Modulation of tear film protein secretion with phosphodiesterase inhibitors

Tear film, contact lenses and tear biomarkers

This article summarises research undertaken since 1993 in the Willcox laboratory at the University of New South Wales, Sydney on the tear film, its interactions with contact lenses, and the use of tears as a source of biomarkers for ocular and non-ocular diseases. The proteome, lipidome and glycome of tears all contribute to important aspects of the tear film, including its structure, its ability to defend the ocular surface against microbes and to help heal ocular surface injuries. The tear film interacts with contact lenses in vivo and interactions between tears and lenses can affect the biocompatibility of lenses, and may be important in mediating discomfort responses during lens wear. Suggestions are made for follow-up research.

Resolvin D2 elevates cAMP to increase intracellular [Ca2+] and stimulate secretion from conjunctival goblet cells

Under physiologic conditions, conjunctival goblet cells (CGCs) secrete mucins into the tear film to preserve ocular surface homeostasis. Specialized proresolving mediators (SPMs), like resolvins (Rvs), regulate secretion from CGCs and actively terminate inflammation. The purpose of this study was to determine if RvD2 stimulated mucin secretion and to investigate the cellular signaling components. Goblet cells were cultured from rat conjunctiva. Secretion was measured by an enzyme-linked lectin assay, change in intracellular [Ca²⁺] ([Ca²⁺]_i) using Fura-2, and cellular cAMP levels by ELISA. RvD2 (10^-11-10^-8 M) stimulated secretion, increased cellular cAMP levels and the [Ca²⁺]_i. RvD2-stimulated increase in [Ca²⁺]_i and secretion was blocked by Ca²⁺ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis and the PKA inhibitor N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride but not by the cAMP exchange protein inhibitor α-[2-(3-chlorophenyl)hydrazinylidene]-5-(1,1-dimethylethyl)-b-oxo-3-isoxazolepropanenitrile. Forskolin, 3-isobutyl-1-methylxanthine, and 8-bromo-cAMP (8-Br-cAMP) increased [Ca²⁺]_i. Increasing cAMP with 8-Br-cAMP inhibited the increase in [Ca²⁺]_i stimulated by the cAMP-independent agonist cholinergic agonist carbachol. In conclusion, RvD2 uses both cellular cAMP and [Ca²⁺]_i to stimulate glycoconjugate secretion from CGCs, but the interaction of cAMP and [Ca²⁺]_i is context dependent. Thus RvD2 likely assists in the maintenance of the mucous layer of the tear film to sustain ocular surface homeostasis and has potential as a novel treatment for dry eye disease.-Botten, N., Hodges, R. R., Li, D., Bair, J. A., Shatos, M. A., Utheim, T. P., Serhan, C. N., Dartt, D. A. Resolvin D2 elevates cAMP to increase intracellular [Ca²⁺] and stimulate secretion from conjunctival goblet cells.

Role of cAMP inhibition of p44/p42 mitogen-activated protein kinase in potentiation of protein secretion in rat lacrimal gland

We previously found that addition of cAMP and a Ca(2+)/PKC-dependent agonist causes synergism or potentiation of protein secretion from rat lacrimal gland acini. In the present study we determined whether cAMP decreases p44/p42 mitogen-activated protein kinase (MAPK) activity in the lacrimal gland. Since we know that activation of MAPK attenuates protein secretion stimulated by Ca(2+)- and PKC-dependent agonists, we also determined whether this activation causes potentiation of secretion. Freshly prepared rat lacrimal gland acinar cells were incubated with dibutyryl cAMP (DBcAMP), carbachol (a cholinergic agonist), phenylephrine (an alpha(1)-adrenergic agonist), or epidermal growth factor (EGF). The latter three agonists are known to activate p44/p42 MAPK. p44/p42 MAPK activity and protein secretion were measured. As measured by Western blot analysis, DBcAMP inhibited both basal and agonist-stimulated p44/p42 MAPK activity. Cellular cAMP levels were increased by 1) using two different cell-permeant cAMP analogs, 2) activating adenylyl cyclase (L-858051), or 3) activation of G(s)-coupled receptors (VIP). The cell-permeant cAMP analogs, L-858051, and VIP inhibited basal p44/p42 MAPK activity by 50, 40, and 40%, respectively. DBcAMP and VIP inhibited carbachol- and EGF-stimulated MAPK activity. cAMP, but not VIP, inhibited phenylephrine-stimulated MAPK activity. Potentiation of secretion was detected when carbachol, phenylephrine, or EGF was simultaneously added with DBcAMP. We conclude that increasing cellular cAMP levels inhibits p44/p42 MAPK activity and that this could account for potentiation of secretion obtained when cAMP was elevated and Ca(2+) and PKC were increased by agonists.

Single eye analysis and contralateral eye comparison of tear proteins in normal and dry eye model rabbits by MALDI-ToF mass spectrometry using wax-coated target plates

A study of rabbit tear protein expression in a dry eye rabbit model was performed to determine if a pattern in expressed proteins could be identified. The uniqueness of the model allows the comparison of normal (control) eye tear protein expression with surgically induced dry eye tear protein expression in individual animals. The sensitivity of the method allows for single eye analysis. One-dimensional mini-gel electrophoresis of the tear proteins did not show substantial differences between band patterns of the normal versus the dry eye, but was used to assess the molecular weight ranges of the major proteins. Specific assignments of some of the predominant proteins were obtained by tandem mass spectrometry (MS) which showed that the lower molecular weight lipid-binding proteins (approximately 10 kDa to 36 kDa) constitute a considerable amount of the observed protein, followed in lesser quantities by the transferrins which have higher molecular weights ranging from 70 kDa to 85 kDa. Enhancement of matrix-assisted laser desorption/ionization time-of-flight (MALDI-ToF) MS linear mode analysis of intact proteins in tear fluid was demonstrated through the use of wax-coated MALDI plates and spot washing. MALDI-ToF MS analysis of the expressed tear proteins illustrates that differences between normal eye tear and dry eye tear protein content are manifested in changes in the lower molecular weight lipid-binding proteins such as lipophilin which exhibits an increase in concentration in the dry eye, and beta-2 microglobulin which undergoes a decrease.

Lacryglobin in human tears, a potential marker for cancer

Lacrygobin has been identified in human tears. This protein has high sequence homology to the mammaglobins, proteins upregulated in breast cancer and in breast cancer metastasis. In order to investigate the utility of tear screening for cancer tear samples were collected from patients with different types of cancer and compared to controls. Tear samples were taken from five controls and eight breast, six lung, five colon, one prostate and three ovary cancer patients. Tears were analysed using 2-D gel electrophoresis (n = 25) and 1-D electrophoresis (n = 3). Lacryglobin was present in the following percentage of patients: breast cancer (88%), lung (83%), colon (100%), ovary (33%), prostate (100%) and controls (60%). Two control patients with lacryglobin had a family history of breast and prostate cancer. Lacryglobin was detected in some but not all tear samples and further studies are warranted to investigate its potential as a marker for cancer.