Aletras et al. BMC Ophthalmology (2018) 18:328 https://doi.org/10.1186/s12886-018-0987-8 RESEARCH ARTICLE Open Access UVB-mediated down-regulation of proteasome in cultured human primary pterygium fibroblasts Alexios J. Aletras1* , Ioannis Trilivas2, Maria-Elpida Christopoulou1, Sotiria Drakouli1,3, Constantine D. Georgakopoulos2 and Nikolaos Pharmakakis2 Abstract Background: Pterygium is a condition characterized by epithelial overgrowth of the cornea, inflammatory cell infiltration and an abnormal extracellular matrix accumulation. Chronic UV exposure is considered as a pathogenic factor of this disease. Proteasome is an intracellular multi-subunit protease complex that degrades intracellular proteins. Among proteasome subunits the β5 (PSMB5), bearing chymotrypsin-like activity. It is considered as the main proteasome subunit and its expression is mediated by Nrf2-ARE pathway in many cell types. This study investigates the expression of PSMB5 in pterygium and the effect of UVB irradiation on its expression and activity in pterygium fibroblasts. Methods: Normal conjunctival and pterygium specimens were obtained from the bulbar conjunctiva of patients undergoing cataract surgery and from patients with pterygium undergoing surgical removal of primary tissue, respectively. Fibroblasts were isolated upon treatment of specimens with clostridium collagenase. The expression of PSMB5 and Nrf2 in tissues and cells was ascertained by RT-PCR analysis and western blotting. Cell survival was measured by the MTT method and the proteasome chymotrypsin-like activity was determined by fluorometry. Results: RT-PCR analysis showed that the expression of PSMB5 was significantly lower in pterygium than in normal conjunctiva. The expression of PSMB5 was mediated by the Nrf2/ARE pathway as indicated by using the Nrf2 activator Oltipraz. The expression of PSMB5 and Nrf2 by pterygium fibroblasts was suppressed in a dose dependent manner following UVB radiation of 0–50 mJ/cm2 doses. The expression of PSMB5, but not of Nrf2, remained at almost the control levels, when UVB exposure was performed after pre-incubation of cells with the src kinases inhibitor PP2. UVB irradiation had very low deleterious effect on fibroblasts survival, while it did not affect the proteasome chymotrypsin-like activity. Conclusion: In pterygium fibroblasts, UVB exposure leads to down-regulation of Nrf2/ARE-mediated PSMB5 gene expression, in which src kinases may be implicated. This effect may be partially responsible for the lower expression of PSMB5 detected in pterygium as compared to normal conjunctiva. Keywords: Pterygium, Fibroblasts, Proteasome, UVB irradiation, Nrf2-ARE pathway, Src kinases * Correspondence: [email protected] 1Laboratory of Biochemistry, Department of Chemistry, University of Patras, 26 504 Patras, Greece Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Aletras et al. BMC Ophthalmology (2018) 18:328 Page 2 of 10 Background oxidized proteins that it exerts, highly contributes in the Pterygium is an eye disorder featuring hyperplasia of the cellular defenses against oxidative stress. In contrast, the cornea epithelium, usually bilateral, overgrowth of stromal impaired function of the proteasome, leads to accumula- fibroblasts and blood vessels, significant neovasculariza- tion of oxidized/misfolded proteins within the cell, and tion, inflammatory cell infiltration and abnormal extracel- may be responsible for the manifestation several degen- lular matrix accumulation, with elastin and collagen being erative diseases [22–26]. the main components. Histopathological studies revealed Previous studies have established that the expression of that the pterygium epithelium, which invades the cornea, the catalytic subunits of the proteasome can be induced exhibits alterations such as squamous metaplasia and gob- by various exogenous stimuli through the Nrf2-ARE let cell overgrowth. An underlying damage of Bowman’s (NF-E2 related factor 2-antioxidant response element) layer was also observed [1]. pathway and it has been proposed that the up-regulation Immuohistologic studies indicate that pterygium is ra- of the proteasome subunits by Nrf2 activators may con- ther a limbal epithelial stem cell disorder, the formation tribute to the protection of cells against oxidative damage and development of which may be triggered from a dam- through the selective degradation of oxidized proteins, age or activation of these stem cells [2]. The exact reasons thus preventing the formation of protein aggregates and of pterygium pathogenesis remain unclear, however ultra- their accumulation within cells [27–33]. violet (UV) radiation is considered to be the most accept- Nrf2 belongs to family of the basic leucine zipper able cause for the initiation of this disorder [3, 4]. NF-E2 (nuclear factor erythroid-derived 2). Under nor- UVB radiation induces damage to cellular DNA, RNA, mal conditions, it occurs as a complex with the protein and extracellular matrix components through activation KEAP1 (Kelch-like erythroid-cell-derived protein with of different pathways [5]. UVB radiation also stimulates CNC homology (ECH)-associated protein), remaining in the expression of cytokines and growth factors in pteryg- the cytoplasm, where it is subsequently degraded by the ium cells, which are considered to play pivotal role in ubiquitin/ proteasome pathway. Upon exposure to oxi- the establishment and development of pterygium. The dative stress, the Nrf2-KEAP1 complex dissociates and expression of cytokines and growth factors, such as IL-6, Nrf2 migrates to the nucleus where interacts with small IL-8, TNF-α, bFGF, TGF-β and PDGF, in pterygium has Maf-family proteins (small musculoaponeurotic fibrosar- been previously reported [6–10]. Extracellular matrix coma TFs required for Nrf2 transactivation) forming het- metalloproteinases (MMPs) may be also involved in the erodimers that bind to AREs in target gene promoters. pathogenesis of pterygium. The expression of MMPs, Various small electrophile molecules, including oltipraz such as MMP-2, MMP-3, MMP-9 and particularly (5-[2-pyrazinyl]-4-methyl-1,2-dithiol-3-thione), inacti- MMP-1, which is the most abundant, has been demon- vate KEAP1 and release the Nrf2, which then migrates strated in pterygium tissues [11–13]. In addition, high to the nucleus [34]. expression of hypoxia inducible factor-1α (HIF-1α) and The implication of proteasome in pterygium pathogen- heat shock proteins (Hsp), such as Hsp90, Hsp70 and esis as well as the effect of UVB irradiation on its ex- Hsp27 in pterygium tissues has been previously reported pression and activity in pterygium cells has not yet been [14, 15]. They may contribute in the cellular defense studied. In the present study we investigated the effect mechanisms against stressful conditions . of UVB irradiation on β5 proteasome subunit (PSMB5) The 26S proteasome is an intracellular proteolytic expression and activity in pterygium fibroblasts. complex of two components: a 20S proteolytic core, which contains the catalytic activity, and a 19S regula- tory subunit [16, 17]. The 20S proteolytic core is con- Methods structed from 28 subunits distributed in four heptameric Specimen selection rings stacked on top of each other, to form a structure Specimens of normal bulbar conjunctival tissues were resembling a barrel. The components of the two outside obtained from five patients (2 male and 3 female) under- rings are called the α-subunits, and those of the two in- going cataract surgery, who did not show symptoms of side rings β-subunits. The proteasome possesses mul- an ocular surface disorder or of dry eyes, whose ages tiple protease activities, such as caspase-like, trypsin-like ranged from 61 to 90 years old (mean age ± SD 75.40 ± and chymotrypsin-like activity, localized on subunits β1, 12.26). Pterygium tissues located nasally were obtained β2andβ5, respectively, and is able to degrade almost all from five patients (3 male and 2 female) subjected to the intracellular abnormal and denaturated proteins, as primary pterygium excision surgery, whose ages ranged well as functional proteins, which have to be recycled. It from 60 to 84 years old (mean age ± SD 71.80 ± 8.73). also plays a role in cell cycle progression, and in cell de- The study population was of Greek origin and all the velopment and death [18–21]. The normal function of specimens were collected in the Department of Ophthal- proteasome and the consequent selective degradation of mology, University Hospital of Patras, Patras, Greece. Aletras et al. BMC Ophthalmology (2018) 18:328 Page 3 of 10 Conjunctival and primary Pterygium fibroblast cultures mM DTT in water for 1 h at 4 °C [39]. Lysates were centri- Pterygium or normal conjunctival specimens immediately fuged at 14,000 g for 30 min at 4 °C and protein concentra- after excision were transferred on ice in the Laboratory tions were determined in supernatants by the