İnflamatuar barsak hastalıklarında peroksizom proliferator aktivite resptör - gamma (PPAR-γ) geninde Pro12Ala polimorfizmin sıklığı

Abstract

ÖZET İnflamatuar Barsak Hastalıkları, genetik yatkınlık, alevlenme- remisyon dönemleri, ekstra-intestinal belirtiler ve uzun süreli hastalıkta görülen kanser riski ile karakterize gastrointestinal sistemin sebebi bilinmeyen kronik inflamatuar hastalıklarıdır. Bu başlık altında yer alan Crohn hastalığı ve ülseratif kolit, tutulum yeri, histolojisi ve immünolojik cevaplardaki temel farklılıklar açısından birbirinden ayrılmaktadır. Epidemiyolojik çalışmalar bu hastalıkları birbirinden ayırmaya yönelik çevresel ve genetik faktörleri araştırma yönünde ilerlemektedir. Bizim çalışmamızda da bu amaca yönelik olarak moleküler biyolojideki hızlı gelişimler sayesinde 1990'h yılların başında tanıştığımız Uganda bağlı aktive olan transkripsiyon faktör ailesine mensup nükleer hormon reseptörlerinden PPAR-y (Peroxisome Proliferator Activated Receptor-gamma)' nın özellikle başta Crohn hastalığı olmak üzere ülseratif kolitte ve sağlıklı Türk popülasyonundaki gen polimorfizmini araştırdık. PPAR-y' lar başlıca adiposit diferansiyasyonu, lipid metabolizması ve glukoz homeostazisinde rol almakla beraber son çalışmalarda anti -inflamatuar etkinlikleri, hücre proliferasyonu ve diferansiyasyonundaki rolleri ortaya konulmuştur. Bu sebeple PPAR-y ligandları, kronik inflamatuar hastalıkların ve çeşitli kanser tiplerinin önlenmesinde ve tedavisinde telaffuz edilir olmaya başlamıştır. Çalışmamızda literatürde ilk defa olarak mezenter adipoz dokuda hipertrofi ile karakterize Crohn hastalığında, adiposit diferansiyasyonunda ve sitokin genlerinin transkripsiyonunda kritik rolü olan PPAR-y geninde, Pro 12 Ala polimorfizmininin sıklığım %15.9 olarak tesbit ettik. Bu polimorfizmin sıklığı ülseratif kolitte % 15.5, sağlıklı Türk popülasy onunda % 13 olarak bulundu. Sonuç olarak İBH' da, PPAR-y geninde, Prol2 Ala polimorfizmi açısından istatistiksel olarak anlamlı bir farklılık tesbit edemedik. Ancak etnik gruplar arasında sıklığı farklılıklar arzeden bu polimorfizmin sağlıklı Türk popülasyonundaki sıklığını % 13 olarak bulduk. Bu oran PPAR-y geninde, Prol2Ala polimorfizminin beyaz ırktaki sıklığı ile paralellik göstermektedir. 52

SUMMARY Peroxisome proliferator-activated receptor (PPAR) is a nuclear hormone receptor that belongs to the family of ligand-activated transcription factors with pleitropic effects on intra- and extracellular lipid metabolism, glucose homeostasis, cell proliferation and control of inflammation. Three PPARs, namely alpha (a), beta/delta (fVÖ), and gamma (y) have been identified with distinct tissue distribution patterns and functions. PPAR- alpha is predominantly expressed in brown adipose tissue, liver, kidney, duodenum, heart, skeletal muscle and vascular endothelial cells and is involved in the control of lipoprotein metabolism, fatty acid oxidation and the cellular uptake of fatty acids. PPAR- gamma is expressed in brown and white adipose tissues, large intestine, retina and some parts of the immune system. PPAR-delta, shows a widespread tissue distribution but its regulation and functions are not known yet. Peroxisome proliferator-activated receptor-gamma ( PPAR-y ), originally shown to play a critical role in adipocyte differentiation and glucose homeostasis, has recently been implicated as a regulator of cellular proliferation and inflammatory responses. The molecular mechanisms mediating the anti-inflammatory action of the PPAR-y are not fully understood at the present time. Several studies have shown that activation of PPAR-y may interfere with several signaling pathways regulating the expression of proinflammatory genes, such as those controlled by stress kinases, NF-kB, signal transducers and activators of transcription ( STATs ), activating protein 1 and the nuclear factor of activated T-cells. Consequent to the inhibition of these signaling pathways, PPAR-y activators modulate the production of inflammatory cytokines, chemokines and cell-adhesion molecules, thereby limiting the recruitment of inflammatory cells. Colonic epithelial cells, which express high levels of PPAR-y protein, have the ability to produce inflammatory cytokines that may play a role in inflammatory bowel disease ( IBD). Despite evidence for anti-inflammatory actions of the PPAR-y in the colon in animal models, the role of PPAR-y in ulcerative colitis and Crohn's disease, the two main forms of BBD in humans has not been fully explored. In patients with ulcerative colitis, Desreumaux et al, recently reported an impaired expression of PPAR-y at the mRNA and protein levels. As peripheral mononuclear cells of ulcerative colitis patients expressed normal levels of PPAR-y, it is likely that factors within the intestinal lumen may contribute to the observed decreased 53expression of PPAR-y in epithelial cells. Crohn's disease, on the other hand, is characterized by a localized hypertrophy of the mesenteric adipose tissue, resulting in the so called fat wrapping of the intestine. Expression of PPAR-y is enhanced in mesenteric white adipose tissue in Crohn's disease. This hypertrophic mesenteric adipose tissue in Crohn's disease is a rich source of TNF-a, which sustains local inflammatory responses. On the basis of the above information, several clinical trials have been initiated to evaluate the therapeutic efficacy of PPAR-y agonists in EBD. Several historical observations are consistent with the possible utility of PPAR-y activation in the treatment of colonic inflammation. First, polyunsaturated fatty acids, such as provided in fish oil, have been shown to be potent activators of PPAR-y and to have efficacy in the treatment of TBD. Second, there is evidence that thiazolidinediones (TZDs) may decrease insulin resistance by decreasing the production of TNF-a by adipose tissue and/or by inhibiting TNF-a mediated signal transduction. Inhibition of TNF-a activity by the use of anti-TNF-a antibodies has been shown to be an effective treatment modality for refractory Crohn's disease. The fact that PPAR-y is expressed in colonic epithelial cells and mesenteric adipose tissue and its ligands have been shown to inhibit tissue injury associated with immune activation in patients with ulcerative colitis, led us to explore polymorphism in the PPAR-y gene in patients with IBD. A number of genetic variants in the PPAR-y gene have been identified. These include a very rare gain of function mutation ( Proll5Gln ) associated with obesity but not insulin resistance, two loss of function mutations ( Val290Met and Pro467Leu ) reported in three individuals with severe insulin resistance but normal body weight, the silent CAC478CAT mutation and the highly prevalent Prol2Ala polymorphism in PPAR-y. The latter is the result of a CCA to GCA missense mutation in codon 12 of exon B of the PPAR-y gene. The Prol2Ala polymorphism in PPAR-y gene was first identified in 1997, and the rare allele frequencies are 11-15 % in Caucasians, 10% in Native Americans, 8% in Samoans, 4% in Japanese, 3% in African-Americans, 1 % in Chinese. In our study, we observed no significant differences in the frequency of the Prol2Ala polymorphism in PPAR-y gene among the cases with Crohn's disease, ulcerative colitis and healthy controls ( % 15.9, %15.5, %13, p>0.05). The frequency of the Prol2Ala polymorphism in PPAR-y gene in our control Turkish population is similar to that observed in other Caucasion population. 54In conclusion, these results of this small series show that mutations in the PPAR-y gene are unlikely to explain the increased expression of PPAR-y in hypertrophic mesenteric adipose tissue in Crohn's disease and the impaired expression of PPAR-y in ulcerative colitis. Thus, someother epigenetic events may be responsible in the pathogenesis of TBD and especially Crohn's disease. 55