Cardiac Output, Venous Return, and Their Regulation
Retinoic acid and ocular surface
1. Retinoic acid and the ocular surface
CHAMEEN 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):183-95
Sydney Eye Hospital, Sydney, New South Wales, Australia; Save Sight Institute, University of Sydney, Sydney,
New South Wales, Australia.
2. 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.
3. Vitamin a deficiency
Leading cause of childhood blindness in developing countries
Manifests in 2 ways:
Night blindness/nyctalopia
Xerophthalmia
Ocular changes include..
Epidermal keratinization
Squamous metaplasia
Corneal ulceration
Night blindness
Retinopathy
4. 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
5. 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
6. 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
7. 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 .
8. 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
9. 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
10. Mechanism of retinoic acid action
Two main mechanisms
1. Nuclear receptor mediated pathway
2. 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
11. 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
12. 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.
13. 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
14. Role in wound healing
1. 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
15. Role in wound healing
2. 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.
16. Role in wound healing
3. 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.
17. 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
18. 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.
19. 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
20. 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.
21. 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
22. 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.
23. 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.
25. 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.
Editor's Notes
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.
Ocular manifestation of vit A deficiency remain the leading cause of childhood blindness in developing countries.
The deficiency of this fat soluble vit or its metabolites (retinoic acid) manifests in 2 ways:
night blindness/nyctalopia
and a spectrum of ocular disease known as xeropthalmia
Retinoic acid promotes incorporation of glucosamine into specific glycoproteins which is significiantly reduced in xeropthalmia
Ocular changes include..
Epidermal keratinization
Squamous metaplasia of the cornea and conjunctiva
Corneal ulceration
Night blindness
Retinopathy
The initial and most common ocular manifestation of vit A def is nyctalopia because the visual pigments of the photoreceptors are derived from this vit
A constant source of this vit is required for optimal photoreceptor function
Retinal electrophysiology can assist in diagnosis and follow up of vitamin deficiency.
Conjunctival pathology typically follows nyctalopia.
The first sign is xerosis (dryness) caused by marked decrease in mucous secreting goblet cells
Epidermal keratinization and squamous metaplasia of mucous memberanes occurs inversely propotional to serum vit A levels
Clinically conjunctiva appears thickened, wrinkled with loss of transparency.
Triangular, perilimbal foamy grayplaques of keratinized conjunctiva overlying an area of dryness known as the Bitot spots are said to be pathognomonic of current vit A deficiency
As the deficiency worsens, the cornea becomes involved.
Instability of the tear film leads to puctate keratopathy which progresses to epithelial defects, keratinization and stromal edema.
Left untreated corneal epithelial defects progress to partial or full thickness ulceration. And may develop bacterial ulceration
Keratomalacia is full thickness liquefactive necrosis of the cornea, is often ass with preceding systemic stressors like measles or severe protein malnutrition.
Corneal stroma can slough leading to either descematocoele or corneal perforation
Lastly theres an uncommon condition known as the xerophthalmic fundus, numerous small yellow dots representing loss of pigmentfrom the RPE
Replenishment of vit A stores typically results in the reversal of night blindness and conjunctival and retinal pathology.
Keratopathy without severe ulceration also respnds favourably .
Produced in body by 2 oxidation steps:from retinol to retinaldehyde and second retinaldehyde to retinoic acid which is the active form of vit A
Retinol is ingested in food and absorbed by intestinal cells is bound to serum retinol-binding protiens and transported to target epithelial cells.
Most organs have the capacity for retinoic acid biosynthesis including corneal epithelial cells
Vit A is transported into they eye via the ocular surface blood vessels and tears.
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
All-trans retinoic acid is inherently unstable because it undergoes photoisomerization.
Retinol/retinyl palmitate, another derivative of vit A is more stable and is the precursor or storage form of vit A
2 mechanisms of action of retinoic acid..nuclear receptor mediated and non nuclear receptor medicated..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.
Regulating the transcriptional level of target genes.
Retinoic acid binds to various nuclear receptors causing conformational changes that increase or decrease gene transcription.
Some effects of this upregulation are mediated by two familes of nuclear receptors which are retinoic acid receptor (RAR) and retinoic X receptors (RXR).
The effects include promotion of ocular surface hydration, epithelial healing and ocular differentiation and development.
In contrast, the trans-repressive activity of retinoic acid 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 matrix metalloproteinases 13 production(which plasys a role in pathogenesis of OSSN)
Down regulation of AP1 transcriptional activity inhibiting oncogenic proliferation and cellular proliferation. This relationship between retinoic acid and AP1 is of particular interest because of its use as adjuvant chemotherapy for conjunctival neoplasia
The non genomic mechanisms include binding to extra nuclear retinol receptors, retinoylation, activation of or interaction with other signaling molecules and the mediation of effects via metabolites
In summary, both nuclear and non nuclear receptor mediated mechanisms lead to
corneal epithelial cell repair via increasing production of hyaluronic acid and pro inflammatory cytokines
maintenance of ocular surface hydration via production of MUC 16
Apoptosis, cellular differentiation and repression of oncogenic proliferation
reduction of keratininzation of ocular surface epithelium
Full thickness penetrating corneal wounds
Studies on rabbit eyes show that application of retinoic acid increases the tensile strength of full thickness corneal wounds.
Retinol palmitate was compared in two doses 0.1 % and 0.5% which showed a significant increase in tensile strength with 0.1 % concentration and
0.5% not only had any effect on wound healing or tensile strength, it also showed an inhibitory effect in high doses.
Recent studies showed an ideal concentration of 10 x 10 -6 M equivalent to 33% increased keratocyte numbers which lead to improved corneal healing and tensile strength.
Non penetrating corneal wounds
increased rate of corneal epithelial wound healing was consistently demonstrated with the use of all-trans-retinoic acid though variable results have been reported with retinol palmatate.
As early as 1954, Agarwal et al demonstrated in rabbits that addition of intramuscular vitamin A acceleratd the healing time for both superficial and deep non penetrating corneal wounds while also decreasing the density of scar formation.
This led to use of intramuscular vitamin A in three groups of human patients; those with non sloughing, sloughing and hypopyon associated corneal ulcers.
Corneal epithelial defects
topical application of retinoic acid benefits epithelial helaing time but it was only found with the use of all-trans-retinoic acid with a concentration of 0.1% and the effect increased with the frequency of application.
Vetrugno et al examined the role of supplementary oral vitamin A & E in re-epithelialization time and corneal haze formation at 1 year post PRK.
Which showed 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 including egg shaped nano particles have been devised.
These showed earlier wound healing but higher concentration was both cytostatic and cytotoxic.
CORNEA
retinoic acid 0.001- 0.1% reverses corneal keratinization and improves hstiological apearence f the cornea.
With an 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% retinoicacid at bedtime.
He also noticed that the corneal surface becomes flatter, more wet able and regular.
Recent studies have shown that excess retinoic acid at concentrations greater than 10 -6 M equivalent to 3.3% induce abnormal differentiation, poor polarity and increase mucin staining.
Thus vitamin A plays a pivotal role in the differentiation of corneal epithelial cells.
CONJUNCTIVA
retinoic acid helps improve conjunctival keratinization and may play a key role in controlling conjunctival fibroblast activity which as implications for cicatrizing conjunctival disease..
Tseng published a small case series of patients with keratoconjunctivitis Sicca, SJS, pseudopemphigoid and surgically induced dry eye whose conjunctival impression cytology improved with the use of retinoic acid.
Studies using retinol palmitate show that higher concentrations (1500 IU/ml) are able to improve goblet cell numbers compared with placebo in a dose dependant manner.
LIMBUS
retinoic acid is vital for correct limbal differentiation but only in correct concentration.
it preferentially differentiates limbal stem cells into transient amplifying cells that go on to epithelize the cornea..
Recent studies have shown an ideal concentration 0.003 – 0.3% required for normal expression of limbal progenators and markers.
In cases of injury where limbal stem cells are irrepairably damaged, a process of trans-differentiation occurs (conjunctival cells undergo differentiation process with loss of goblet cells and cornea like morphology is adopted).
In the presence of viable limbal stem cells, retinoic acid acts to encourage corneal regeneration from this source.
However, when all stem cells are destroyed, re-epitheliazation occurs across the limbus from conjunctival cells and is once again influenced by retinoic acid.
ANTI-TUMOUR EFFECT
studies have shown positive effect of all-trans-retinoic acid in reversing squamous metaplasia.
It was postulated that 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 therefore synergistic combination with other agents such as interferon @2b is done for management of ocular surface dysplasias
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 causing atrophy of the acini and hypo-secretion of oil effecting tear osmolality and evaporation leading to significant dry eye symptoms.
Recently, it has been realized that mebomian gland dysfunction is the most frequent cause of dry eye secondary to increased evaporation of aqueous layer.
On impression cytology, there was a reversal of squamous metaplasia along with an increase in goblet cell density critical for the improved comfort and wetting of the ocular suface with use of topical retinoic acid 0.001 to 0.1%
Studies have compared retinyl palmatate 0.05% with cyclosporine A 0.05% for treating the inflammatory component of dry eye disease.
In 2011 international workshop on mebomian gland dysfunction, it was discussed that Hyperkeratinization of the orifice and ductal epithelium led to mebomian gland obstruction.
This obstruction led to increased internal pressure causing atrophic changes and squamous metaplasia within the mebomian acini causing a secondary hypo-secretion.
For the treatment of dry eye, retinoic acid only has a role if ocular surface keratinization is the predominant mechanism and not in mebomian gland dysfunction.
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 and possibly in the establishment of immune tolerance in the eye.
In animal studies, retinoic acid decreased the severity of optic neuritis and auto-immune uveo-retinitis.
It also inhibited human lens epithelial cell proliferation raising the possibility of its use for posterior capsular opacification.
Successful treatments used .01 to 0.1% all trans retinoic acid
Higher doses 0.25% were found to be ineffective
Recent studies have sought to use the lowest possible conc which is effective and with minimum possible frequency
Fewer studies have used retinol palmitate, it is used as 1000 IU/ml QID for 4 weeks with good results and fewer side effects
Retinoic acid is known to improve cutaneous wound healing and, in recent years, its application in ophthalmology has been investigated.
Retinoic acid can be produced synthetically, and its mechanism of action includes both nuclear and non-nuclear receptor mediated pathways.
Retinoic acid plays a critical role in cell differentiation at the cornea, conjunctiva, and limbus, and may have an anti-tumor role.