Secretome of primary cultures of human adipose-derived stem cells: modulation of serpins by adipogenesis

Effect of weight loss on serum pigment epithelium-derived factor levels

BACKGROUND

Cumulating evidence suggests that the broadly acting neurotrophic pigment epithelium-derived factor is associated with visceral adiposity, the metabolic syndrome, diabetes and exerts beneficial effects on atherosclerosis. To further elucidate the relationship between pigment epithelium-derived factor and metabolic perturbations characteristic of obesity, we examined the effect of pronounced weight loss on serum levels of pigment epithelium-derived factor.

MATERIALS AND METHODS

Thirty-six severely obese adults were examined before and 18 months after bariatric surgery. Abdominal fat distribution was determined by ultrasound, metabolic parameters by standard methods, pro-inflammatory biomarkers and serum pigment epithelium-derived factor levels by enzyme-linked immunosorbent assay.

RESULTS

Bariatric surgery resulted in a mean body mass index (BMI) reduction of 9·0 ± 5·0 kg m(-2) and concomitant improvements in glucose homoeostasis and lipid profile. Pigment epithelium-derived factor serum levels decreased from a median 11·0 μg mL(-1) (interquartile range: 3·8) to 9·2 μg mL(-1) (interquartile range: 4·5) (P < 0·0001). In univariate analysis, relative change in pigment epithelium-derived factor levels was significantly associated with change in weight, BMI, fat mass, visceral fat diameter, insulin, homoeostasis model for insulin resistance, triglyceride and leptin levels (all r > 0·370, P < 0·05). No associations were observed for C-reactive protein, interleukin-6 or tumour necrosis factor alpha. After adjustment for age, sex and smoking status, associations remained significant.

CONCLUSIONS

The beneficial effects of bariatric surgery-induced pronounced weight loss on glucose homoeostasis may partially be attributable to visceral adipose tissue reduction and concomitantly decreasing pigment epithelium-derived factor concentrations.

Pigment epithelial-derived factor and melanoma differentiation associated gene-7 cytokine gene therapies delivered by adipose-derived stromal/mesenchymal stem cells are effective in reducing prostate cancer cell growth

Adipose-derived stromal/mesenchymal stem cells (ASC) have gained interest as promising tools for delivering cancer therapy. Adipose tissue can be obtained readily in amounts sufficient for ASC isolation, which can be expanded rapidly, allowing its use at low passage numbers, and can be transduced by viral and nonviral means. Our goal was to examine the potential of ASC to deliver cytokine gene therapies melanoma differentiation associated gene-7 (MDA-7) or pigment epithelial-derived factor (PEDF) to cancer cells. These novel cytokines are a potent proapoptotic and an antiangiogenesis mediator, respectively, with potential as antitumor agents. Expression of cytokine therapies did not adversely affect ASC biology, and these cells were still able to differentiate and retain normal viability. The ASC cytokine therapies were efficient in reducing tumor cell growth in coculture and also in suppressing in vitro angiogenesis phenotypes. We also observed that ASC retained their innate ability to migrate toward tumor cells in coculture, and this ability could be blocked by inhibition of CXCR4 signaling. The ASC were found to be nontumorigenic in vitro using a soft agar assay, as well as in vivo, utilizing 2 prostate cancer xenograft models. The ASC-MDA7 only reduced tumor growth in the TRAMP-C2-Ras (TC2Ras) prostate cancer model. The ASC-PEDF, however, reduced growth in both the TC2Ras and the PC3 highly aggressive prostate cancer models, and it was able to completely prevent prostate tumor establishment in vivo. In conclusion, ASC expressing PEDF and MDA7 could effectively reduce prostate tumor growth in vivo, suggesting ASC-cytokine therapies might have translational applications, especially the PEDF modality.

Neurotoxicity of microglial cathepsin D revealed by secretome analysis

Microglia-driven inflammatory responses have both neuroprotective and neurotoxic effects in the CNS. The excessive and chronic activation of microglia, however, may shift the balance towards neurotoxic effects. In this regard, proteins secreted from activated microglia likely play a key role in the neurotoxic effects. To characterize secreted proteins of activated microglia, conditioned media obtained from BV-2 mouse microglia cells were analyzed by two-dimensional gel electrophoresis or liquid chromatography coupled with tandem mass spectrometry. Among many proteins identified in the secretome of activated microglia, an aspartic endoprotease cathepsin D has been found to mediate microglial neurotoxicity based on the following results: (i) the expression of cathepsin D protein was markedly increased in lipopolysaccharide/interferon-γ-stimulated microglia compared with resting microglia as determined by western blot analysis of conditioned media; (ii) knockdown of cathepsin D expression in microglia using short hairpin RNA diminished the neurotoxicity in the coculture of microglia and neuroblastoma cells and (iii) recombinant procathepsin D protein exerted cytotoxic effects toward cultured neurons. In conclusion, cathepsin D appears to play a central role in the microglial neurotoxicity, and could be a potential biomarker or drug target for the diagnosis and treatment of neurodegenerative diseases that are associated with excessive microglial activation and subsequent neurotoxic inflammation.

Heat shock protein B1 and its regulator genes are negatively correlated with intramuscular fat content in the longissimus thoracis muscle of Hanwoo (Korean cattle) steers

In previous proteomic studies, heat shock protein β 1 (HSPB1) was detected as a candidate protein related to meat quality in cattle. This study sought to determine if its gene expression was associated with intramuscular fat content in the longissimus thoracis muscle of Korean cattle (Hanwoo). Tissue from two groups of 10 steers each, low-marbling (mean intramuscular fat content, 7.4 ± 1.5%) and high-marbling (23.5 ± 2.8%), were used for immunoblotting, real-time PCR, and statistical analyses. HSPB1 expression in both mRNA and protein was shown to be negatively related to intramuscular fat content (P < 0.05). Pathway analysis found two genes, TNF receptor superfamily member 6 (FAS) and angiotensinogen (AGT), that were regulators of the HSPB1 gene. The expression of the two genes showed a negative correlation with intramuscular fat content (P < 0.05). These results suggest that HSPB1, FAS, and AGT may be good candidate genes associated with intramuscular fat content in the longissimus muscle of Korean cattle.

Pigment epithelium-derived factor regulates lipid metabolism via adipose triglyceride lipase

OBJECTIVE

Pigment epithelium-derived factor (PEDF) is an adipocyte-secreted factor involved in the development of insulin resistance in obesity. Previous studies have identified PEDF as a regulator of triacylglycerol metabolism in the liver that may act through adipose triglyceride lipase (ATGL). We used ATGL(-/-) mice to determine the role of PEDF in regulating lipid and glucose metabolism.

RESEARCH DESIGN AND METHODS

Recombinant PEDF was administered to ATGL(-/-) and wild-type mice, and whole-body energy metabolism was studied by indirect calorimetry. Adipose tissue lipolysis and skeletal muscle fatty acid metabolism was determined in isolated tissue preparations. Muscle lipids were assessed by electrospray ionization-tandem mass spectrometry. Whole-body insulin sensitivity and skeletal muscle glucose uptake were assessed.

RESULTS

PEDF impaired the capacity to adjust substrate selection, resulting in a delayed diurnal decline in the respiratory exchange ratio, and suppressed daily fatty acid oxidation. PEDF enhanced adipocyte lipolysis and triacylglycerol lipase activity in skeletal muscle. Muscle fatty acid uptake and storage were unaffected, whereas fatty acid oxidation was impaired. These changes in lipid metabolism were abrogated in ATGL(-/-) mice and were not attributable to hypothalamic actions. ATGL(-/-) mice were also refractory to PEDF-mediated insulin resistance, but this was not related to changes in lipid species in skeletal muscle.

CONCLUSIONS

The results are the first direct demonstration that 1) PEDF influences systemic fatty acid metabolism by promoting lipolysis in an ATGL-dependent manner and reducing fatty acid oxidation and 2) ATGL is required for the negative effects of PEDF on insulin action.

Proteome of human subcutaneous adipose tissue stromal vascular fraction cells versus mature adipocytes based on DIGE

Adipose tissue contains a heterogeneous population of mature adipocytes, endothelial cells, immune cells, pericytes, and preadipocytic stromal/stem cells. To date, a majority of proteomic analyses have focused on intact adipose tissue or isolated adipose stromal/stem cells in vitro. In this study, human subcutaneous adipose tissue from multiple depots (arm and abdomen) obtained from female donors was separated into populations of stromal vascular fraction cells and mature adipocytes. Out of 960 features detected by 2-D gel electrophoresis, a total of 200 features displayed a 2-fold up- or down-regulation relative to each cell population. The protein identity of 136 features was determined. Immunoblot analyses comparing SVF relative to adipocytes confirmed that carbonic anhydrase II was up-regulated in both adipose depots while catalase was up-regulated in the arm only. Bioinformatic analyses of the data set determined that cytoskeletal, glycogenic, glycolytic, lipid metabolic, and oxidative stress related pathways were highly represented as differentially regulated between the mature adipocytes and stromal vascular fraction cells. These findings extend previous reports in the literature with respect to the adipose tissue proteome and the consequences of adipogenesis. The proteins identified may have value as biomarkers for monitoring the physiology and pathology of cell populations within subcutaneous adipose depots.

Effects of combination of sibutramine and L-carnitine compared with sibutramine monotherapy on inflammatory parameters in diabetic patients

The aim of the study was to evaluate the effects of 12-month treatment with sibutramine plus L-carnitine compared with sibutramine alone on body weight, glycemic control, insulin resistance, and inflammatory state in type 2 diabetes mellitus patients. Two hundred fifty-four patients with uncontrolled type 2 diabetes mellitus (glycated hemoglobin [HbA(1c)] >8.0%) in therapy with different oral hypoglycemic agents or insulin were enrolled in this study and randomized to take sibutramine 10 mg plus L-carnitine 2 g or sibutramine 10 mg in monotherapy. We evaluated at baseline and after 3, 6, 9, and 12 months these parameters: body weight, body mass index, HbA(1c), fasting plasma glucose, postprandial plasma glucose, fasting plasma insulin, homeostasis model assessment of insulin resistance index, total cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, triglycerides, leptin, tumor necrosis factor-α, adiponectin, vaspin, and high-sensitivity C-reactive protein. Sibutramine plus L-carnitine gave a faster improvement of fasting plasma glucose, postprandial plasma glucose, lipid profile, leptin, tumor necrosis factor-α, and high-sensitivity C-reactive protein compared with sibutramine alone. Furthermore, there was a better improvement of body weight, HbA(1c), fasting plasma insulin, homeostasis model assessment of insulin resistance index, vaspin, and adiponectin with sibutramine plus L-carnitine compared with sibutramine alone. Sibutramine plus L-carnitine gave a better and faster improvement of all the analyzed parameters compared with sibutramine alone without giving any severe adverse effect.

Bidirectional signaling of mammary epithelium and stroma: implications for breast cancer--preventive actions of dietary factors

The mammary gland is composed of two major cellular compartments: a highly dynamic epithelium that undergoes cycles of proliferation, differentiation and apoptosis in response to local and endocrine signals and the underlying stroma comprised of fibroblasts, endothelial cells and adipocytes, which collectively form the mammary fat pad. Breast cancer originates from subversions of normal growth regulatory pathways in mammary epithelial cells due to genetic mutations and epigenetic modifications in tumor suppressors, oncogenes and DNA repair genes. Diet is considered a highly modifiable determinant of breast cancer risk; thus, considerable efforts are focused on understanding how certain dietary factors may promote resistance of mammary epithelial cells to growth dysregulation. The recent indications that stromal cells contribute to the maintenance of the mammary epithelial 'niche' and the increasing appreciation for adipose tissue as an endocrine organ with a complex secretome have led to the novel paradigm that the mammary stromal compartment is itself a relevant target of bioactive dietary factors. In this review, we address the potential influence of dietary factors on mammary epithelial-stromal bidirectional signaling to provide mechanistic insights into how dietary factors may promote early mammary epithelial differentiation to decrease adult breast cancer risk.

Mapping of the secretome of primary isolates of mammalian cells, stem cells and derived cell lines

Within a mammalian organism, the interaction among cells both at short and long distances is mediated by soluble factors released by cells into the extracellular environment. The secreted proteins may involve extracellular matrix proteins, proteinases, growth factors, protein hormones, immunoregulatory cytokines, chemokines or other bioactive molecules that have a direct impact on target cell phenotype. Stem cells of mesenchymal, adipose, neural and embryonic origin, fibroblast feeder cells as well as primary isolates of astrocytes, endothelial and muscle cells have recently become targets of intensive secretome profiling with the search for proteins regulating cell survival, proliferation, differentiation or inflammatory response. Recent advances and challenges of the stem cell and primary cell secretome analysis together with the most relevant results are discussed in this review.

Conditioned media from cell lines: a complementary model to clinical specimens for the discovery of disease-specific biomarkers

In the strictest sense, the cell secretome (conditioned media) refers to the collection of proteins that contain a signal peptide and are processed via the endoplasmic reticulum and Golgi apparatus through the classical secretion pathway. More generally, the secretome also encompasses proteins shed from the cell surface and intracellular proteins released through non-classical secretion pathway or exosomes. These secreted proteins include numerous enzymes, growth factors, cytokines and hormones or other soluble mediators. They are fundamental in the processes of cell growth, differentiation, invasion and angiogenesis by regulating cell-to-cell and cell-to-extracellular matrix interactions. The main aim of this review is to provide a synopsis of findings from the analysis of the secretome taking diabetes, cancer and neurodegenerative diseases as examples. We will also discuss the preparation of conditioned media and on the main proteomic-based methodological approaches that have been developed for the study of secreted/shed proteins.

Characterization of the Human Adipocyte Proteome and Reproducibility of Protein Abundance by One-Dimensional Gel Electrophoresis and HPLC-ESI-MS/MS

Abnormalities in adipocytes play an important role in various conditions, including the metabolic syndrome, type 2 diabetes mellitus and cardiovascular disease, but little is known about alterations at the protein level. We therefore sought to (1) comprehensively characterize the human adipocyte proteome for the first time and (2) demonstrate feasibility of measuring adipocyte protein abundances by one-dimensional SDS-PAGE and high performance liquid chromatography-electron spray ionization-tandem mass spectrometry (HPLC-ESI-MS/MS). In adipocytes isolated from approximately 0.5 g of subcutaneous abdominal adipose tissue of three healthy, lean subjects, we identified a total of 1493 proteins. Triplicate analysis indicated a 22.5% coefficient of variation of protein abundances. Proteins ranged from 5.8 to 629 kDa and included a large number of proteins involved in lipid metabolism, such as fatty acid transport, fatty acid oxidation, lipid storage, lipolysis, and lipid droplet maintenance. Furthermore, we found most glycolysis enzymes and numerous proteins associated with oxidative stress, protein synthesis and degradation as well as some adipokines. 22% of all proteins were of mitochondrial origin. These results provide the first detailed characterization of the human adipocyte proteome, suggest an important role of adipocyte mitochondria, and demonstrate feasibility of this approach to examine alterations of adipocyte protein abundances in human diseases.

Gel-based and gel-free proteomic technologies

Proteomics refers to the analysis of expression, localization, functions, posttranslational modifications, and interactions of proteins expressed by a genome at a specific condition and at a specific time. Mass spectrometry (MS)-based proteomic methods have emerged as a key technology for unbiased systematic and high-throughput identification and quantification of complex protein mixtures. These methods have the potential to reveal unknown and novel changes in protein interactions and assemblies that regulate cellular and physiological processes. Both gel-based (one-dimensional [1D] gel electrophoresis, two-dimensional [2D] polyacrylamide gel electrophoresis, 2D difference in-gel electrophoresis [DIGE]) and gel-free (liquid chromatography [LC], capillary electrophoresis) approaches have been developed and utilized in a variety of combinations to separate proteins prior to mass spectrometric analysis. Detailed protocols for global proteomic analysis from adipose-derived stem cells (ASCs) using two central strategies, 2D-DIGE-MS and 2D-LC-MS, are presented here.

Intrinsic depot-specific differences in the secretome of adipose tissue, preadipocytes, and adipose tissue-derived microvascular endothelial cells

OBJECTIVE

Visceral adipose tissue (VAT) is more closely linked to insulin resistance than subcutaneous adipose tissue (SAT). We conducted a quantitative analysis of the secretomes of VAT and SAT to identify differences in adipokine secretion that account for the adverse metabolic consequences of VAT.

RESEARCH DESIGN AND METHODS

We used lectin affinity chromatography followed by comparison of isotope-labeled amino acid incorporation rates to quantitate relative differences in the secretomes of VAT and SAT explants. Because adipose tissue is composed of multiple cell types, which may contribute to depot-specific differences in secretion, we isolated preadipocytes and microvascular endothelial cells (MVECs) and compared their secretomes to those from whole adipose tissue.

RESULTS

Although there were no discrete depot-specific differences in the secretomes from whole adipose tissue, preadipocytes, or MVECS, VAT exhibited an overall higher level of protein secretion than SAT. More proteins were secreted in twofold greater abundance from VAT explants compared with SAT explants (59% versus 21%), preadipocytes (68% versus 0%), and MVECs (62% versus 15%). The number of proteins in the whole adipose tissue secretome was greater than the sum of its cellular constituents. Finally, almost 50% of the adipose tissue secretome was composed of factors with a role in angiogenesis.

CONCLUSIONS

VAT has a higher secretory capacity than SAT, and this difference is an intrinsic feature of its cellular components. In view of the number of angiogenic factors in the adipose tissue secretome, we propose that VAT represents a more readily expandable tissue depot.

Effects of weight reduction on serum vaspin concentrations in obese subjects: modification by insulin resistance

Visceral adipose tissue-derived serpin (vaspin) has been regarded as a novel adipokine with potential insulin sensitizing properties. We investigated the changes of serum vaspin concentration in response to weight reduction, and the associations between changes in serum vaspin concentrations and changes of anthropometric and metabolic variables in obese subjects after weight reduction. We performed a longitudinal clinical intervention study on 63 obese persons enrolled in a 12-week weight reduction program that included lifestyle modification and adjuvant treatment with the antiobesity agent orlistat. Anthropometric variables, lipid profiles, fasting glucose, fasting insulin, and serum vaspin concentrations were measured. Statistical analyses were performed according to the homeostasis model assessment of insulin resistance (HOMA(IR)). Serum vaspin concentrations decreased significantly in responders (≥2% reduction in baseline weight), but not in nonresponders (<2% reduction in baseline weight). Changes in serum vaspin concentrations were significantly correlated with body weight, BMI, waist circumference, and hip circumference in the higher, but not in the lower, HOMA(IR) group. In multivariate linear regression analysis, change in serum vaspin concentrations in the higher, but not in the lower, HOMA(IR) group was positively correlated with change in BMI and negatively correlated with initial HOMA(IR) level. The associations between changes in serum vaspin concentrations and changes in anthropometric and metabolic parameters differed according to insulin resistance status in obese subjects. These relationships were more prominent in the higher HOMA(IR) group. Insulin resistance may influence the correlations between changes in serum vaspin concentration and related metabolic variables.

Temporal profiling of the secretome during adipogenesis in humans

Adipose tissue plays a key role as a fat-storage depot and as an endocrine organ. Although mouse adipogenesis has been studied extensively, limited studies have been conducted to characterize this process in humans. We carried out a temporal proteomic analysis to interrogate the dynamic changes in the secretome of primary human preadipocytes as they differentiate into mature adipocytes. Using iTRAQ-based quantitative proteomics, we identified and quantified 420 proteins from the secretome of differentiated human adipocytes. Our results revealed that the majority of proteins showed differential expression during the course of differentiation. In addition to adipokines known to be differentially secreted in the course of adipocyte differentiation, we identified a number of proteins whose dynamic expression in this process has not been previously documented. They include collagen triple helix repeat containing 1, cytokine receptor-like factor 1, glypican-1, hepatoma-derived growth factor, SPARC related modular calcium binding protein 1, SPOCK 1, and sushi repeat-containing protein. A bioinformatics analysis using Human Protein Reference Database and Human Proteinpedia revealed that of the 420 proteins identified, 164 proteins possess signal peptides and 148 proteins are localized to the extracellular compartment. Additionally, we employed antibody arrays to quantify changes in the levels of 182 adipokines during human adipogenesis. This is the first large-scale quantitative proteomic study that combines two platforms, mass spectrometry and antibody arrays, to analyze the changes in the secretome during the course of adipogenesis in humans.

The secretome of adipocytes is regulated during the differentiation of preadipocytes into adipocytes. Using iTRAQ-based quantitative proteomics, we identified and quantified 420 proteins. Additionally, we employed antibody arrays to quantify changes in the levels of 182 adipokines. This is the first large-scale quantitative proteomic study that combines two platforms, mass spectrometry and antibody arrays, to analyze the changes in the secretome during the course of adipogenesis in humans.

Adipose tissue-derived stem cells secrete CXCL5 cytokine with chemoattractant and angiogenic properties

Adipose tissue-derived stem cells (ADSC) secreted CXCL5 cytokine abundantly and higher passaged ADSC up to passage 6 (P6) secreted more CXCL5 than lower passaged ADSC. Higher passaged ADSC also appeared to express higher levels of CXCL5 receptor, i.e., CXCR2. Both CXCL5 and CXCR2 were localized in the tunica intima and tunica adventitia of blood vessels in adipose tissue. Colocalization with CD34 further indicates their association with the putative ADSC in tunica adventitia. Migration assay indicates chemoattractant effects of CXCL5 on ADSC and HUVEC endothelial cells. CXCL5 also enhanced matrigel-based endothelial tube-like formation of HUVEC.

Circulating pigment epithelium-derived factor levels are associated with insulin resistance and decrease after weight loss

OBJECTIVE

We aimed to study circulating pigment epithelium-derived factor (PEDF) in vivo in association with insulin resistance and in vitro in human adipocytes.

METHODS

Circulating PEDF (ELISA) and metabolic profile were assessed in 125 Caucasian men. PEDF levels were also assessed in an independent cohort of subjects (n = 33) to study the effects of changing insulin action. PEDF gene expression and secretion were measured during differentiation of human preadipocytes.

RESULTS

In all subjects, PEDF was positively associated with body mass index (r = 0.326; P < 0.0001), waist-to-hip ratio (r = 0.380; P < 0.0001), HbA(1c), and fasting triglycerides and negatively with insulin sensitivity (r = -0.320; P < 0.0001). PEDF levels were significantly increased in subjects with altered glucose tolerance and type 2 diabetes. Of the inflammatory markers measured, PEDF levels were positively associated with serum soluble TNF-α receptor 1 and IL-10 in obese subjects. Interestingly, weight loss led to significantly decreased PEDF concentration from 34.8 ± 19.3 to 22.5 ± 14.2 μg/ml (P < 0.0001). Multiple linear regression analyses revealed that insulin sensitivity contributed independently to explain 14% of the variance in PEDF levels after controlling for the effects of body mass index, age, and log fasting triglycerides. Differences in PEDF observed after weight loss were related to changes in obesity, insulin resistance, and blood pressure measures. PEDF gene expression and secretion increased during differentiation of human preadipocytes.

CONCLUSION

Circulating PEDF is associated with insulin sensitivity. The findings show, for the first time in humans, that PEDF concentrations decrease significantly after weight loss in association with blood pressure. PEDF seems to be involved in human adipocyte biology.

Proteomic analysis of tumor necrosis factor-alpha-induced secretome of human adipose tissue-derived mesenchymal stem cells

Human adipose tissue-derived mesenchymal stem cells (hASCs) are useful for regeneration of inflamed or injured tissues. To identify secreted hASC proteins during inflammation, hASCs were exposed to tumor necrosis factor-alpha (TNF-alpha) and conditioned media derived from hASCs were analyzed by liquid chromatography coupled with tandem mass spectrometry. We identified 187 individual proteins as secreted proteins (secretome) in hASC-conditioned media; 118 proteins were secreted at higher levels upon TNF-alpha treatment. The TNF-alpha-induced secretome included a variety of cytokines and chemokines such as interleukin-6 (IL-6), IL-8, chemokine (C-X-C motif) ligand 6, and monocyte chemotactic protein-1 (MCP-1). TNF-alpha also increased expression of various proteases including cathepsin L, matrix metalloproteases and protease inhibitors, and induced secretion of long pentraxin 3, a key inflammatory mediator implicated in innate immunity. TNF-alpha-conditioned media stimulated migration of human monocytes, which play a key role in inflammatory responses. This migration was abrogated by pretreatment with neutralizing anti-IL-6, anti-IL-8, and anti-MCP-1 antibodies, suggesting that IL-6, IL-8, and MCP-1 are involved in migration of monocytes. Taken together, these results suggest that TNF-alpha-induced secretome may play a pivotal role in inflammatory responses and that shotgun proteomic analysis will be useful for elucidation of the paracrine functions of mesenchymal stem cells.

High-throughput and targeted in-depth mass spectrometry-based approaches for biofluid profiling and biomarker discovery

Proteomics aims to create a link between genomic information, biological function and disease through global studies of protein expression, modification and protein-protein interactions. Recent advances in key proteomics tools, such as mass spectrometry (MS) and (bio)informatics, provide tremendous opportunities for biomarker-related clinical applications. In this review, we focus on two complementary MS-based approaches with high potential for the discovery of biomarker patterns and low-abundant candidate biomarkers in biofluids: high-throughput matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy-based methods for peptidome profiling and label-free liquid chromatography-based methods coupled to MS for in-depth profiling of biofluids with a focus on subproteomes, including the low-molecular-weight proteome, carrier-bound proteome and N-linked glycoproteome. The two approaches differ in their aims, throughput and sensitivity. We discuss recent progress and challenges in the analysis of plasma/serum and proximal fluids using these strategies and highlight the potential of liquid chromatography-MS-based proteomics of cancer cell and tumor secretomes for the discovery of candidate blood-based biomarkers. Strategies for candidate validation are also described.

Adipocyte extracellular matrix composition, dynamics and role in obesity

The central role of the adipose tissue in lipid metabolism places specific demands on the cell structure of adipocytes. The protein composition and dynamics of the extracellular matrix (ECM) is of crucial importance for the functioning of those cells. Adipogenesis is a bi-phasic process in which the ECM develops from a fibrillar to a laminar structure as cells move from the commitment phase to the growth phase characterized by storage of vast amounts of triglycerides. Mature adipocytes appear to spend a lot of energy on the maintenance of the ECM. ECM remodeling is mediated by a balanced complement of constructive and destructive enzymes together with their enhancers and inhibitors. ECM remodeling is an energy costing process regulated by insulin, by the energy metabolism, and by mechanical forces. In the obese, overgrowth of adipocytes may lead to instability of the ECM, possibly mediated by hypoxia.

Responses of adipose-derived stem cells during hypoxia: enhanced skin-regenerative potential

Mesenchymal stem cells within the stromal-vascular fraction of subcutaneous adipose tissue (i.e., adipose-derived stem cells (ASCs)), have been used for tissue engineering. In addition to serving a building-block function, ASCs are reported to secrete growth factors that are essential for their function. Increasing evidence indicates that ASCs play a significant role in skin regeneration, a function that is enhanced by hypoxia through upregulating secretion of growth factors. Although the anatomical sites of ASCs in the body are relatively oxygen-deficient, ASCs are usually cultured under normoxic conditions (i.e., atmospheric oxygen levels). Culturing ASCs under physiologically relevant low-oxygen-tension conditions may uniquely benefit the expansion, differentiation, adhesion, growth factor secretion and regenerative potential of ASCs. Therefore, understanding the response and adaptation of ASCs to hypoxia may be invaluable for developing novel cell- and cyto-therapy strategies. This review highlights our current understanding of cellular and molecular responses of ASCs to hypoxia, focusing on the enhancement of ASC function and secretory activity by hypoxic culture conditions.

Nanozeolite-driven approach for enrichment of secretory proteins in human hepatocellular carcinoma cells

Given the importance of secreted proteins as a source for early detection and diagnosis of disease, secreted proteins have been arousing considerable attention. However, the analysis of secreted proteins represents a challenge for current proteomic techniques. One of the difficulties in secretomic study is to concentrate proteins from large volume of growth media, particularly, the low abundant and low molecular weight proteins (molecular weight <30 kDa). Herein, we describe a novel strategy for harvesting secretory proteins. In this approach, proteins secreted from the human hepatocellular carcinoma cell line were enriched by zeolite LTL nanocrystals, followed by 1-D SDS-PAGE for protein fractionation and then by LC-ESI-MS/MS for protein identification. In total, 1474 unique proteins were confidently identified, including 505 low molecular weight proteins, and covered a broad range of pI and molecular weight. Furthermore, this study not only offered an efficient and powerful method for the enrichment of secretory proteins but also allowed in-depth study of secretome of hepatocellular carcinoma cells. The reported work is expected to represent one of the most comprehensive secretomic analyses so far.

Pigment epithelium-derived factor suppresses adipogenesis via inhibition of the MAPK/ERK pathway in 3T3-L1 preadipocytes

We previously reported that circulating levels of pigment epithelium-derived factor (PEDF), a newly identified adipokine, are increased in patients with type 2 diabetes, correlating with body mass index. However, the role of PEDF in adipogenesis remains elusive. In the present study, we have investigated the effects and mechanisms of PEDF on adipocyte differentiation in 3T3-L1 preadipocytes. Differentiation of 3T3-L1 preadipocytes was induced in the presence or absence of human recombinant PEDF protein. The effects of PEDF on adipogenic gene expression, mitotic clonal expansion (MCE), and MAPK activation were investigated. Physiological concentrations of human PEDF protein inhibited adipocyte differentiation, evidenced by decreased lipid accumulation, downregulation of adipocyte markers, and inhibition of master adipogenic transcription factors such as C/EBP-alpha and PPARgamma. The antiadipogenic effects of PEDF were observed only when PEDF was added to the cells on day 0, but not on day 3 during differentiation, suggesting that PEDF targets some early adipogenic events. Similarly, overexpression of PEDF by adenovirus attenuated adipocyte differentiation. Further studies revealed that PEDF, or U-0126, a specific MAPK/ERK inhibitor, sequentially inhibited the early activation of ERK and MCE. Moreover, PEDF attenuated expression and the phosphorylation of C/EBP-beta at Thr(188), an essential step for transcriptional activation of C/EBP-beta. In addition, PEDF expression was decreased significantly in the first 24 h during adipocyte differentiation, suggesting that downregulation of PEDF may be essential for the initiation of MCE and adipogenesis. We conclude that PEDF inhibits adipogenesis in 3T3-L1 preadipocytes partially because of inhibition of the MAPK/ERK signaling pathway and MCE.

Quantitative analysis of secretome from adipocytes regulated by insulin

Adipocyte is not only a central player involved in storage and release of energy, but also in regulation of energy metabolism in other organs via secretion of peptides and proteins. During the pathogenesis of insulin resistance and type 2 diabetes, adipocytes are subjected to the increased levels of insulin, which may have a major impact on the secretion of adipokines. We have undertaken cleavable isotope-coded affinity tag (cICAT) and label-free quantitation approaches to identify and quantify secretory factors that are differentially secreted by 3T3-L1 adipocytes with or without insulin treatment. Combination of cICAT and label-free results, there are 317 proteins predicted or annotated as secretory proteins. Among these secretory proteins, 179 proteins and 53 proteins were significantly upregulated and down-regulated, respectively. A total of 77 reported adipokines were quantified in our study, such as adiponectin, cathepsin D, cystatin C, resistin, and transferrin. Western blot analysis of these adipokines confirmed the quantitative results from mass spectrometry, and revealed individualized secreting patterns of these proteins by increasing insulin dose. In addition, 240 proteins were newly identified and quantified as secreted proteins from 3T3-L1 adipocytes in our study, most of which were up-regulated upon insulin treatment. Further comprehensive bioinformatics analysis revealed that the secretory proteins in extracellular matrix-receptor interaction pathway and glycan structure degradation pathway were significantly upregulated by insulin stimulation.

Identification of astrocyte secreted proteins with a combination of shotgun proteomics and bioinformatics

Astrocytes are important regulators of normal brain function in mammals, including roles in synaptic signaling, synapse formation, and neuronal health and survival. Many of these functions are executed via secreted proteins. To analyze the astrocyte secretome, a combination of shotgun proteomics and bioinformatics was employed to analyze conditioned media from primary murine astrocyte cultures. Both two- and one-dimensional LC-MS/MS were used to analyze astrocyte secreted proteins, resulting in the identification of over 420 proteins. To refine our results, the intracellular protein contaminants were removed in silico using a cytoplasmic control. In additional rounds of refinement, putative secreted proteins were subjected to analysis by SignalP, SecretomeP, and gene ontology analysis, yielding a refined list of 187 secreted proteins. In conclusion, the use of shotgun proteomics combined with multiple rounds of data refinement produced a high quality catalog of astrocyte secreted proteins.

Gene profiling of human adipose tissue during evoked inflammation in vivo

OBJECTIVE

Adipose inflammation plays a central role in obesity-related metabolic and cardiovascular complications. However, few human adipose-secreted proteins are known to mediate these processes. We hypothesized that microarray mRNA profiling of human adipose during evoked inflammation could identify novel adipocytokines.

RESEARCH DESIGN AND METHODS

Healthy human volunteers (n = 14) were treated with intravenous endotoxin (3 ng/kg lipopolysaccharide [LPS]) and underwent subcutaneous adipose biopsies before and after LPS. On Affymetrix U133Plus 2.0 arrays, adipose mRNAs modulated >1.5-fold (with P < 0.00001) were selected. SignalP 3.0 and SecretomeP 2.0 identified genes predicted to encode secreted proteins. Of these, 86 candidates were chosen for validation in adipose from an independent human endotoxemia protocol (N = 7, with 0.6 ng/kg LPS) and for exploration of cellular origin in primary human adipocytes and macrophages in vitro.

RESULTS

Microarray identified 776 adipose genes modulated by LPS; 298 were predicted to be secreted. Of detectable prioritized genes, 82 of 85 (96% [95% CI 90-99]) were upregulated (fold changes >1.0) during the lower-dose (LPS 0.6 ng/kg) validation study and 51 of 85 (59% [49-70]) were induced greater than 1.5-fold. Treatment of primary adipocytes with LPS and macrophage polarization to M1 proinflammatory phenotype increased expression by 1.5-fold for 58 and 73% of detectable genes, respectively.

CONCLUSIONS

We demonstrate that evoked inflammation of human adipose in vivo modulated expression of multiple genes likely secreted by adipocytes and monocytes. These included established adipocytokines and chemokines implicated in recruitment and activation of lymphocytes, adhesion molecules, antioxidants, and several novel genes with unknown function. Such candidates may represent biomarkers and therapeutic targets for obesity-related complications.

Getting the message across: mechanisms of physiological cross talk by adipose tissue

Obesity is associated with resistance of skeletal muscle to insulin-mediated glucose uptake, as well as resistance of different organs and tissues to other metabolic and vascular actions of insulin. In addition, the body is exquisitely sensitive to nutrient imbalance, with energy excess or a high-fat diet rapidly increasing insulin resistance, even before noticeable changes occur in fat mass. There is a growing acceptance of the fact that, as well as acting as a storage site for surplus energy, adipose tissue is an important source of signals relevant to, inter alia, energy homeostasis, fertility, and bone turnover. It has also been widely recognized that obesity is a state of low-grade inflammation, with adipose tissue generating substantial quantities of proinflammatory molecules. At a cellular level, the understanding of the signaling pathways responsible for such alterations has been intensively investigated. What is less clear, however, is how alterations of physiology, and of signaling, within one cell or one tissue are communicated to other parts of the body. The concepts of cell signals being disseminated systemically through a circulating "endocrine" signal have been complemented by the view that local signaling may similarly occur through autocrine or paracrine mechanisms. Yet, while much elegant work has focused on the alterations in signaling that are found in obesity or energy excess, there has been less attention paid to ways in which such signals may propagate to remote organs. This review of the integrative physiology of obesity critically appraises the data and outlines a series of hypotheses as to how interorgan cross talk takes place. The hypotheses presented include the "fatty acid hypothesis,", the "portal hypothesis,", the "endocrine hypothesis,", the "inflammatory hypothesis,", the "overflow hypothesis,", a novel "vasocrine hypothesis," and a "neural hypothesis," and the strengths and weaknesses of each hypothesis are discussed.

Temporal profiling of the adipocyte proteome during differentiation using a five-plex SILAC based strategy

The adipose tissue has important secretory and endocrine functions in humans. The regulation of adipocyte differentiation has been actively pursued using transcriptomic methods over the last several years. Quantitative proteomics has emerged as a promising approach to obtain temporal profiles of biological processes such as differentiation. Stable isotope labeling with amino acids in cell culture (SILAC) is a simple and robust method for labeling proteins in vivo. Here, we describe the development and application of a five-plex SILAC experiment using four different heavy stable isotopic forms of arginine to study the nuclear proteome and the secretome during the course of adipocyte differentiation. Tandem mass spectrometry analysis using a quadrupole time-of-flight instrument resulted in identification of a total 882 proteins from these two proteomes. Of these proteins, 427 were identified on the basis of one or more arginine-containing peptides that allowed quantitation. In addition to previously reported molecules that are differentially expressed during the process of adipogenesis (e.g., adiponectin and lipoprotein lipase), we identified several proteins whose differential expression during adipocyte differentiation has not been documented previously. For example, THO complex 4, a context-dependent transcriptional activator in the T-cell receptor alpha enhancer complex, showed highest expression at middle stage of adipogenesis, while SNF2 alpha, a chromatin remodeling protein, was downregulated upon initiation of adipogenesis and remained so during subsequent time points. This study using a 5-plex SILAC to investigate dynamics illustrates the power of this approach to identify differentially expressed proteins in a temporal fashion.

Adipose proteome analysis: focus on mediators of insulin resistance

As is well known, adipose tissue is an important site for lipid metabolism and insulin-responsive glucose uptake. The recent discovery of the endocrine function of adipose tissue and the association of obesity with chronic low-grade inflammation in adipose tissue has reinforced the concept of the central role of adipose tissue in mediating obesity-linked insulin resistance and metabolic dysregulation. The study of adipose cells has provided new insights into the mechanism underlying insulin resistance as well as the therapeutic strategies for diabetes. Numerous efforts have been made in identifying key molecular regulators of insulin action and metabolism, including the utilization of advanced proteomics technology. Various proteomic approaches have been applied to identify the adipose secretome, protein-expression profiling and post-translational modifications in adipose cells in the pathological state. In this review, we summarize the recent advances in the proteomics of adipose tissue, and discuss the identified proteins that potentially play important roles in insulin resistance and diabetes.

Ethanol exposure depletes hepatic pigment epithelium-derived factor, a novel lipid regulator

BACKGROUND & AIMS

Ethanol abuse can lead to hepatic steatosis and evolve into cirrhosis and hepatocellular carcinoma. Pigment epithelium-derived factor (PEDF) is a multifunctional secreted glycoprotein that is expressed by hepatocytes. Proteomic, experimental, and clinical studies implicate PEDF's role in lipid regulation. Because matrix metalloproteinase (MMP)-2/9 activity regulates PEDF levels, we investigated whether PEDF degradation by MMPs has a permissive role in ethanol-induced hepatic steatosis.

METHODS

PEDF levels were examined in liver biopsy specimens from patients with ethanol-induced steatosis. Hepatic PEDF levels and MMP activity were assessed in 2 animal models of ethanol feeding (rats on an alcohol-containing liquid diet and mice given intragastric infusion of ethanol). The consequences of PEDF depletion in the liver were examined in PEDF-null mice.

RESULTS

Liver biopsy samples from patients with ethanol-induced steatosis had reduced PEDF levels, compared with normal liver samples. Ethanol-fed animals had histologic steatosis and increased liver triglyceride content (P< .05), as well as reduced levels of hepatic PEDF and increased MMP-2/9 activity. Ethanol-exposed hepatic lysates degraded PEDF in a MMP-2/9-dependent manner, and liver sections demonstrated abundant MMP-2/9 activity in situ. Addition of recombinant PEDF to PEDF-null hepatocytes, reduced their triglyceride content.

CONCLUSIONS

Ethanol exposure leads to marked loss of hepatic PEDF in human livers and in 2 animal models of ethanol feeding. Loss of PEDF contributes to the accumulation of lipids in ethanol-induced hepatic steatosis.

Increased secretion and expression of myostatin in skeletal muscle from extremely obese women

OBJECTIVE

Obesity is associated with endocrine abnormalities that predict the progression of insulin resistance to type 2 diabetes. Because skeletal muscle has been shown to secrete proteins that could be used as biomarkers, we characterized the secreted protein profile of muscle cells derived from extremely obese (BMI 48.8 +/- 14.8 kg/m(2); homeostasis model assessment [HOMA] 3.6 +/- 1.0) relative to lean healthy subjects (BMI 25.7 +/- 3.2 kg/m(2); HOMA 0.8 +/- 0.2).

RESEARCH DESIGN AND METHODS

We hypothesized that skeletal muscle would secrete proteins that predict the severity of obesity. To test this hypothesis, we used a "bottom-up" experimental design using stable isotope labeling by amino acids in culture (SILAC) and liquid chromatography/mass spectometry/mass spectometry (LC-MS/MS) to both identify and quantify proteins secreted from cultured myotubes derived from extremely obese compared with healthy nonobese women.

RESULTS

Using SILAC, we discovered a 2.9-fold increase in the secretion of myostatin from extremely obese human myotubes. The increased secretion and biological activity of myostatin were validated by immunoblot (3.16 +/- 0.18, P < 0.01) and a myoblast proliferation assay using conditioned growth medium. Myostatin was subsequently shown to increase in skeletal muscle (23%, P < 0.05) and plasma (35%, P < 0.05) and to correlate (r(2) = 0.6, P < 0.05) with the severity of insulin resistance.

CONCLUSIONS

Myostatin is a potent antianabolic regulator of muscle mass that may also play a role in energy metabolism. These findings show that increased expression of myostatin in skeletal muscle with obesity and insulin resistance results in elevated circulating myostatin. This may contribute to systemic metabolic deterioration of skeletal muscle with the progression of insulin resistance to type 2 diabetes.

Novel insights into adipogenesis from omics data

Obesity, the excess accumulation of adipose tissue, is one of the most pressing health problems in both the Western world and in developing countries. Adipose tissue growth results from two processes: the increase in number of adipocytes (hyperplasia) that develop from precursor cells, and the growth of individual fat cells (hypertrophy) due to incorporation of triglycerides. Adipogenesis, the process of fat cell development, has been extensively studied using various cell and animal models. While these studies pointed out a number of key factors involved in adipogenesis, the list of molecular components is far from complete. The advance of high-throughput technologies has sparked many experimental studies aimed at the identification of novel molecular components regulating adipogenesis. This paper examines the results of recent studies on adipogenesis using high-throughput technologies. Specifically, it provides an overview of studies employing microarrays for gene expression profiling and studies using gel based and non-gel based proteomics as well as a chromatin immunoprecipitation followed by microarray analysis (ChIP-chip) or sequencing (ChIP-seq). Due to the maturity of the technology, the bulk of the available data was generated using microarrays. Therefore these data sets were not only reviewed but also underwent meta analysis. The review also shows that large-scale omics technologies in conjunction with sophisticated bioinformatics analyses can provide not only a list of novel players, but also a global view on biological processes and molecular networks. Finally, developing technologies and computational challenges associated with the data analyses are highlighted, and an outlook on the questions not previously addressed is provided.

An enhanced mass spectrometry approach reveals human embryonic stem cell growth factors in culture

The derivation and long-term maintenance of human embryonic stem cells (hESCs) has been established in culture formats that are both dependent and independent of support (feeder) cells. However, the factors responsible for preserving the viability of hESCs in a nascent state remain unknown. We describe a mass spectrometry-based method for probing the secretome of the hESC culture microenvironment to identify potential regulating protein factors that are in low abundance. Individual samples were analyzed several times, using successive mass (m/z) and retention time-directed exclusion, without sampling the same peptide ion twice. This iterative exclusion -mass spectrometry (IE-MS) approach more than doubled protein and peptide metrics in comparison to a simple repeat analysis method on the same instrument, even after extensive sample pre-fractionation. Furthermore, implementation of the IE-MS approach was shown to enhance the performance of an older quadrupole time of flight (Q-ToF) MS. The resulting number of identified peptides approached that of a parallel repeat analysis on a newer LTQ-Orbitrap MS. The combination of the results of both instruments proved to be superior to that achieved by a single instrument in the identification of additional proteins. Using the IE-MS strategy, combined with complementary gel- and solution-based fractionation methods, the hESC culture microenvironment was extensively probed. Over 10 to 12 times more extracellular proteins were observed compared with previously published surveys. The detection of previously undetectable growth factors, present at concentrations ranging from 10(-9) to 10(-11) g/ml, highlights the depth of our profiling. The IE-MS approach provides a simple and reliable technique that greatly enhances instrument performance by increasing the effective depth of MS-based proteomic profiling. This approach should be widely applicable to any LC-MS/MS instrument platform or biological system.

A Novel Adipocytokine, Visceral Adipose Tissue-derived Serine Protease Inhibitor (Vaspin), and Obesity

Visceral adipose tissue-derived serine protease inhibitor (vaspin) is an interesting novel adipocytokine with insulin-sensitizing effects. Some studies have suggested that vaspin could play an important role in the development of obesity and metabolic disorders. The induction of vaspin mRNA expression could represent a compensatory mechanism associated with obesity, severe insulin resistance and type 2 diabetes mellitus, however it is unclear whether a correlation exists between human vaspin serum levels and markers of insulin sensitivity and glucose or lipid metabolism. Vaspin serum concentrations have been shown to be lower in lean subjects and competitive sportsmen with long-term physical training, but they are increased with weight loss associated with a physical training programme. In conclusion, there is at present no clear proof of a causal link between vaspin and visceral fat accumulation, or insulin resistance. This article reviews the role of vaspin in obesity-associated diseases and its potential as a new biomarker for obesity and impaired insulin sensitivity.

Subcutaneous adipose tissue-derived stem cells facilitate colonic mucosal recovery from 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colitis in rats

BACKGROUND

Adipose tissue-derived stem cells (ADSCs) can be easily obtained from subcutaneous adipose tissue, and ADSCs can be demonstrated to display multilineage developmental plasticity. In this study, using TNBS-induced colitis rats, we show the feasibility of repairing injured intestinal mucosa with adipose tissue-derived stem cells.

METHODS

The subcutaneous adipose tissue of F344 rats was obtained and digested by collagenase. The digested tissue was cultured in DMEM containing 10% FBS for 1 month. ADSCs were confirmed to differentiate under appropriate conditions into various lineages of cells, including bone, neural cells, adipocytes, and epithelial cells. HGF, VEGF, TGF-beta, and adiponectin in the culture supernatants of ADSCs were determined by ELISA. ADSCs (10(7) cells) were injected into the submucosa of the colon to examine their capacity to repair intestinal mucosa injured by TNBS.

RESULTS

In the experimental colitis model, the injection of ADSCs facilitated colonic mucosal repair and reduced the infiltration of inflammatory cells. High levels of HGF, VEGF, and adiponectin were detected in the culture supernatants of ADSCs. Moreover, injected ADSCs distributed to several layers of the colon, and some of them differentiated into mesodermal lineage cells.

CONCLUSIONS

ADSCs can accelerate the regeneration of injured regions in experimental colitis. HGF, VEGF, and adiponectin might be responsible for the regeneration of injured regions in the colon.

In vitro Differentiation Potential of Mesenchymal Stem Cells

SUMMARY: Mesenchymal stem cells (MSCs) represent a class of multipotent progenitor cells that have been isolated from multiple tissue sites. Of these, adipose tissue and bone marrow offer advantages in terms of access, abundance, and the extent of their documentation in the literature. This review focuses on the in vitro differentiation capability of cells derived from adult human tissue. Multiple, independent studies have demonstrated that MSCs can commit to mesodermal (adipocyte, chondrocyte, hematopoietic support, myocyte, osteoblast, tenocyte), ectodermal (epithelial, glial, neural), and endodermal (hepatocyte, islet cell) lineages. The limitations and promises of these studies in the context of tissue engineering are discussed.

Identification of paracrine neuroprotective candidate proteins by a functional assay-driven proteomics approach

Glial cells support neuronal survival and function by secreting neurotrophic cytokines. Retinal Mueller glial cells (RMGs) support retinal neurons, especially photoreceptors. These highly light-sensitive sensory neurons receive vision, and their death results in blinding diseases. It has been proposed that RMGs release factors that support photoreceptor survival, but the nature of these factors remains to be elucidated. To discover such neurotrophic factors, we developed an integrated work flow toward systematic identification of neuroprotective proteins, which are, like most cytokines, expressed only in minute amounts. This strategy can be generally applied to identify secreted bioactive molecules from any body fluid once a recipient cell for this activity is known. Toward this goal we first isolated conditioned medium (CM) from primary porcine RMGs cultured in vitro and tested for survival-promoting activity using primary photoreceptors. We then developed a large scale, microplate-based cellular high content assay that allows rapid assessment of primary photoreceptor survival concomitant with biological activity in vitro. The enrichment strategy of bioactive proteins toward their identification consists of several fractionation steps combined with tests for biological function. Here we combined 1) size fractionation, 2) ion exchange chromatography, 3) reverse phase liquid chromatography, and 4) mass spectrometry (Q-TOF MS/MS or MALDI MS/MS) for protein identification. As a result of this integrated work flow, the insulin-like growth factor-binding proteins IGFBP5 and IGFBP7 and connective tissue growth factor (CTGF) were identified as likely candidates. Cloning and stable expression of these three candidate factors in HEK293 cells produced conditioned medium enriched for either one of the factors. IGFBP5 and CTGF, but not IGFBP7, significantly increased photoreceptor survival when secreted from HEK293 cells and when added to the original RMG-CM. This indicates that the survival-promoting activity in RMG-CM is multifactorial with IGFBP5 and CTGF as an integral part of this activity.