Tumour necrosis factor α-induced adipose-related protein (TIARP): co-localization with caveolin-1

The Role of STAMP2 in Pathogenesis of Chronic Diseases Focusing on Nonalcoholic Fatty Liver Disease: A Review

Nonalcoholic fatty liver disease (NAFLD) is a major health issue. NAFLD can progress from simple hepatic steatosis to nonalcoholic steatohepatitis (NASH). NASH can progress to cirrhosis or hepatocellular carcinoma. Unfortunately, there is no currently approved pharmacologic therapy for NAFLD patients. The six transmembrane protein of prostate 2 (STAMP2), a metalloreductase involved in iron and copper homeostasis, is well known for its critical role in the coordination of glucose/lipid metabolism and inflammation in metabolic tissues. We previously demonstrated that hepatic STAMP2 could be a suitable therapeutic target for NAFLD. In this review, we discuss the emerging role of STAMP2 in the dysregulation of iron metabolism events leading to NAFLD and suggest therapeutic strategies targeting STAMP2.

STEAP4: its emerging role in metabolism and homeostasis of cellular iron and copper

Preserving energy homeostasis in the presence of stressors such as proinflammatory cytokines and nutrient overload is crucial to maintaining normal cellular function. Six transmembrane epithelial antigen of the prostate 4 (STEAP4), a metalloreductase involved in iron and copper homeostasis, is thought to play a potentially important role in the cellular response to inflammatory stress. Genome-wide association studies have linked various mutations in STEAP4 with the development of metabolic disorders such as obesity, metabolic syndrome and type 2 diabetes. Several studies have shown that expression of Steap4 is modulated by inflammatory cytokines, hormones and other indicators of cellular stress and that STEAP4 may protect cells from damage, helping to maintain normal metabolic function. STEAP4 appears to be particularly relevant in metabolically oriented cells, such as adipocytes, hepatocytes and pancreatic islet cells. These cells struggle to maintain their function in iron or copper overloaded states, presumably due to increased oxidative stress, suggesting STEAP4's role in metal homeostasis is critical to the maintenance of cellular homeostasis in general, and in preventing the onset of metabolic disease. In this review, we explore genetic associations of STEAP4 with metabolic disorders, and we examine STEAP4 tissue expression, subcellular localization, regulation, structure and function as it relates to metabolic diseases. We then examine how STEAP4's role as a regulator of cellular iron and copper may relate to type 2 diabetes.

STAMPing at the crossroads of normal physiology and disease states

The six transmembrane protein of prostate (STAMP) proteins, also known as six transmembrane epithelial antigen of prostate (STEAPs), comprises three members: STAMP1-3. Their expression is regulated by a variety of stimuli, including hormones and cytokines, in varied settings and tissues with important roles in secretion and cell differentiation. In addition, they are implicated in metabolic and inflammatory diseases and cancer. Here, we review the current knowledge on the role of STAMPs in both physiological and pathological states.

Steap4 attenuates high glucose and S100B-induced effects in mesangial cells

Six-transmembrane epithelial antigen of prostate 4 (Steap4)-knockout mice develop hyperglycaemia and inflammation whereas Steap4 overexpression attenuates atherosclerosis in diabetic mice. Thus, we studied the roles of Steap4 in high glucose (HG, 27.5 mM) or S100B (1 μM, a ligand for the receptor for advanced glycation end-product or RAGE)-induced effects in mouse mesangial (MES13) cells. We found that HG-induced Steap4 protein expression was dependent on S100B. HG increased cell membrane, but not cytosolic, Steap4 protein expression. HG increased protein-protein interaction between Steap4 and S100B, which was confirmed by mass spectrometry of immunoprecipitated S100B. SP600125, LY294002 and AG490 attenuated S100B-induced Steap4 protein expression or gene transcriptional activity. A mutation in signal transducer and activator of transcription 3 (Stat3) site 2 of the Steap4 promoter constructs resulted in a marked decrease in HG or S100B-induced activation of Steap4 gene transcription. Overexpression of Steap4 attenuates HG or S100B-induced collagen IV, fibronectin and cyclooxygenase 2 protein expression. Overexpression of Steap4 attenuates HG or S100B-induced transforming growth factor-β (TGF-β). Moreover, overexpression of Steap4 attenuates S100B-induced signalling. Finally, overexpressing Steap4 attenuated renal expression of fibronectin, S100B, TGF-β, type IV collagen, p-Akt, p-extracellular signal regulated kinase 1/2 and p-Stat3 in streptozotocin-diabetic mice. Thus, overexpression of Steap4 attenuated HG or S100B-induced effects in MES13 cells and attenuated some of S100B-induced effects in diabetic mouse kidneys.

STAMPing into Mitochondria

Six transmembrane protein of prostate 2 (STAMP2) is a protein that has been extensively studied due to its association with prostate cancer. Currently, STAMP2 is well known for its critical role in metabolism and modulating inflammatory signals. Even so, the molecular mechanism of STAMP2 activity and its downstream effectors are still largely unknown. Here, we review the current knowledge of STAMP2, and suggest possible explanations for some of its less well-understood features. A few studies suggest that STAMP2 may interact with mitochondria. Considering STAMP2 functions as a potential component of mitochondrial biology may yield valuable insight into this protein.

The STEAP protein family: versatile oxidoreductases and targets for cancer immunotherapy with overlapping and distinct cellular functions

The human six-transmembrane epithelial antigen of the prostate (STEAP) protein family contains at least five homologous members. The necessity of multiple homologous STEAP proteins is still unclear, but their peculiar and tissue-specific expression suggests that they are assigned to distinct functional tasks. This concept is supported by the fact that especially STEAP1, and to a lesser extent STEAP2 and -4, are highly over-expressed in many different cancer entities, while being only minimally expressed in a few normal tissues. Despite their very similar domain organisation, STEAP3 seems to act as a potent metalloreductase essential for physiological iron uptake and turnover, while in particular STEAP4 appears to be rather involved in responses to nutrients and inflammatory stress, fatty acid and glucose metabolism. Moreover, individual STEAP proteins possess overlapping functions important for growth and survival of cancer cells. Due to their membrane-bound localisation and their high expression in many different cancers such as prostate, breast and bladder carcinoma as well as Ewing's sarcoma, STEAP proteins have been recognised and utilised as promising targets for cell- and antibody-based immunotherapy. This review summarises our present knowledge of the individual members of the human STEAP family and highlights the functional differences between them.

Evaluating caveolin interactions: do proteins interact with the caveolin scaffolding domain through a widespread aromatic residue-rich motif?

Caveolins are coat proteins of caveolae, small flask-shaped pits of the plasma membranes of most cells. Aside from roles in caveolae formation, caveolins recruit, retain and regulate many caveolae-associated signalling molecules. Caveolin-protein interactions are commonly considered to occur between a ∼20 amino acid region within caveolin, the caveolin scaffolding domain (CSD), and an aromatic-rich caveolin binding motif (CBM) on the binding partner (фXфXXXXф, фXXXXфXXф or фXфXXXXфXXф, where ф is an aromatic and X an unspecified amino acid). The CBM resembles a typical linear motif - a short, simple sequence independently evolved many times in different proteins for a specific function. Here we exploit recent improvements in bioinformatics tools and in our understanding of linear motifs to critically examine the role of CBMs in caveolin interactions. We find that sequences conforming to the CBM occur in 30% of human proteins, but find no evidence for their statistical enrichment in the caveolin interactome. Furthermore, sequence- and structure-based considerations suggest that CBMs do not have characteristics commonly associated with true interaction motifs. Analysis of the relative solvent accessible area of putative CBMs shows that the majority of their aromatic residues are buried within the protein and are thus unlikely to interact directly with caveolin, but may instead be important for protein structural stability. Together, these findings suggest that the canonical CBM may not be a common characteristic of caveolin-target interactions and that interfaces between caveolin and targets may be more structurally diverse than presently appreciated.

TNF-alpha potentiates protein-tyrosine nitration through activation of NADPH oxidase and eNOS localized in membrane rafts and caveolae of bovine aortic endothelial cells

A major source of reactive oxygen species (ROS) in endothelial cells is the NADPH oxidase enzyme complex. The selective distributions of any enzyme within cells have important implications in regulating enzyme effectiveness through facilitation of access to local substrates and/or product targets. Because membrane rafts provide a spatially preferable environment for a variety of enzyme systems, we sought to determine whether NADPH oxidase is present and functional in this plasma membrane compartment in endothelial cells. We found that, in resting endothelial cells, NADPH oxidase subunits were preassembled and the enzyme functional in membrane rafts, specifically in caveolae. Stimulation with TNF-alpha induced additional recruitment of the p47(phox) regulatory subunit to raft-localized NADPH oxidase and enhanced ROS production within raft domains. TNF-alpha also induced nitric oxide production through activation of endothelial nitric oxide synthase (eNOS) present in the same membrane compartment. The dual activation of superoxide and nitric oxide-generating systems provided a spatially favorable environment for nitration of tyrosine-containing proteins localized to rafts. Perturbation of membrane raft structural integrity with cholesterol-sequestering compounds caused the delocalization of NADPH oxidase subunits and eNOS from the rafts and inhibited TNF-alpha-induced ROS production and protein tyrosine nitration. Together, these data provide evidence that membrane rafts and caveolae play a role in the spatial regulation of NADPH oxidase and subsequent ROS/reactive nitrogen species in endothelial cells.

Lipid rafts mediate H2O2 prosurvival effects in cultured endothelial cells

Reactive oxygen species (ROS) generated during pathological events, such as inflammation and ischemia-reperfusion, activates both proapoptotic and antiapoptotic signaling programs in endothelial cells. Because cholesterol-rich, plasma membrane rafts serve as platforms for organizing and integrating signaling transduction processes, we asked whether these membrane regions play a mechanistic role in H2O2-induced responses. Bovine aortic endothelial cell cultures exposed to a 500-microM bolus of H2O2 showed progressive activation of caspase 3 and an increase in the number of TUNEL-positive cells. Pretreatment with either wortmannin or PD 098059 heightened these apoptotic responses, demonstrating that both PI3 kinase/Akt and ERK1/2 serve as signaling mediators to alleviate H2O2 cytotoxic effects. To investigate the role of lipid rafts in these signaling processes, endothelial cells were pretreated with methyl-beta-cyclodextrin (CD) or filipin to ablate raft structures. H2O2-induced phosphorylation of Akt and ERK 1/2 was attenuated, while caspase 3 and the number of TUNEL positive cells was enhanced in CD-pretreated cells exposed to H2O2. Reconstitution of raft domains restored H2O2-induced Akt and ERK1/2 phosphorylation, which was concomitant with reduction of caspase 3 activation and DNA fragmentation. Taken together, our findings suggest that plasma membrane compartments rich in cholesterol participate in signal transduction pathways activated by oxidative stress.