Defining the Regulated Secreted Proteome of Rodent Adipocytes upon the Induction of Insulin Resistance

Abstract

Insulin resistance defines the metabolic syndrome and precedes, as well is the hallmark of, type II diabetes. Adipocytes, besides being a major site for energy storage, are endocrine in nature and secrete a variety of proteins, adipocytokines (adipokines), that can modulate insulin sensitivity, inflammation, obesity, hypertension, food intake (anorexigenic and orexigenic), and general energy homeostasis. Recent data demonstrates that increased intracellular glycosylation of proteins via O-GlcNAc can induce insulin resistance and that a rodent model with genetically elevated O-GlcNAc levels in muscle and fat displays hyperleptinemia. The link between O-GlcNAc levels, insulin resistance, and adipocytokine secretion is further explored here. First, with the use of immortalized and primary rodent adipocytes, the secreted proteome of differentiated adipocytes is more fully elucidated by the identification of 97 and 203 secreted proteins, respectively. Mapping of more than 80 N-linked glycosylation sites on adipocytokines from the cell lines further defines this proteome. Importantly, adipocytokines that are modulated when cells are shifted from insulin responsive to insulin resistant conditions are determined. By the use of two protocols for inducing insulin resistance, classical hyperglycemia with chronic insulin exposure and pharmacological elevation of O-GlcNAc levels, several proteins are identified that are regulated in a similar fashion under both conditions including HCNP, Quiescin Q6, Angiotensin, lipoprotein lipase, matrix metalloproteinase 2, and slit homologue 3. Detection of these potential prognostic/diagnostic biomarkers for metabolic syndrome, type II diabetes, and the resulting complications of both diseases further establishes the central role of the O-GlcNAc modification of intracellular proteins in the pathophysiology of these conditions.

Raf Kinase Inhibitory Protein (RKIP): Functional Pleiotropy in the Mammalian Brain

“…18 There is some recent evidence to indicate that HCNP, and potentially RKIP itself, is secreted under certain conditions, although how this occurs and through which receptors the secreted peptide acts are outstanding questions. 19,20 Within the cell, both the work of Bernier et al and the subsequent elucidation of the crystal structure of RKIP suggest strongly that RKIP is associated with the inner leaflet of the plasma membrane, where it is ideally situated to transduce signals from the membrane to the cytoplasm. 2,21 On the other hand, immunohistochemical studies have revealed that RKIP may be expressed in multiple subcellular compartments, depending on the cell type or condition: RKIP is localized predominantly in the cytoplasm and at the plasma membrane, with occasional expression detected within the nucleus.…”

Role of Adipose Secreted Factors and Kisspeptin in the Metabolic Control of Gonadotropin Secretion and Puberty

“…Recent proteomic studies of human and rat adipocytes have revealed the true scope of the adipose tissue secretome (Chen et al, 2005;Kheterpal et al, 2011;Lehr et al, 2012;Lim et al, 2008;Zhong et al, 2010). With refined and advanced proteomics techniques, these studies have revealed that many of the adipose tissue secreted factors identified at the gene level do indeed encode secreted proteins (Chen et al, 2005;Kheterpal et al, 2011;Lehr et al, 2012;Lim et al, 2008;Zhong et al, 2010).…”

“…With refined and advanced proteomics techniques, these studies have revealed that many of the adipose tissue secreted factors identified at the gene level do indeed encode secreted proteins (Chen et al, 2005;Kheterpal et al, 2011;Lehr et al, 2012;Lim et al, 2008;Zhong et al, 2010). The presence of an N-terminal secretion signal peptide validates secreted proteins in conditioned media (Renes et al, 2009).…”

“…In many of these studies, the presence or absence of a signal peptide was used to validate or identify truly secreted adipocyte proteins (Chen et al, 2005;Lehr et al, 2012;Lim et al, 2008;Zhong et al, 2010). In these studies, the percentage of total apparent secreted proteins that were considered secreted (+ signal peptide) ranged from 39 to 75% and the total number of secreted proteins ranged from 164 to 263 (Chen et al, 2005;Lehr et al, 2012;Lim et al, 2008;Zhong et al, 2010). However, the signal peptide approach could underestimate the adipocyte derived proteins present in the extracellular space (review, Renes et al, 2009).…”

Structural Basis for Nucleotide Hydrolysis by the Acid Sphingomyelinase-like Phosphodiesterase SMPDL3A

“…The substrate and function of SMPDL3A, however, are less clear. This enzyme is secreted by osteoclasts (8), adipocytes (9), astrocytes (10), and macrophages (11), where it is up-regulated by the activation of the liver X receptor (12,13). In the serum, SMPDL3A interacts with apolipoprotein A1 (14,15).…”