The Matricellular Protein SPARC/Osteonectin as a Newly Identified Factor Up-regulated in Obesity

Abstract

Alterations in the expression level of genes may contribute to the development and pathophysiology of obesity. To find genes differentially expressed in adipose tissue during obesity, we performed suppression subtractive hybridization on epididymal fat mRNA from goldthioglucose (GTG) obese mice and from their lean littermates. We identified the secreted protein acidic and rich in cysteine (SPARC), a protein that mediates cell-matrix interactions and plays a role in modulation of cell adhesion, differentiation, and angiogenesis. SPARC mRNA expression in adipose tissue was markedly increased (between 3-and 6-fold) in three different models of obesity, i.e. GTG mice, ob/ob mice, and AKR mice, after 6 weeks of a high fat diet. Immunoblotting of adipocyte extracts revealed a similar increase in protein level. Using a SPARC-specific ELISA, we demonstrated that SPARC is secreted by isolated adipocytes. We found that insulin administration to mice increased SPARC mRNA in the adipose tissue. Food deprivation had no effect on SPARC expression, but after high fat refeeding SPARC mRNA levels were significantly increased. Our results reveal both hormonal and nutritional regulation of SPARC expression in the adipocyte, and importantly, its alteration in obesity. Finally, we show that purified SPARC increased mRNA levels of plasminogen activator inhibitor 1 (PAI-1) in cultured rat adipose tissue suggesting that elevated adipocyte expression of SPARC might contribute to the abnormal expression of PAI-1 observed in obesity. We propose that SPARC is a newly identified autocrine/paracrine factor that could affect key functions in adipose tissue physiology and pathology.

Matrix Metalloproteinases Are Differentially Expressed in Adipose Tissue during Obesity and Modulate Adipocyte Differentiation

“…Animals were maintained in a temperature-controlled facility (22°C) on a 12-h light-dark cycle with regular unrestricted diet. Induction of obesity by high fat diet in AKR mice was carried out as described previously (5). Animals were killed by cervical dislocation, and epididymal fat pads were rapidly dissected and processed for RNA or protein analysis.…”

“…Northern blotting was conducted as previously described (5). Briefly, 10 g of denatured RNA was resolved by electrophoresis on 1.2% agarose gels and transferred to positively charged nylon membrane.…”

“…Rat cDNA fragments for gelatinase A (MMP-2), gelatinase B (MMP-9), stromelysin-1 (MMP-3), stromelysin-3 (MMP-11), MT1-MMP (MMP-14), collagenase-3 (MMP-13), TIMP-1, TIMP-2, and TIMP-3 were used to probe for the corresponding mRNAs. cDNA probes for mouse metalloelastase (MMP-12) and TIMP-4 were obtained by reverse transcriptase-PCR from mouse epididymal white adipose tissue RNA as described for other probes (5). cDNA for aP2 was provided by C. Dani (CNRS, UMR 6543, Nice, France).…”

“…Adipose Tissue Fractionation-Epididymal fat pads from male Wistar rats or from male C57BL/6 and ob/ob mice were minced and digested with the Liberase Blendzyme 3 (Roche Molecular Biochemicals) as previously described (5). Isolated adipocytes were separated from stromal-vascular cells (S-V) by centrifugation.…”

“…Although numerous studies have examined the regulation and intracellular events that occur during adipogenesis, only limited information is available about molecular mechanisms underlying ECM remodeling, proteolytic events, and cell-matrix interactions during obesity-related fat mass development. In a course of differential gene expression analysis between lean and obese adipose tissues, we recently identified osteonectin/ SPARC, a protein involved in dynamic interactions between cell and matrix, up-regulated in obesity (5). This suggested that alteration in the expression level of matrix proteins may contribute to the development of obesity-associated adipose tissue growth.…”