Retinoic acid and the ocular surfaceCHAMEEN SAMARAWICKRAMA, BSC(MED), MBBS, PHD, SKY CHEW, BSC(MED), MBBS, PHD STEPHANIE WATSON, BSC(MED), MBBS, PHD, FRANZCO

Surv Ophthalmol. 2015 May-Jun;60(3):183-95Sydney Eye Hospital, Sydney, New South Wales, Australia; Save Sight Institute, University of Sydney,

Sydney, New South Wales, Australia.

Introduction

Vit A has been known to improve cutaneous wound healing. It accelerates epithelial migration, granulation tissue formation

and reversal of the retardation of healing induced by corticosteroids

This review explores the international literature on ophthalmic use of retinoic acid on the ocular surface.

Vitamin a deficiency Leading cause of childhood blindness in developing countriesManifests in 2 ways:

Night blindness/nyctalopia Xerophthalmia

Ocular changes include.. Epidermal keratinization Squamous metaplasia Corneal ulceration Night blindness Retinopathy

Vitamin a deficiency

Initial and most common ocular manifestation of vitamin A deficiency is nyctalopia.

Retinal electrophysiology can assist in diagnosis and follow up of vitamin deficiency

Vitamin A deficiency

Conjunctival pathology typically follows nyctalopia. First sign is xerosis (dryness) Conjunctiva appears thickened, wrinkled with loss of

transparency. Bitot spots- triangular, perilimbal foamy gray plaques of

keratinized conjunctiva overlying an area of dryness -pathognomonic

Vitamin A deficiency Puctate keratopathy which progresses to epithelial defects,

keratinization and stromal edema. Corneal epithelial defects progress to partial or full thickness

ulceration Keratomalacia is often associated with preceding systemic

stressors like measles or severe protein malnutrition. Descematocoele/corneal perforation

Vitamin A deficiency

Xerophthalmic fundus-numerous small yellow dots representing loss of pigment from the RPE

Replenishment of vitamin A stores typically results in the reversal of night blindness and conjunctival and retinal pathology.

Keratopathy without severe ulceration also responds favorably .

Production of retinoic acid

NATURAL PRODUCTION IN THE BODY Produced in body by 2 oxidation steps:

retinol retinaldehyde Retinaldehyde retinoic acid

Retinol ingested in food absorbed by intestinal cells bound to serum retinol-binding protiens transported to target epithelial cells.

Most organs have the capacity for retinoic acid biosynthesis including corneal epithelial cells

Production of retinoic acid

SYNTHETIC PRODUCTION 2 methods known in literature for synthetic production of retinoic

acid both for therapeutic and research purposes Both lead to formation of all-trans retinoic acid It is inherently unstable because it undergoes

photoisomerization. Retinol/ retinyl palmitate, another derivative of vitamin A is more

stable and is the precursor or storage form of vitamin A

Mechanism of retinoic acid action

Two main mechanisms1. Nuclear receptor mediated pathway2. Non nuclear receptor medicated pathway

Studies have shown nuclear receptor pathway to be the primary pathway.

Retinoic acid binds to nuclear receptors that act as ligand activated transcriptional factors, resulting in either Transcriptional activation or Repression of retinoid controlled genes

Mechanism of retinoic acid action Nuclear receptor mediated mechanism Regulates the transcriptional level of target genes Retinoic acid binds to nuclear receptors conformational changes gene

transcription Increased gene transcription upregulation of protiens transactivation Decreased gene transcription downregulation of protiens transrepression

TRANSACTIVATION is mediated by 2 families of nuclear receptors RAR* and RXR Promotion of ocular surface hydration, Epithelial healing Ocular differentiation and development

Mechanism of retinoic acid action TRANSREPRESSION results in a reduction of keratinization,

protection of the cornea from dissolution, and suppression of oncogenic proliferation and neoplasia

Transrepression leads to Reduction of keratin production Inhibition of collagenases production Possible down regulation of MMP 13 production(which plasys a role

in pathogenesis of OSSN) Down regulation of AP1 transcriptional activity inhibiting oncogenic

proliferation and cellular proliferation.

Mechanism of retinoic acid action

Non nuclear receptor mediated mechanism Binding to extra nuclear retinol receptors, Retinoylation, Activation of or interaction with other signaling molecules Mediation of effects via metabolites

To summarize..* Corneal epithelial cell repair Maintenance of ocular surface hydration - MUC 16 Apoptosis, cellular differentiation and repression of oncogenic proliferation Reduction of keratininzation of ocular surface epithelium

Role in wound healing1. ROLE IN FULL THICKNESS CORNEAL WOUNDS

Application of retinoic acid increases the tensile strength Retinol palmitate was compared in two doses 0.1 % and 0.5%

which showed a significant increase in tensile strength with 0.1 % concentration

0.5% not only had any effect on wound healing or tensile strength, it also showed an inhibitory effect in high doses.

An ideal concentration of 10 x 10 -6 m equivalent to 33% increased keratocyte numbers

Role in wound healing2. ROLE IN NON PENETRATING CORNEAL WOUNDS

Increased rate of corneal epithelial wound healing with the use of all-trans-retinoic acid -variable results have been reported with retinol palmatate

1954- Agarwal et al - intramuscular vitamin A accelerated the healing time for both superficial and deep non penetrating corneal wounds while also decreasing the density of scar formation.

Use of IM vitamin A in 3 groups of human patients;1. Non sloughing, 2. Sloughing 3. Hypopyon associated corneal ulcers.

Role in wound healing3. ROLE IN CORNEAL EPITHELIAL DEFECTS

• Topical application of retinoic acid benefits epithelial healing time

• Vetrugno et al - role of oral vitamin A & E in re-epithelialization time and corneal haze formation at 1 year post PRK - significantly improved re- epitheliazation rates and reduction in formation of corneal haze which was more pronounced in high myopic corrections

• Novel methods of delivery of all-trans-retinoic acid -egg shaped Nano particles

• Showed earlier wound healing but higher concentration was both cytostatic and cytotoxic.

Role in cell differentiation-cornea

Retinoic acid 0.001- 0.1% reverses corneal keratinization and improves histological appearance of the cornea.

Improvement of surface keratinization in the untreated contralateral eye. Wright & Herbort et al showed an improvement in persistent epithelial

defects with use of 0.1% retinoic acid at bedtime Corneal surface becomes flatter, more wet able and regular Retinoic acid concentrations greater than 10 -6 M equivalent to 3.3% induce

Abnormal differentiation, Poor polarity and Increase mucin staining

Role in cell differentiation-conjunctiva

Retinoic acid helps improve conjunctival keratinization Controlling conjunctival fibroblast activity which as implications for

cicatrizing conjunctival disease Tseng - pts whose conjunctival impression cytology improved with the use of

retinoic acid: KCS SJS Pseudo-pemphigoid Surgically induced dry eye

Retinol palmitate - higher concentrations (1500 IU/ml) improve goblet cell numbers compared with placebo in a dose dependent manner.

Role in cell differentiation-limbus

Retinoic acid is vital for correct limbal differentiation but only in correct concentration

It differentiates limbal stem cells into transient amplifying cells that go on to epithelize the cornea

Ideal concentration 0.003 – 0.3% required for normal expression of limbal progenators and markers.

Where limbal stem cells are irreparably damaged, a process of trans-differentiation occurs

In the presence of viable limbal stem cells, retinoic acid acts to encourage corneal regeneration from this source.

When all stem cells are destroyed, re-epithelization occurs across the limbus from conjunctival cells and is once again influenced by retinoic acid

Role in cell differentiation-anti tumour effect

Positive effect of all-trans-retinoic acid in reversing squamous metaplasia Retinoic acid changes keratinocyte membrane glycoconjugates and this may

alter intra-cellular adhesions that control growth. Retinoic acid has the potential to contain but not cure neoplastic lesions Synergistic combination with other agents such as interferon alpha2b is

done for management of ocular surface dysplasias.

Role in cell differentiation-mebomian glands

Main limiting factor to the use of retinoic acid is its dramatic effect on mebomian gland.

Even systemic use of retinoic acid decreases mebomian gland function Atrophy of the acini Hyposecretion of oil Tear osmolality and Evaporation

Role in dry eye

Mebomian gland dysfunction is the most frequent cause of dry eye. On impression cytology, reversal of squamous metaplasia, increase

in goblet cell density with use of topical retinoic acid 0.001 to 0.1% Retinyl palmatate 0.05% vs cyclosporine A 0.05% for treating the

inflammatory component of dry eye disease 2011- international workshop on mebomian gland dysfunction-

hyperkeratinization of the orifice and ductal epithelium led to mebomian gland obstruction.

Retinoic acid only has a role if ocular surface keratinization is the predominant mechanism of dry eye and not in mebomian gland dysfunction.

Recent developments

Various studies have shown that retinoic acid is involved in the Photo-receptor differentiation and development, Lens development and regeneration Barrier function and trans-differentiation of retinal pigment epithelial

cells, Prevention of micro ophthalmia Establishment of immune tolerance in the eye

In animal studies, retinoic acid decreased the severity of optic neuritis and auto-immune uveo-retinitis.

Inhibited human lens epithelial cell proliferation raising the possibility of its use for pcos.

Drug dosage

conclusion

This review looks at the role of retinoic acid on the ocular surface. It has been shown to improve full and partial thickness corneal lacerations as well as corneal epithelial defects. Its positive effect is only achieved at the correct concentration, however; excess concentrations of retinoic acid have a deleterious effect. The main limiting factor of retinoic acid use is its detrimental effect on meibomian glands, resulting in cell death, atrophy of acini, hyposecretion of oils, and altered gene expression, eventually resulting in dry eye symptoms. This effect is reversible on discontinuation of the drug.