Central role of SREBP-2 in the pathogenesis of osteoarthritis.
BACKGROUND
Recent studies have implied that osteoarthritis (OA) is a metabolic disease linked to deregulation of genes involved in lipid metabolism and cholesterol efflux. Sterol Regulatory Element Binding Proteins (SREBPs) are transcription factors regulating lipid metabolism with so far no association with OA. Our aim was to test the hypothesis that SREBP-2, a gene that plays a key role in cholesterol homeostasis, is crucially involved in OA pathogenesis and to identify possible mechanisms of action.


RESULTS
We performed a genetic association analysis using a cohort of 1,410 Greek OA patients and healthy controls and found significant association between single nucleotide polymorphism (SNP) 1784G>C in SREBP-2 gene and OA development. Moreover, the above SNP was functionally active, as normal chondrocytes' transfection with SREBP-2-G/C plasmid resulted in interleukin-1β and metalloproteinase-13 (MMP-13) upregulation. We also evaluated SREBP-2, its target gene 3-hydroxy-3-methylglutaryl-coenzymeA reductase (HMGCR), phospho-phosphoinositide3-kinase (PI3K), phospho-Akt, integrin-alphaV (ITGAV) and transforming growth factor-β (TGF-β) mRNA and protein expression levels in osteoarthritic and normal chondrocytes and found that they were all significantly elevated in OA chondrocytes. To test whether TGF-β alone can induce SREBP-2, we treated normal chondrocytes with TGF-β and found significant upregulation of SREBP-2, HMGCR, phospho-PI3K and MMP-13. We also showed that TGF-β activated aggrecan (ACAN) in chondrocytes only through Smad3, which interacts with SREBP-2. Finally, we examined the effect of an integrin inhibitor, cyclo-RGDFV peptide, on osteoarthritic chondrocytes, and found that it resulted in significant upregulation of ACAN and downregulation of SREBP-2, HMGCR, phospho-PI3K and MMP-13 expression levels.


CONCLUSIONS
We demonstrated, for the first time, the association of SREBP-2 with OA pathogenesis and provided evidence on the molecular mechanism involved. We suggest that TGF-β induces SREBP-2 pathway activation through ITGAV and PI3K playing a key role in OA and that integrin blockage may be a potential molecular target for OA treatment.
Department of Cytogenetics and Molecular Genetics, University of Thessaly School of Medicine, Larissa, Greece.
Fotini
F
Kostopoulou
Vasiliki
V
Gkretsi
Konstantinos N
KN
Malizos
Dimitrios
D
Iliopoulos
Pagona
P
Oikonomou
Lazaros
L
Poultsides
Aspasia
A
Tsezou
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D053674:Aggrecans / Q000235:genetics; D000483:Alleles; D002478:Cells, Cultured; D019902:Chondrocytes / Q000187:drug effects / Q000378:metabolism; D024042:Collagen Type I / Q000378:metabolism; D024043:Collagen Type II / Q000378:metabolism; D005260:Female; D005786:Gene Expression Regulation / Q000187:drug effects; D020022:Genetic Predisposition to Disease; D005838:Genotype; D006579:Heterozygote; D006801:Humans; D006888:Hydroxycholesterols / Q000494:pharmacology; D006903:Hydroxymethylglutaryl CoA Reductases / Q000235:genetics / Q000378:metabolism; D039564:Integrin alphaV / Q000235:genetics; D008297:Male; D008875:Middle Aged; D009842:Oligopeptides / Q000494:pharmacology; D010003:Osteoarthritis / Q000235:genetics* / Q000378:metabolism; D010456:Peptides, Cyclic / Q000494:pharmacology; D010641:Phenotype; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation; D020641:Polymorphism, Single Nucleotide; D011485:Protein Binding; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D018125:Receptors, Transforming Growth Factor beta / Q000037:antagonists & inhibitors; D051900:Smad3 Protein / Q000378:metabolism; D051782:Sterol Regulatory Element Binding Protein 2 / Q000235:genetics* / Q000378:metabolism; D016212:Transforming Growth Factor beta / Q000235:genetics / Q000378:metabolism
C508319:ACAN protein, human; D053674:Aggrecans; D024042:Collagen Type I; D024043:Collagen Type II; D006888:Hydroxycholesterols; D039564:Integrin alphaV; D009842:Oligopeptides; D010456:Peptides, Cyclic; D018125:Receptors, Transforming Growth Factor beta; D051900:Smad3 Protein; D051782:Sterol Regulatory Element Binding Protein 2; D016212:Transforming Growth Factor beta; C007997:25-hydroxycholesterol; C047981:arginyl-glycyl-aspartic acid; D006903:Hydroxymethylglutaryl CoA Reductases; D019869:Phosphatidylinositol 3-Kinases; D051057:Proto-Oncogene Proteins c-akt