated gene products, such as cyclin D1, Bcl-2, Bcl-xL, survivin, Mcl-1, and vascular endothelial growth factor. Finally, ␤-escin inhibited proliferation and also substantially potentiated the apoptotic effects of paclitaxel and doxorubicin in HCC cells. Overall, these results suggest that ␤-escin is a novel blocker of STAT3 activation that may have potential in the suppression of proliferation and chemosensitization in HCC.Hepatocellular carcinoma (HCC) is one of the most common solid tumors, ranking fifth in incidence and third in mortality worldwide (Bruix et al., 2004). Although epidemiologic studies have shown that chronic viral infections and hepatotoxic agents are the major risk factors, the molecular pathogenesis of HCC is quite complex with the involvement of several oncogenes and tumor suppressor genes . Surgery remains the first choice for treatment of HCC; however, tumor size, hepatic functional reserve, and/or portal hypertension all may restrict surgical ablation (Kerr and Kerr, 2009). Currently, first-line drugs used for HCC include doxorubicin, fuorouracil, cisplatin, and mitomycin, but most of them are nonselective cytotoxic molecules with significant side effects (Kerr and Kerr, 2009). Hence, novel agents that are cheap, nontoxic, and efficacious are urgently needed.STAT proteins were originally discovered as latent cytoplasmic transcription factors more than a decade ago (Ihle, 1996). There are seven known mammalian STAT proteins,

Butyrate, a 4-carbon fatty acid, has been shown to cause growth arrest and apoptosis of cancer cells in vitro and in vivo. The signaling pathways leading to changes in cell growth are unclear. We used a functional proteomics approach to delineate the pathways and mediators involved in butyrate action in HT-29 cells at 24 hr posttreatment. Using 2-dimensional gel electrophoresis, we showed that butyrate treatment resulted in alterations in the proteome of HT-29 cells. MALDI-TOF mass spectrometry was used to identify butyrate-regulated spots. First, our results revealed that the expression of various components of the ubiquitin-proteasome system was altered with butyrate treatment. This suggests that, in addition to the regulation of gene expression through the histone deacetylase pathway, proteolysis could be a means by which butyrate may regulate the expression of key proteins in the control of cell cycle, apoptosis and differentiation. Second, we found that both proapoptotic proteins (capase-4 and cathepsin D) and antiapoptotic proteins (hsp27, antioxidant protein-2 and pyruvate dehydrogenase E1) were simultaneously upregulated in butyrate-treated cells. Western blotting was carried out to confirm butyrate regulation of the spots. Both cathepsin D and hsp27 showed a time-dependent increase in expression with butyrate treatment in HT-29 cells. However, in HCT-116 cells, which were 5-fold more sensitive to butyrate-induced apoptosis, the upregulation of cathepsin D with time was not accompanied by a similar increase in hsp27 levels. Thus, the simultaneous upregulation of both proapoptotic and antiapoptotic proteins in HT-29 cells may account for their relative resistance to butyrateinduced apoptosis.

Colorectal cancer is one of the most common cancers in developed countries, and its incidence is negatively associated with high dietary fiber intake. Butyrate, a shortchain fatty acid fermentation by-product of fiber induces cell maturation with the promotion of growth arrest, differentiation, and/or apoptosis of cancer cells. The stimulation of cell maturation by butyrate in colonic cancer cells follows a temporal progression from the early phase of growth arrest to the activation of apoptotic cascades. Previously we performed two-dimensional DIGE to identify differentially expressed proteins induced by 24-h butyrate treatment of HCT-116 colorectal cancer cells. Herein we used quantitative proteomics approaches using iTRAQ (isobaric tags for relative and absolute quantitation), a stable isotope labeling methodology that enables multiplexing of four samples, for a temporal study of HCT-116 cells treated with butyrate. In addition, cleavable ICAT, which selectively tags cysteine-containing proteins, was also used, and the results complemented those obtained from the iTRAQ strategy. Selected protein targets were validated by real time PCR and Western blotting. A model is proposed to illustrate our findings from this temporal analysis of the butyrateresponsive proteome that uncovered several integrated cellular processes and pathways involved in growth arrest, apoptosis, and metastasis. These signature clusters of butyrate-regulated pathways are potential targets for novel chemopreventive and therapeutic drugs for treatment of colorectal cancer. Molecular & Cellular Proteomics 7:1174 -1185, 2008.In developed countries, colorectal cancer is a prevalent disease with high mortality and morbidity rates (1). This disease has emerged as the top malignancy in Singapore. Environmental factors are responsible for about 80% of the cases, whereas genetic predisposition accounts for the minority 20% of cases. Epidemiological evidence suggests that high intake of dietary fiber reduces the incidence and risk of this neoplasm (2, 3). A wealth of studies has shown that butyrate produced from anaerobic fermentation of indigestible carbohydrate is the molecule responsible for the chemopreventive properties of a fiber-rich diet (4 -6).Although butyrate serves as an energy source for normal colonocytes, in vivo and in vitro studies have shown that at physiological concentrations this natural short-chain fatty acid mediates cell maturation with the promotion of growth arrest followed by differentiation and/or apoptosis of cancer cells (7-11). These biological effects are crucial in colorectal cancer therapy as colonic transformation is characterized by multistage alterations of tissue homeostasis resulting in aberrant cell division and/or cell death (12, 13). Butyrate has been purported as a potential anticancer agent. This initiated notable research in identifying proteins that contribute to its biological effects (14, 15). However, most of these investigations focused on one target at any one time and were thus unable to systematica...

Butyrate, a 4-carbon short chain fatty acid, is responsible for the protective effects of fiber in colorectal cancer prevention. To better understand the 'blueprint' of butyrate's chemopreventive role in this disease, we performed 2-dimensional difference gel electrophoresis (2-D DIGE) of butyrate-treated HCT-116 colorectal cancer cells after pre-fractionation using heparin affinity chromatography. A combination of this enrichment step with overlapping narrow range IPGs (pH 4-7 and pH 6-11) in 2-D DIGE resulted in the detection of 46 differentially expressed spots. Twenty-four of these were identified by MS analyses, and 5 spots were found to be heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1). Three isoforms of 38 kDa were down-regulated while two with Mr approximately 26 kDa were up-regulated. These represent phosphorylated isoforms of hnRNP A1 as verified by immunoblotting with anti-phosphotyrosine and anti-phosphoserine antibodies. Using 2-DE, subcellular fractionation and western blot analysis, we further showed that full-length hnRNP A1 underwent down-regulation, cleavage and cytoplasmic retention upon butyrate treatment. These indicate that modulations of hnRNP A1 may play a significant role in the mediation of growth arrest and apoptosis by butyrate.