Journal of Ophthalmology

Journal of Ophthalmology / 2016 / Article
Special Issue

Lacrimal Gland, Ocular Surface, and Dry Eye

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Research Article | Open Access

Volume 2016 |Article ID 8350134 |

Hee-Jung Ju, Yong-Soo Byun, Jee-Won Mok, Choun-Ki Joo, "The Blockade of IL6 Counterparts the Osmolar Stress-Induced Apoptosis in Human Conjunctival Epithelial Cells", Journal of Ophthalmology, vol. 2016, Article ID 8350134, 7 pages, 2016.

The Blockade of IL6 Counterparts the Osmolar Stress-Induced Apoptosis in Human Conjunctival Epithelial Cells

Academic Editor: Edit Tóth-Molnár
Received12 Jan 2016
Accepted04 May 2016
Published31 Jul 2016


To determine the effect of hyperosmolarity on cell survival/apoptosis of conjunctival epithelial cells and evaluate the possible role of IL6, Wong-Kilbourne derivative of Chang conjunctival cell line (WKD) was used in this study. Confluent cells were incubated under different osmolarity (290 mOsm and 500 mOsm) with or without neutralizing IL6 antibody (50 ng/mL). The expression of IL6 level was measured in the supernatant of each conditioned medium. Cell viability/apoptosis assay was performed using Annexin V/Propidium Iodide (PI) and Cell Counting Kit-8 (CCK-8). Western blot was conducted to measure the abundance of apoptotic markers and IL6 related downstream signaling pathway. The concentration of IL6 showed time-dependent increase in cells treated with 500 mOsm. Although apoptosis of WKD cell is increased in treated 500 mOsm for 24 h, apoptosis reduced in WKD cell treated 500 mOsm with anti-IL6 for 24 h. Anti-IL6 inhibited the activation of JAK-STAT signaling pathway, which was induced by hyperosmolarity. Hyperosmolar condition induced apoptosis in conjunctival epithelial cells, along with increase of IL6 production. IL6 neutralizing antibody inhibited apoptosis and JAK-STAT signaling in hyperosmolar condition. These findings suggested that IL6 may be involved in apoptotic change and in hyperosmolarity.

1. Introduction

Dry eye syndrome is a multifactorial disease, which is caused by a vicious cycle: abnormalities of tear film and lacrimal hyposecretion induce the break-up of tear film, and the following inflammation of ocular surface deteriorates the secretion and the composition of tears [1]. Hyperosmolarity is induced by lacrimal hyposecretion or the increase of evaporation evokes desquamation, decreased intercellular connections, blunting and loss of microplicae, cell membrane disruption, and cellular swelling with decreased cytoplasmic density in the corneal epithelium [2]. Moreover, hyperosmolarity provokes squamous metaplasia, loss of goblet cells, and inflammation in the conjunctival epithelium [35]. These phenomena decrease the production of mucin for lubricating corneal epithelium, and the reduction of mucin aggravates dry eye [6]. Histologic findings of dry eye in patients with Sjögren syndrome and immunosuppressant patients, such as postbone marrow transplantation state, were reported as a reduction of goblet cells, increment of inflammatory cells in cornea and conjunctiva, and inflammation with fibrosis of the lacrimal gland [710]. Dry eye also induces the secretion of cytokines such as IL6, IL-1β, TNF-α, and IFN-γ [11]. Cyclosporin, one of the drugs suppressing those cytokines, reduces the infiltration of conjunctival cells and IL6, regulates the necrosis of conjunctival epithelial cells, and elevates the number of goblet cells by preventing the loss of the cells [12].

IL6 has been known as a representative cytokine with increased expression in tears and the conjunctival epithelium of eyes with dry eye syndrome. It has also been reported to have pro- and anti-inflammatory effects. As evidence of the proinflammatory effects, a report revealed that IL6 treatment reduced the survival of liver cancer cells [13] and tocilizumab, an IL6 blocker, has been used as a therapeutic drug for autoimmune diseases in rheumatology. Anti-inflammatory effects of IL6 were revealed by the following evidence: IL6 treatment on cells increased migration [14], IL6 induced cell migration and wound healing of mouse biliary epithelial cells [15], blocking of IL6 reduced inflammatory-related molecules in mouse alkali burn model [16], and IL6-deficient mice showed delayed wound healing after skin resection [17].

Although IL6 has been well known as an important cytokine on disease progression and severity associated with immune mechanisms, its role in dry eye was still vague except for increased expression on the ocular surface. Therefore, this research investigated the role of IL6 on apoptosis of conjunctival epithelial cells after hyperosmolarity.

2. Material and Methods

2.1. In Vitro Osmolar Stress Experiment

The Wong-Kilbourne derivative of Chang conjunctival cells (WKD, ATCC CCL-20, Manassas, VA, USA) were cultured in Dulbecco’s modified Eagle’s Medium F12 (1 : 3) culture medium (Invitrogen, Waltham, MA, USA), supplemented with 1% penicillin and streptomycin (WELGENE, Daegu, Seoul) and 5% heat-inactivated fetal bovine serum (WISENT, Quebec, Canada). When cells were approximately 80–90% confluence, culture medium was replaced with fresh medium with added 1 M NaCl to increase the osmolarity (corresponding to 290, 500 mOsm) for 24 h. Cells were incubated for 24 h before protein extraction and conditioned medium collection.

The blockade of IL6 was done 24 h before incubation using the neutralizing Anti-Human IL6 antibody (anti-IL6, Clone 1936, R&D systems, Minneapolis, MN, USA) for neutralizing the IL6. To determine whether IL6 cytokine has protected effect in the NaCl exposure or not, conjunctiva epithelial cells were incubated with IL6 for 24 h and then exposed to 500 mOsm NaCl for 24 h.

2.2. Measurement of Cytokine Production

Cell supernatant was collected 290 mOsm, 500 mOsm NaCl incubation after 24 h, and then centrifuged 12,000 rpm for 3 min at 4°C. Cytokine levels in supernatant were determined using the Human IL-1β, IL6, TNF-α, and IFN-γ Quantikine Enzyme-Linked Immunosorbent Assay Kit (R&D systems, Minneapolis, MN, USA) and we followed the manufacturer’s protocol. Briefly, for IL6 kit 100 μL of standard and samples were incubated in antibody-coated plate at 2 hrs. After being washed four times, add 200 μL conjugate solution being incubated at 2 hrs. After being washed four times, add 200 μL substrate solution being incubated at 20 min. Add 50 μL stop solution and read at 450 nm with reader within 30 min.

2.3. Cell Viability Assay

Cell viability was determined using the Cell Counting Kit-8 (Dojindo, Kumamoto, Japan) according to the manufacturer’s instructions. Cells ( cells/mL, 100 μL) were seeded in 96-well plate. When cells were confluent 80%–90%, cells were treated with anti-IL6 for 24 h. After treatment for 24 h, add 10 μL of CCK-8 solution and incubate the plate for 1 h and cover the plate to protect from light, and then measure the absorbance using a microplate reader at 450 nm. The experiment was performed three times.

2.4. Flow Cytometry

Cells were seeded at cells in a well of 60 mm plates and cultured until confluent 80–90%. Cells were exposed to 290 mOsm, 500 mOsm NaCl with/without anti-IL6 50 ng for 24 h. After 24 h, cells were collected and resuspended. For labeling the Annexin V/Propidium Iodide (PI), we were using the Annexin V/PI kit (Invitrogen, Eugene, Oregon, USA). The suspension cells were labeled with Annexin V/PI and analyzed by flow cytometry (BD, Franklin, New Jersey).

2.5. Western Blot Assay

Cells were harvested by scraping with RIPA buffer. Extracts were incubated for 2 h at 4°C and obtained by centrifugation (13,000 rpm for 20 min at 4°C). Protein concentrations were determined using the BCA assay Kit (Thermostat, Hercules, CA, USA), and whole-cell extracts were adjusted to same amount of total protein (20 μg). Samples were electrophoresed in 10% SDS-PAGE. Proteins were then transferred onto a PVDF membrane (Millipore Corporation, Billerica, MA, USA) at 300 mA for 90 min at 4°C, and the membranes were incubated with 3% BSA (Sigma-Aldrich, St. Louis, MO, USA) in TBST to block nonspecific binding. Primary antibodies were incubated over night at 4°C. And we washed five times with TBST (0.5% tween 20 in 1x TBS) the secondary antibodies conjugated horseradish peroxidase (HRP) (Santa Cruz, Dallas, Texas, USA) that was applied and incubated for 1 h at RT. After five times of washing with TBST (0.5% tween 20 in 1x TBS), the membrane followed by chemiluminescent detection using Immobilon Western Substrate (Millipore Corporation, Billerica, MA, USA) with the ChemiDoc MP Imaging system (Bio-Rad Laboratories Inc., Hercules, California, USA). The antibodies diluents were shown at Table 1.

Primary antibodyHostDilutionManufacturer

JAK-STAT signaling markersp-STAT3Mouse1 : 1000Santa Cruz
STAT3Rabbit1 : 1000Santa Cruz
p-ERK1/2Rabbit1 : 1000Santa Cruz
ERK1/2Mouse1 : 1000Santa Cruz
p-mTORRabbit1 : 1000Santa Cruz
mTORGoat1 : 1000Santa Cruz

Apoptosis markersBaxRabbit1 : 500Santa Cruz
Bcl2Mouse1 : 500Santa Cruz
Cleaved caspase-3Rabbit1 : 500Abcam
Caspase-3Mouse1 : 1000Santa Cruz

Housekeeping gene markerBeta-actinRabbit1 : 4000Abcam

2.6. Statistical Analysis

All results were indicated as means ± SEM. The results were analyzed by Kruskal-Wallis analysis, followed by a Mann-Whitney analysis. A value of less than 0.05 was considered to be statistically significant.

3. Results

3.1. Hyperosmolarity Induces IL6 Levels in the WKD Cells

WKD cells were cultured under 290 mOsm and 500 mOsm for 24 h and the cell viability was analyzed. There were no definite apoptotic cell deaths in 290 mOsm, while cell viability was significantly reduced after 12 h in 500 mOsm ( at 12 h, at 24 h, and at 48 h, resp., at 12 h, 24 h) (Figure 1(a)). We assessed the levels of IL-1β, TNF-α, IFN-γ, and IL6 from medium of cells cultured under 290 mOsm and 500 mOsm to identify the inflammatory cytokines which are well known to increase in epithelial cells by hyperosmolarity stress. Quantitative analysis showed that, whereas IL-1β ( in 290 mOsm, in 500 mOsm), TNF-α ( in 290 mOsm, in 500 mOsm), and IFN-γ ( in 290 mOsm, in 500 mOsm) showed no significant change, IL6 ( in 290 mOsm, in 500 mOsm) were increased (, Figure 1(b)). When measuring the time-dependent expression of IL6 in 290 mOsm and 500 mOsm, the expression of IL6 in 500 mOsm increased from 0.012 pg/mL at 1 h to 2.1 pg/mL at 24 h, which was higher than that in 290 mOsm (, Figure 1(c)).

3.2. IL6 Mediates Hyperosmolarity Induced Apoptotic Cell Death

To evaluate whether IL6 was secreted to protect cells or induce cell death in apoptosis by hyperosmolarity, 50 ng of anti-IL6 was used for blocking IL6. When investigating change of morphology and cell distributions in 290 mOsm, 500 mOsm, and 500 mOsm with anti-IL6, the cells in 500 mOsm showed shrinkage and more apoptosis than in 290 mOsm. Nevertheless, cells in 500 mOsm with anti-IL6 showed less apoptosis and shrinkage than those in 500 mOsm without anti-IL6 (Figure 2(a)).

We have given a 500 mOsm on the conjunctival epithelial cells and the effect of IL6 was examined using Annexin V/PI. The amount of the cells was significantly increased in the cell pretreated with the anti-IL6. The apoptosis rates are increased from in 290 mOsm to in 500 mOsm and in 500 mOsm NaCl with anti-IL6. Treated anti-IL6 was very effectively decreased for the apoptosis rates in 500 mOsm NaCl treated (Figure 2(b)).

There was significant difference in the proportions of apoptotic cells among 290 mOsm, 500 mOsm, and 500 mOsm with anti-IL6 (Figure 2(c)), which implies that IL6 could promote apoptosis by hyperosmolarity.

3.3. Anti-IL6 Inhibits JAK-STAT Pathway Activation in Hyperosmolarity Induced Cell Apoptosis

Evaluating JAK-STAT signaling and apoptosis markers by Western blot, the expressions of p-STAT3, p-ERK1/2, and p-mTOR were higher in 500 mOsm (resp., , , and ) than 290 mOsm (resp., , , and ); however, those were lower in 500 mOsm with anti-IL6 (resp., , , and ) than in 500 mOsm.

Bax and cleaved-caspase-3 showed higher expression in 500 mOsm (resp., , ) than in 290 mOsm (resp., , ), and those were also lower in 500 mOsm with anti-IL6 (resp., , ) than in 500 mOsm. Bcl2 showed lowest expression in 500 mOsm (), followed by that in 500 mOsm with anti-IL6 () and 290 mOsm () (Figure 3(a)). The significant difference of expressions of p-STAT3/total STAT3, p-ERK/total ERK, and p-mTOR/total mTOR was revealed between those in 500 mOsm and 500 mOsm with anti-IL6 () Apoptosis markers such as Bax, Bcl2, and cleaved caspase-3/caspase-3 showed significant discrepancy between those in 500 mOsm and 500 mOsm with anti-IL6 (, Figure 3(b)).

4. Discussion

It has been reported that hyperosmolarity stress could induce apoptosis and promote the secretion of diverse proinflammatory cytokines [18]. The association between conjunctival epithelial cells and IL6 has been described in many studies: increased expression of IL-1β, IL8, IL6, and TNF-α in conjunctival epithelial cells and tear fluid in the patients with dry eye syndrome [19, 20] and increase of IL-1β, IL8, and, especially, IL6 in impression cytology [21, 22]. Those studies suggested conjunctiva epithelial cells could aggravate dry eye and IL6 had an important role in pathomechanism of dry eye. A previous study showed that blocking IL6 could increase goblet cells in conjunctival epithelium and reduce inflammatory cells, which could soothe the symptoms and prevent the chronic progression of dry eye [12]. Flow cytometry analysis showed that the blocking of IL6 significantly suppressed apoptosis observed at 500 mOsm, which suggests blocking IL6 could modify apoptosis following hyperosmolarity.

Cytokines such as IL6 can activate the JAK-STAT signaling pathway [23]. IL6 can be activated though the membrane-bound IL6 receptor (classical pathway) or the soluble type of IL6 receptor (trans-signaling). The IL6 attached with receptor could initiate cascade via JAK activation, and activated JAK kinase phosphorylate induces forming of dimer by phosphorylation of STAT3 [24, 25]. This signaling pathway promotes proliferation, differentiation, migration, and apoptosis of cells [26]. Although the activation of the JAK-STAT signal by IL6 was observed in cancer [27], there have been few studies for the association between conjunctival epithelial cells and IL6, especially about the role of increased IL6 in a hyperosmolar state of conjunctival epithelial cells. This research revealed that the IL6/JAK/STAT signaling pathway was associated with apoptosis induced by hyperosmolarity. Western blot showed that 500 mOsm promoted the expression of STAT3 (direct signal), ERK, and mTOR (indirect signal), and IL6 reduced the increased expressions. Bax and caspase-3 as apoptosis markers showed higher expression in hyperosmolar state (500 mOsm), which was suppressed by blocking IL6. The inverse pattern of Bcl-2 expression related to IL6 and hyperosmolarity was also revealed. These suggested that JAK-STAT signaling pathway of apoptosis was associated with IL6.

In this study, blocking IL6 inhibited the apoptosis of conjunctival epithelial cells under hyperosmolar condition, and the process was associated with the JAK-STAT signal pathway (Figure 4). This indicated that IL6 could be one of the important cytokines affecting the pathogenesis of dry eye. Therefore, in dry eye patients, IL6 could be a biologic marker, and regulating IL6 though the IL6/JAK/STAT3 signaling pathway could be an effective therapeutic target.

5. Conclusion

Hyperosmolarity induced apoptosis in conjunctival epithelial cells was suppressed by blocking IL6, which suggests that IL6 may play an important role in the pathogenesis of dry eye disease.

Competing Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.


  1. C. Baudouin, P. Aragona, E. M. Messmer et al., “Role of hyperosmolarity in the pathogenesis and management of dry eye disease: proceedings of the OCEAN group meeting,” The Ocular Surface, vol. 11, no. 4, pp. 246–258, 2013. View at: Publisher Site | Google Scholar
  2. J. P. Gilbard, J. B. Carter, D. N. Sang, M. F. Refojo, L. A. Hanninen, and K. R. Kenyon, “Morphologic effect of hyperosmolarity on rabbit corneal epithelium,” Ophthalmology, vol. 91, no. 10, pp. 1205–1212, 1984. View at: Publisher Site | Google Scholar
  3. J. P. Gilbard, S. R. Rossi, K. L. Gray, L. A. Hanninen, and K. R. Kenyon, “Tear film osmolarity and ocular surface disease in two rabbit models for keratoconjunctivitis sicca,” Investigative Ophthalmology & Visual Science, vol. 29, no. 3, pp. 374–378, 1988. View at: Google Scholar
  4. J. P. Gilbard, S. R. Rossi, K. L. Gray, and L. A. Hanninen, “Natural history of disease in a rabbit model for keratoconjunctivitis sicca,” Acta Ophthalmologica, vol. 67, supplement 192, pp. 95–101, 1989. View at: Google Scholar
  5. S. C. Pflugfelder, S. C. G. Tseng, K. Yoshino, D. Monroy, C. Felix, and B. L. Reis, “Correlation of goblet cell density and mucosal epithelial membrane mucin expression with rose bengal staining in patients with ocular irritation,” Ophthalmology, vol. 104, no. 2, pp. 223–235, 1997. View at: Publisher Site | Google Scholar
  6. I. K. Gipson and T. Inatomi, “Cellular origin of mucins of the ocular surface tear film,” Advances in Experimental Medicine and Biology, vol. 438, pp. 221–227, 1998. View at: Publisher Site | Google Scholar
  7. W.-L. Fei, J.-Q. Chen, X. Du et al., “Dry eye in graft-versus-host disease,” Chinese Journal of Ophthalmology, vol. 39, no. 11, pp. 686–690, 2003. View at: Google Scholar
  8. Y. Ogawa, S. Okamoto, M. Wakui et al., “Dry eye after haematopoietic stem cell transplantation,” British Journal of Ophthalmology, vol. 83, no. 10, pp. 1125–1130, 1999. View at: Publisher Site | Google Scholar
  9. T. Igarashi, C. Fujimoto, H. Suzuki et al., “Short-time exposure of hyperosmolarity triggers interleukin-6 expression in corneal epithelial cells,” Cornea, vol. 33, no. 12, pp. 1342–1347, 2014. View at: Publisher Site | Google Scholar
  10. M. A. Lemp, C. Baudouin, J. Baum et al., “The definition and classification of dry eye disease: report of the definition and classification subcommittee of the international Dry Eye WorkShop (2007),” The Ocular Surface, vol. 5, no. 2, pp. 75–92, 2007. View at: Publisher Site | Google Scholar
  11. K.-C. Yoon, I.-Y. Jeong, Y.-G. Park, and S.-Y. Yang, “Interleukin-6 and tumor necrosis factor-α levels in tears of patients with dry eye syndrome,” Cornea, vol. 26, no. 4, pp. 431–437, 2007. View at: Publisher Site | Google Scholar
  12. K. Turner, S. C. Pflugfelder, Z. Ji, W. J. Feuer, M. Stern, and B. L. Reis, “Interleukin-6 levels in the conjunctival epithelium of patients with dry eye disease treated with cyclosporine ophthalmic emulsion,” Cornea, vol. 19, no. 4, pp. 492–496, 2000. View at: Publisher Site | Google Scholar
  13. Y. Liu, P.-K. Li, C. Li, and J. Lin, “Inhibition of STAT3 signaling blocks the anti-apoptotic activity of IL-6 in human liver cancer cells,” The Journal of Biological Chemistry, vol. 285, no. 35, pp. 27429–27439, 2010. View at: Publisher Site | Google Scholar
  14. I. A. Valsero, L. S. Romaní, L. G. Posadas, A. L. García, and Y. Diebold, “IL-6 as a corneal wound healing mediator in an invitro scratch assay,” Experimental Eye Research, vol. 125, pp. 183–192, 2014. View at: Publisher Site | Google Scholar
  15. G.-X. Jiang, X.-Y. Zhong, Y.-F. Cui et al., “IL-6/STAT3/TFF3 signaling regulates human biliary epithelial cell migration and wound healing in vitro,” Molecular Biology Reports, vol. 37, no. 8, pp. 3813–3818, 2010. View at: Publisher Site | Google Scholar
  16. T. Sakimoto, S. Sugaya, A. Ishimori, and M. Sawa, “Anti-inflammatory effect of IL-6 receptor blockade in corneal alkali burn,” Experimental Eye Research, vol. 97, no. 1, pp. 98–104, 2012. View at: Publisher Site | Google Scholar
  17. Z.-Q. Lin, T. Kondo, Y. Ishida, T. Takayasu, and N. Mukaida, “Essential involvement of IL-6 in the skin wound-healing process as evidenced by delayed wound healing in IL-6-deficient mice,” Journal of Leukocyte Biology, vol. 73, no. 6, pp. 713–721, 2003. View at: Publisher Site | Google Scholar
  18. L. Luo, D.-Q. Li, and S. C. Pflugfelder, “Hyperosmolarity-induced apoptosis in human corneal epithelial cells is mediated by cytochrome c and MAPK pathways,” Cornea, vol. 26, no. 4, pp. 452–460, 2007. View at: Publisher Site | Google Scholar
  19. S. C. Pflugfelder, D. Jones, Z. Ji, A. Afonso, and D. Monroy, “Altered cytokine balance in the tear fluid and conjunctiva of patients with Sjogren's syndrome keratoconjunctivitis sicca,” Current Eye Research, vol. 19, no. 3, pp. 201–211, 1999. View at: Publisher Site | Google Scholar
  20. A. Solomon, D. Dursun, Z. Liu, Y. Xie, A. Macri, and S. C. Pflugfelder, “Pro- and anti-inflammatory forms of interleukin-1 in the tear fluid and conjunctiva of patients with dry-eye disease,” Investigative Ophthalmology and Visual Science, vol. 42, no. 10, pp. 2283–2292, 2001. View at: Google Scholar
  21. D. T. Jones, D. Monroy, Z. Ji, S. S. Atherton, and S. C. Pflugfelder, “Sjogren's syndrome: cytokine and Epstein-Barr viral gene expression within the conjunctival epithelium,” Investigative Ophthalmology & Visual Science, vol. 35, no. 9, pp. 3493–3504, 1994. View at: Google Scholar
  22. J. Zhang, X. Yan, and H. Li, “Analysis of the correlations of mucins, inflammatory markers, and clinical tests in dry eye,” Cornea, vol. 32, no. 7, pp. 928–932, 2013. View at: Publisher Site | Google Scholar
  23. D. Hebenstreit, J. Horejs-Hoeck, and A. Duschl, “JAK/STAT-dependent gene regulation by cytokines,” Drug News and Perspectives, vol. 18, no. 4, pp. 243–249, 2005. View at: Publisher Site | Google Scholar
  24. R. J. Stefan, “Il-6 trans-signaling via the soluble IL-6 receptor: importance for the proinflammatory activities of IL-6,” International Journal of Biological Sciences, vol. 8, no. 9, pp. 1237–1247, 2012. View at: Publisher Site | Google Scholar
  25. A. J. Brooks, W. Dai, M. L. O'Mara et al., “Mechanism of activation of protein kinase JAK2 by the growth hormone receptor,” Science, vol. 344, no. 6185, Article ID 1249783, 2014. View at: Publisher Site | Google Scholar
  26. J. S. Rawlings, K. M. Rosler, and D. A. Harrison, “The JAK/STAT signaling pathway,” Cell Science, vol. 117, pp. 1280–1283, 2004. View at: Publisher Site | Google Scholar
  27. S. C. Afford, J. Pongracz, R. A. Stockley, J. Crocker, and D. Burnett, “The induction by human interleukin-6 of apoptosis in the promonocytic cell line U937 and human neutrophils,” The Journal of Biological Chemistry, vol. 267, no. 30, pp. 21612–21616, 1992. View at: Google Scholar

Copyright © 2016 Hee-Jung Ju et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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