HISTONE DEACETYLASES IN INFLAMMATORY BOWEL DISEASES - INFLUENCE ON THE ANTIMICROBIAL BARRIER

Abstract

The development and clinical behavior of the two major inflammatory bowel diseases (IBD) subgroups Crohn’s disease (CD) and ulcerative colitis (UC), are determined by multiple underlying factors leading to an impaired antimicrobial barrier and chronic inflammation. Contributing environmental influences on the onset of the diseases such as the intestinal microbiota, antibiotics use, smoking, or nutrition have more and more entered the limelight of the field. The increasing incidence of IBD, the big variance in disease progression, and the discrepancy between monocygotic twins urgently impose the question how exactly environmental factors might impact on IBD risk and progression. The emerging field of epigenetics offers a mechanistic framework to dissect the present interplay between environment and genome in the context of IBD and will allow a better understanding of disease pathology. Histone deacetylases (HDACs) are important epigenetic factors implicated in intestinal tissue homeostasis. They deacetylate histones but also a vast number of non-histone proteins, e.g. the transcription factor NF-κB, thereby impacting on transcriptional regulation on different levels. A role for HDACs in the epigenetically-mediated modulation of hBDs has been demonstrated in a number of studies, underscoring a potential involvement of HDACs in the β-defensin-related defects found in colonic IBD. In addition, inhibiting HDACs has been proposed as an IBD intervention, making more detailed studies on the role of HDACs in gut antimicrobial barrier function indespensible. Firstly, in this work, a systematic overview of class I HDAC mRNA intestinal expression has been performed in a large cohort of IBD patients. First insights on the expression pattern on the protein level are also given. Secondly, this study aimed to contribute to the small body of knowledge on HDAC-mediated epigenetic control of antimicrobial peptide (AMP) expression; focusing on human β-defensin 2 (hBD2). Herein, emphasis has been laid on the therapeutically relevant probiotic E. coli Nissle 1917 (EcN) as a potent hBD2-inducing factor in addition to the pro-inflammatory cytokine IL1β and the bacterial membrane component LPS. In vitro HDAC inhibition (HDACi) in colonic epithelial cells generated a strong, NF-κB-dependent enhancement of EcN- and LPS-induced hBD2 expression, but also of the pro-inflammatory cytokine IL8. For IL1β-induced hBD2, the observed augmentation seems to be mediated by additional transducing factors and conditions depending on which HDACs are inhibited. In the case of IL1β, the enhancing effect of HDACi on hBD2 could be evinced in a second colonic epithelial cell line. In an effort to closer mimic the in vivo situation, an ex vivo human colonic biopsy culture has been established with almost identical culture conditions using IBD and healthy control tissue. This was aimed to allow a comparison to the in vitro results, since it is of substantial importance to learn how a more complex, non-tumorous human tissue compound reacts to HDACi. Strikingly, in this context, an opposing impact of HDACi on EcN-stimulated hBD2 expression was observed. Inhibiting HDAC function using pan-HDAC inhibitors hindered hBD2 expression instead of enhancing it, but did not impede IL8 expression. Whether these contrary results could be due to the malignant nature of the in vitro cell lines was investigated using ex vivo treated human colorectal cancer tissue showing the same response as non-cancerous intestinal biopsies. In addition, first insights into the reactivity of a primary, non-transformed gingival epithelial cell line towards HDACi showed a comparable hBD2 response upon IL1β-stimulation as did the cancerous intestinal cell lines. Furthermore, ex vivo fold induction levels of hBD2 in CD patients have been reduced hinting towards a disturbance in hBD2 inducibility in response to EcN. In this study, differential intestinal mRNA expression patterns have been unveiled for class I HDACs. Importantly, a strong regulatory influence of HDACs on the expression of hBD2 could be demonstrated. The simultaneous upregulation of IL8 in epithelial cells and the missing downregulation of the same in biopsy culture under HDACi, advises caution in considering HDACi as therapeutic in IBD. Furthermore, a dependency of the HDACi-induced hBD2 enhancement on NF-κB could be found. Utilizing different culture approaches, the obtained results argue for a cellular context-dependent modulation of the epigenetic regulation of hBD2 expression by HDACs. Overall, this work promotes the understanding of epigenetics as the conjoining integrative mechanism between genome and environment, bridging the way to answering many yet elusive questions in the pathogenesis of IBD.