Brain Tumor Microvesicles: Insights into Intercellular Communication in the Nervous System

Brain tumors are heterogeneous tumors composed of differentiated tumor cells that resemble various neural cells and a small number of multipotent cancer stem cells. These tumors modify normal cells in their environment to promote tumor growth, invasion and metastases by various ways. Recent publications show that glioblastoma cells release microvesicles that contain a select subset of cellular proteins and RNAs. These microvesicles are avidly taken up by normal cells in cell culture and can change the translational profile of these cells through delivery of tumor-derived mRNAs, which are translated into functional proteins. In addition to mRNA and proteins, microvesicles have been shown to contain microRNAs, non-coding RNAs and DNA. This commentary explores the recent advances in this novel intercellular communication route and discusses the potential physiological role of microvesicles in brain tumorigenesis.     

Evidences against vesicle-dependent trafficking and involvement of extracellular proteasomes into cell-to-cell communications

The ubiquitin proteasome system is involved in the regulation of most basic intracellular processes, and deregulation of this system can results in certain kinds of human diseases. Proteolytic core this system, the 20S proteasome, has been found in physiological fluids of both healthy humans and patients suffering from a variety of inflammatory, autoimmune, and neoplastic diseases. The concentration of these extracellular proteasomes has been found to correlate with the diseased state, being of a prognostic significance. The transport mechanisms and functions of these proteasomes, however, are largely unclear. Previous studies revealed that the transport of extracellular proteasomes may occur via microvesicles and exosomes, which led to the hypothesis that extracellular proteasomes are implicated in cell-to-cell communication process. Here we show that microvesicles and exosomes, two major known types of intercellular vehicles, contain no detectable proteasomes. Moreover, neither affinity purified nor naturally released into conditioned medium by donor cells 20S proteasomes could penetrate recipient HeLa cells. Taken together, these results suggest that extracellular proteasomes are unlikely to be involved in the cell-to-cell communication and that their release by cells serve other biological purposes.     

Adiponectin is partially associated with exosomes in mouse serum

Exosomes are membrane vesicles 30–120 nm in diameter that are released by many cell types and carry a cargo of proteins, lipids, mRNA, and microRNA. Cultured adipocytes reportedly release exosomes that may play a role in cell-to-cell communication during the development of metabolic diseases. However, the characteristics and function of exosomes released from adipocytes in vivo remain to be elucidated. Clearly, adipocyte-derived exosomes could exist in the circulation and may be associated with adipocyte-specific proteins such as adipocytokines. We isolated exosomes from serum of mice by differential centrifugation and analyzed adiponectin, leptin, and resistin in the exosome fraction. Western blotting detected adiponectin but no leptin and only trace amounts of resistin in the exosome fraction. The adiponectin signal in the exosome fraction was decreased by proteinase K treatment and completely quenched by a combination of proteinase K and Triton X-100. Quantitative ELISA showed that the exosome fraction contains considerable amounts of adiponectin, but not leptin or resistin. The concentration of adiponectin in the serum and the ratio of adiponectin to total protein in the exosome fraction were lower in obese mice than in lean mice. These results suggest that a portion of adiponectin exists as a transmembrane protein in the exosomes in mouse serum. We propose adiponectin as a marker of exosomes released from adipocytes in vivo.     

Inflammation Activation Contributes to Adipokine Imbalance in Patients with Acute Coronary Syndrome

Inflammation can be activated as a defensive response by the attack of acute coronary syndrome (ACS) for ischemic tissue injury. The aim of the present study was to investigate the impact of ACS-activated inflammation on adipokine imbalance and the effects of statins on the crosstalk between inflammation and adipokine imbalance during ACS. In this study, 586 subjects were categorized into: (1) control group; (2) SA (stable angina) group; and (3) ACS group. Circulating levels of hs-CRP, adiponectin and resistin were measured by ELISA. Furthermore, forty C57BL/6 mice were randomized into: sham, AMI, low-statin (atorvastatin, 2 mg/kg/day) and high-statin (atorvastatin, 20 mg/kg/day) group. After 3 weeks, AMI models were established by surgical coronary artery ligation. Circulating levels and adipose expressions of adiponectin and resistin were assessed in animals. Besides, we investigate the effects of atorvastatin on ox-LDL-induced adipokine imbalance in vitro. As a result, we found that ACS patients had higher hs-CRP and resistin levels and lower adiponectin levels. Our correlation analysis demonstrated hs-CRP concentrations were positively correlated with resistin but negatively with adiponectin levels in humans. Our animal findings indicated higher circulating hs-CRP and resistin levels and lower adiponectin levels in AMI mice. Atorvastatin pre-treatment dose-dependently decreased hs-CRP and resistin levels but increased adiponectin levels in mice. The consistent findings were observed about the adipose expressions of resistin and adiponectin in mice. In study in vitro, ox-LDL increased cellular resistin expressions and otherwise for adiponectin expressions, which dose-dependently reversed by the addition of atorvastatin. Therefore, our study indicates that the ACS attack activates inflammation leading to adipokine imbalance that can be ameliorated by anti-inflammation of atorvastatin.     

Direct reciprocal effects of resistin and adiponectin on vascular endothelial cells: a new insight into adipocytokine-endothelial cell interactions

Resistin is an adipocytokine which plays a role in the development of insulin resistance. In this study, we investigated the direct effect of resistin on vascular endothelial cells. Resistin induced the expression of adhesion molecules such as VCAM-1 and ICAM-1, and long pentraxin 3, a marker of inflammation. The induction of VCAM-1 by resistin was inhibited partially by pitavastatin. Moreover, the induction of VCAM-1 and ICAM-1 by resistin was inhibited by adiponectin, an adipocytokine that improves insulin resistance. Taken together, these results suggest that the balance in the concentrations of adipocytokines such as resistin and adiponectin determines the inflammation status of vasculature, and in turn the progress of atherosclerosis.     

Adiponectin is stimulated by adrenalectomy in ob/ob mice and is highly correlated with resistin mRNA

Plasma levels of the adipocyte product adiponectin, a putative insulin-sensitizing agent, are reduced in obesity, whereas plasma levels of resistin, an agent that some believe to confer insulin resistance, are thought to increase with obesity. Because adrenalectomy can increase insulin sensitivity, we hypothesized that adrenalectomy would increase expression of adiponectin and decrease expression of resistin. Therefore, we measured adiponectin mRNA, adiponectin peptide, and resistin mRNA in adrenalectomized ob/ob mice. Adrenalectomy restored adiponectin expression in ob/ob mice to wild-type levels and stimulated adiponectin peptide to above wild-type levels. Surprisingly, expression of adiponectin and resistin was highly positively correlated even after statistical removal of effects of insulin, glucose, and adiposity. In addition, adiponectin and resistin expression were also highly correlated in diet-induced obese mice. The data support a role for adiponectin in mediating some effects of adrenalectomy on insulin sensitivity.