Distribution and regulation of plasminogen activator inhibitor-1 in murine adipose tissue in vivo. Induction by tumor necrosis factor-alpha and lipopolysaccharide

Elevated expression of transforming growth factor-beta in adipose tissue from obese mice

BACKGROUND

Tumor necrosis factor-alpha (TNF-alpha) is chronically elevated in the adipose tissue from obese humans and mice. This increase in TNF-alpha contributes to the insulin resistance, elevated plasminogen activator inhibitor-1 (PAI-1) levels, and cardiovascular complications associated with obesity and noninsulin-dependent diabetes (NIDDM). PAI-1 gene expression in adipose tissue is also stimulated by transforming growth factor-beta (TGF-beta). Experiments were performed to determine whether TGF-beta is regulated by TNF-alpha and elevated in obesity.

MATERIALS AND METHODS

The concentration of TGF-beta and PAI-1 mRNA in murine adipose tissue and cultured 3T3-L1 adipocytes was determined by quantitative reverse transcription polymerase chain reaction (RT-PCR), and the cellular localization of these molecules was evaluated using in situ hybridization and cell fractionation. Total TGF-beta protein was determined by employing an ELISA assay.

RESULTS

TGF-beta mRNA and protein were increased in the adipose tissue from two different strains of genetically obese mice (i.e., ob/ob and db/db), compared with their lean counterparts. This increase in TGF-beta may result from TNF-alpha since TNF-alpha increased TGF-beta mRNA expression in the adipose tissue of lean mice and stimulated TGF-beta production by cultured adipocytes. Administration of TGF-beta increased PAI-1 antigen in the plasma and PAI-1 mRNA in the adipocytes of lean mice, and enhanced the rate of PAI-1 synthesis by adipocytes in vitro.

CONCLUSIONS

TNF-alpha contributes to the elevated TGF-beta expression demonstrated in the adipose tissue of obese mice. A potential role for TGF-beta in the increased PAI-1 and vascular pathologies associated with obesity/NIDDM is suggested.

Adipocyte differentiation: a transcriptional regulatory cascade

The adipose cell is now known to play a complex role in energy homeostasis, energy storage and signaling to other tissues concerning the state of energy balance. The past few years have seen an explosive increase in our knowledge of the transcriptional basis of adipocyte differentiation. Factors such as peroxisome proliferator-activated receptor gamma, the CCAAT/enhancer binding protein family members, and adipocyte determination- and differentiation-dependent factor 1 play important regulatory roles in this process. Furthermore, these factors provide a focus for beginning to understand how various hormones and metabolites influence the development of adipose tissue in vivo.

Tissue distribution and regulation of plasminogen activator inhibitor-1 in obese mice

BACKGROUND

Although elevated plasminogen activator inhibitor-1 (PAI-1) is associated with obesity and may be a risk factor for cardiovascular disease, the mechanism(s) that lead to this elevation, and the tissue/cellular origins of this increase, remain to be defined. In this report, we have addressed these questions using genetically obese mice (ob/ob) and their lean counterparts (+/?).

MATERIALS AND METHODS

PAI-1 activity and antigen levels were determined using a tissue-type plasminogen activator (t-PA) binding assay and Western blotting. The concentration of PAI-1 mRNA in tissues was determined by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), and the cellular localization of PAI-1 was evaluated using in situ hybridization, immunohistochemistry, and cell fractionation.

RESULTS

PAI-1 activity was approximately 4-fold higher in plasma from ob/ob mice than in that obtained from their lean counterparts, and this difference increased further with age (i.e., 6-fold at 3 months). PAI-1 mRNA levels were elevated 4- to 5-fold in the adipose tissues of obese mice, and these differences in mRNA also increased with age. The elevated PAI-1 mRNA in the adipose tissues of obese mice was localized to mature adipocytes as well as to vascular smooth muscle cells and occasional endothelial cells. Obesity is often associated with hyperinsulinemia, and acute injection of insulin into lean mice increased PAI-1 mRNA 6- to 8-fold in the epididymal fat in cells that morphologically resembled adipocytes. Insulin did not increase PAI-1 in large vessel endothelial or smooth muscle cells. The adipocyte response to insulin was confirmed in cell culture studies where PAI-1 synthesis by mature 3T3-L1 adipocytes was increased 5- to 6-fold by insulin.

CONCLUSIONS

Our results suggest that elevated PAI-1 associated with obesity may result in part from insulin-induced induction of PAI-1 specifically by adipocytes within the fat itself.

Fibrin deposition in tissues from endotoxin-treated mice correlates with decreases in the expression of urokinase-type but not tissue-type plasminogen activator

The primary hypothesis of this report is that the formation and subsequent removal of fibrin in specific tissues during pathologic processes reflects temporal changes in the local expression of key procoagulant and fibrinolytic genes. To begin to test this hypothesis, we have used quantitative PCR assays and in situ hybridization analysis to examine the effects of endotoxin on the expression of specific genes in murine tissues, and to relate these changes to fibrin deposition/dissolution using immunohistochemical approaches. Endotoxin caused large increases in plasminogen activator inhibitor-1 mRNA and modest increases in tissue factor mRNA in most tissues examined. However, fibrin was only detected in the kidneys and adrenals of endotoxin-treated mice, and it was transient. Unexpectedly, changes in urokinase-type plasminogen activator mRNA but not tissue-type plasminogen activator mRNA correlated with fibrin deposition/dissolution in these tissues. Pretreatment of mice with the fibrinolytic inhibitor epsilon-aminocaproic acid before endotoxin increased both the number of fibrin-positive tissues and the duration of fibrin deposition in the kidneys and adrenals. These results suggest that the absence of fibrin in some tissues reflects ongoing local fibrinolysis, and that increases in plasminogen activator inhibitory and tissue fac- tor gene expression and decreases in urokinase-type plasminogen activator expression are necessary for tissue-specific fibrin deposition. Changes in tissue-type plasminogen activator gene expression do not appear to be essential for fibrin deposition/dissolution in this murine model of sepsis.