Retinoic acid receptor gamma mediates topical retinoid efficacy and irritation in animal models

Cosmetic retinoid use in photoaged skin: A review of the compounds, their use and mechanisms of action

The inevitable attrition of skin due to ultraviolet radiation, termed photoaging, can be partially restored by treatment with retinoid compounds. Photoaged skin in lightly pigmented individuals, clinically presents with the appearance of wrinkles, increased laxity, and hyper- and hypopigmentation. Underlying these visible signs of ageing are histological features such as epidermal thinning, dermal-epidermal junction flattening, solar elastosis and loss of the dermal fibrillin microfibrillar network, fibrillar collagen and glycosaminoglycans. Retinoid compounds are comprised of three main generations with the first generation (all-trans retinoic acid, retinol, retinaldehyde and retinyl esters) primarily used for the clinical and cosmetic treatment of photoaging, with varying degrees of efficacy, tolerance and stability. All-trans retinoic acid is considered the 'gold standard' for skin rejuvenation; however, it is a prescription-only product largely confined to clinical use. Therefore, retinoid derivatives are readily incorporated into cosmeceutical formulations. The literature reported in this review suggests that retinol, retinyl esters and retinaldehyde that are used in many cosmeceutical products, are efficacious, safe and well-tolerated. Once in the skin, retinoids utilize a complex signalling pathway that promotes remodelling of photoaged epidermis and dermis and leads to the improvement of the cutaneous signs of photoaging.

Topical retinoids: Novel derivatives, nano lipid-based carriers, and combinations to improve chemical instability and skin irritation

BACKGROUND

Retinoids, defined as synthetic or natural derivatives of vitamin A, have been extensively studied as anti-aging molecules that are widely applied in cosmetics. However, due to their physicochemical property, retinoids are highly unstable and extremely sensitive to light, oxygen, and temperature. Moreover, topical application of retinoids often leads to cutaneous irritation. These instabilities and irritant properties of retinoids limit their application in cosmetic and pharmaceutical products.

AIM

Our study aimed to provide a systematic review to summarize the mechanisms underlying the instability and irritant properties of retinoids, as well as recent developments in addressing these challenges.

METHODS

A comprehensive PubMed search was conducted using the following keywords: retinoids, chemical instability, skin irritation, retinoid derivatives, nano lipid-based carriers, liposomes, penetration-enhancer vesicles, ethosomes, niosomes, nanoemulsions, solid lipid nanoparticles, vitamins, soothing and hydrating agents, antioxidants and metal chelator and retinol combinations. Relevant researches published between 1968 and 2023 and studies related to these reports were reviewed.

RESULTS

The development of new retinoid derivatives, the utilization of new delivery systems like nano lipid-based carriers and the combination with other compounds like vitamins, soothing agents, antioxidants and metal chelator have been explored to improve the stability, bioavailability, and toxicity of the retinoid family.

CONCLUSIONS

Through advancements in formulation techniques, structure modification of retinoid derivatives and development of novel nano lipid-based carriers, the chemical instability and skin irritation of retinoids has been mitigated, ensuring their efficacy and potency over extended periods.

Skin ageing and topical rejuvenation strategies

Skin ageing is a complex process involving the additive effects of skin's interaction with its external environment, predominantly chronic sun exposure, upon a background of time-dependent intrinsic ageing. Skin health and beauty is considered one of the principal factors perceived to represent overall 'health and wellbeing'; thus, the demand for skin rejuvenation strategies has rapidly increased, with a worldwide annual expenditure expected to grow from $US24.6 billion to around $US44.5 billion by 2030 (https://www.databridgemarketresearch.com/reports/global-facial-rejuvenation-market). Skin rejuvenation can be achieved in several ways, ranging from laser and device-based treatments to chemical peels and injectables; however, topical skin care regimes are a mainstay treatment for ageing skin and all patients seeking skin rejuvenation can benefit from this relatively low-risk intervention. While the most efficacious topical rejuvenation treatment is application of tretinoin (all-trans retinoic acid) - a prescription-only medicine considered to be the clinical 'gold standard' - a hybrid category of 'cosmeceutical' products at the midpoint of the spectrum of cosmetics and pharmaceutical has emerged. This article reviews the clinical manifestations of skin ageing and the available topical treatments for skin rejuvenation, including retinoids, peptides and antioxidants.

Multifaceted amelioration of cutaneous photoageing by (0.3%) retinol

BACKGROUND

Although retinol skin care products improve the appearance of photoaged skin, there is a need for an effective retinol concentration that provides skin benefits without irritation.

OBJECTIVE

To compare the efficacy of topical 0.1%, 0.3% and 1% retinol in remodelling the cutaneous architecture in an in vivo experimental patch test study, and to determine tolerance of the most effective formulations when used in a daily in-use escalation study.

METHODS

For the patch test study, retinol products were applied under occlusion, to the extensor forearm of photoaged volunteers (n = 5; age range 66-84 years), and 3 mm skin biopsies obtained after 12 days. Effects of different retinol concentrations, and a vehicle control, on key epidermal and dermal biomarkers of cellular proliferation and dermal remodelling were compared to untreated baseline. Separately, participants (n = 218) recorded their tolerance to 0.3% or 1% retinol over a six-week, approved regimen, which gradually increased the facial applications to once nightly.

RESULTS

Retinol treatment induced a stepwise increase in epidermal thickness and induced the expression of stratum corneum proteins, filaggrin and KPRP. 0.3% retinol and 1% retinol were comparably effective at inducing keratinocyte proliferation in the epidermis, whilst reducing e-cadherin expression. Fibrillin-rich microfibril deposition was increased following treatment with 0.3% and 1% retinol (p < 0.01); other dermal components remained unaltered (e.g., fibronectin, collagen fibrils, elastin), and no evidence of local inflammation was detected. The in-use study found that 0.3% retinol was better tolerated than 1% retinol, with fewer and milder adverse events reported (χ² (1) = 23.97; p < 0.001).

CONCLUSIONS

This study suggests that 1% and 0.3% retinol concentrations were similarly effective at remodelling photodamaged skin in an in vivo model of long-term use. Use of 0.3% retinol in the escalation study was associated with fewer adverse reactions when applied daily. Hence, 0.3% retinol may be better tolerated than 1% retinol, thereby allowing longer-term topical application.

Anti-Irritant Strategy against Retinol Based on the Genetic Analysis of Korean Population: A Genetically Guided Top-Down Approach

Retinol, one of the most powerful cosmetic materials for anti-aging supported by a solid scientific background, exhibits a wide range of type and severity of irritation while showing limited user compliance. The lack of understanding of the mechanism of retinol-induced irritation has been the main hurdle in the development of anti-irritation strategies. Here, we identified 30 genetic markers related to the susceptibility to retinol-induced irritation in the Korean population. Based on the genetic analysis, a novel formula against retinol-induced irritation was developed, which mitigated the molecular pathogenesis—as indicated by the genetic markers—of the retinol-induced irritation. In human tests, this formula effectively decreased retinol-induced irritation. Furthermore, a polygenic risk score model for irritation was constructed and validated. Our comprehensive approach for the analysis of retinol-induced irritation will not only aid the development of anti-irritation strategies to ensure higher user compliance but also contribute to improving the current knowledge about the biological effects of retinoids.

50 Years of Topical Retinoids for Acne: Evolution of Treatment

Since the US Food and Drug Administration (FDA) approved tretinoin in 1971, retinoids alone or combined with other agents have become the mainstay of acne treatment. Retinoids act through binding to retinoic acid receptors, altering expression levels of hundreds of cellular proteins affecting multiple pathways involved in acne pathogenesis. Retinoids have evolved from first-generation agents, such as tretinoin, through chemical modifications resulting in a second generation (etretinate and acitretin for psoriasis), a third generation (adapalene and tazarotene) and, most recently, a fourth (trifarotene). For all topical retinoids, local irritation has been associated with poor tolerability and suboptimal adherence. Efforts to improve tolerability have utilized novel delivery systems and/or novel agents. This qualitative literature review summarizes the evolution of the four topical single-agent retinoids available for the treatment of acne in the US today and their various formulations, presenting the rationale behind their development and data from key studies.

Strategies to generate functionally normal neutrophils to reduce infection and infection-related mortality in cancer chemotherapy

Neutrophils form an essential part of innate immunity against infection. Cancer chemotherapy-induced neutropenia (CCIN) is a condition in which the number of neutrophils in a patient's bloodstream is decreased, leading to increased susceptibility to infection. Granulocyte colony-stimulating factor (GCSF) has been the only approved treatment for CCIN over two decades. To date, CCIN-related infection and mortality remain a significant concern, as neutrophils generated in response to administered GCSF are functionally immature and cannot effectively fight infection. This review summarizes the molecular regulatory mechanisms of neutrophil granulocytic differentiation and innate immunity development, dissects the biology of GCSF in myeloid expansion, highlights the shortcomings of GCSF in CCIN treatment, updates the recent advance of a selective retinoid agonist that promotes neutrophil granulocytic differentiation, and evaluates the benefits of developing GCSF biosimilars to increase access to GCSF biologics versus seeking a new mode to fundamentally advance GCSF therapy for treatment of CCIN.

Nonclinical and human pharmacology of the potent and selective topical retinoic acid receptor-γ agonist trifarotene

BACKGROUND

First- and third-generation retinoids are the main treatment for acne. Even though efficacious, they lack full selectivity for retinoic acid receptor (RAR) γ, expressed in the epidermis and infundibulum.

OBJECTIVES

To characterize the in vitro metabolism and the pharmacology of the novel retinoid trifarotene.

MATERIALS AND METHODS

In vitro assays determined efficacy, potency and selectivity on RARs, as well as the activity on the expression of retinoid target genes in human keratinocytes and ex vivo cultured skin. In vivo studies investigated topical comedolytic, anti-inflammatory and depigmenting properties. The trifarotene-induced gene expression profile was investigated in nonlesional skin of patients with acne and compared with ex vivo and in vivo models. Finally, the metabolic stability in human keratinocytes and hepatic microsomes was established.

RESULTS

Trifarotene is a selective RARγ agonist with > 20-fold selectivity over RARα and RARβ. Trifarotene is active and stable in keratinocytes but rapidly metabolized by human hepatic microsomes, predicting improved safety. In vivo, trifarotene 0·01% applied topically is highly comedolytic and has anti-inflammatory and antipigmenting properties. Gene expression studies indicated potent activation of known retinoid-modulated processes (epidermal differentiation, proliferation, stress response, retinoic acid metabolism) and novel pathways (proteolysis, transport/skin hydration, cell adhesion) in ex vivo and in vivo models, as well as in human skin after 4 weeks of topical application of trifarotene 0·005% cream.

CONCLUSIONS

Based on its RARγ selectivity, rapid degradation in human hepatic microsomes and pharmacological properties including potent modulation of epidermal processes, topical treatment with trifarotene could result in good efficacy and may present a favourable safety profile in acne and ichthyotic disorders.

Emerging Therapies for Acne Vulgaris

As we gain a greater understanding of acne pathogenesis, both new agents as well as new uses for established drugs are being considered for the treatment of acne vulgaris. Multiple clinical trials assessing new formulations or combinations of established acne treatments have been conducted, and novel uses of antimicrobials such as modified diallyl disulfide oxide and nitric oxide are being assessed in clinical trials. There are also a multitude of new therapies currently being studied that target the inflammatory cascade of acne pathogenesis, including sebosuppressive and anti-inflammatory phytochemicals, and  small molecule inhibitors targeting sebaceous glands and enzymes, among others. Laser and light therapy is also being modified for the treatment of acne through combination methods with metal nanoshells and vacuum assistance. Probiotics have gained popularity in medicine as greater knowledge of the microbiome and its effects on multiple organ systems is being elucidated. Studies describing the positive effects of certain ammonia-oxidizing bacterial strains in the regulation of the skin's inflammatory response are ongoing. Therapies for acne are constantly evolving and current gold-standard acne therapy may be supplemented with novel treatment modalities in the near future.

In vivo long-term effects of retinoic acid exposure in utero on induced tumours in adult mouse skin

BACKGROUND

Retinoic acid (RA) and its analogues (retinoids) are promising agents in skin cancer prevention following either topical application or oral administration. However, long-term in vivo effects of RA on chemically induced hyperplastic epidermal foci in adult mouse skin have also been described, casting some doubt with regard to its chemopreventive activity.

HYPOTHESIS/OBJECTIVES

To characterize chemically induced skin tumours and to investigate the in vivo long-term action and preventive effect of RA on adult mouse skin carcinogenesis.

ANIMALS

Fifty-six adult Naval Medical Research Institute mice, exposed (n = 28) or not exposed (n = 28) to RA in utero.

METHODS

Mice were treated with a standard two-stage skin carcinogenesis protocol, which included an initiating application of 7,12-dimethylbenz(a)anthracene followed by promotion with 12-O-tetradecanoylphorbol 13-acetate.

RESULTS

Retinoic acid administered to pregnant mice showed a long-term inhibitory action on cell differentiation and development of chemically induced tumours on the adult skin of their offspring, as well as a stimulatory effect on cell proliferation and expression of an early marker of malignant progression (keratin 13).

CONCLUSIONS AND CLINICAL IMPORTANCE

The results suggest that RA exposure in utero confers long-lasting effects on adult mouse skin carcinogenesis. These include chemopreventive activity (reduced number of tumours), as well as enhancement of squamous papilloma progression, which appears to be due to enhanced keratinocyte proliferation and suppression of epidermal maturation. The clinical significance of these findings is not known for other routes of RA administration at this time.

Retinoid Induces the Degradation of Corneodesmosomes and Downregulation of Corneodesmosomal Cadherins: Implications on the Mechanism of Retinoid-induced Desquamation

BACKGROUND

Topical retinoids induce skin fragility. As corneodesmosomes are important adhesion structures in the epidermal cohesion, an effect of retinoids on corneodesmosomes has been suspected.

OBJECTIVE

The aim of this study was to investigate the effect of retinoid on the expression of corneodesmosomal components including desmoglein (DSG) 1, desmocollin (DSC) 1, corneodesmosin (CDSN) and kallikrein (KLK)s.

METHODS

2% all-trans-retinol or ethanol was applied to the back of hairless mice for five days, and the structure of the stratum corneum was examined by transmission electron microscopy. The cultured human keratinocytes were treated with all-trans-retinoic acid (RA) in low or high calcium media for 24 hours.

RESULTS

Topical retinol increased corneocyte detachment and degradation of corneodesmosomes. RA significantly decreased DSG1 and DSC1 expression at the mRNA and protein levels in keratinocytes that were cultured in both low- and high-calcium media. On the other hand, CDSN mRNA levels did not decrease in low-calcium media or increase in high-calcium media after RA treatment. KLK5 and KLK7 expression did not increase after RA treatment.

CONCLUSION

Our results indicate that DSG1 and DSC1 downregulation by RA could be related to the increased degradation of corneodesmosomes and consequent desquamation induced by retinoids.

Regulation of keratin expression by retinoids

Vitamin A and its natural and synthetic metabolites (retinoids) affect growth and differentiation of human skin and among the genes affected by retinoids in epidermis are keratin genes. Keratins are intermediate filament proteins that have essential functions in maintaining the structural integrity of epidermis and its appendages. Their expressions are under strict control to produce keratins that are optimally adapted to their environment. In this article, retinoid regulation of keratin expression in cultured human epidermal keratinocytes and in human skin in vivo will be reviewed. The direct and indirect mechanisms involved will be discussed and novel therapeutic strategies will be proposed for utilizing retinoids in skin disorders due to keratin mutations (e.g., epidermolysis bullosa simplex and epidermolytic ichthyosis).

Oral administration of a retinoic Acid receptor antagonist reversibly inhibits spermatogenesis in mice

Here we investigated a pharmacological approach to inhibit spermatogenesis in the mouse model by manipulating retinoid signaling using low doses of the pan-retinoic acid receptor (RAR) antagonist BMS-189453. Spermatogenesis was disrupted, with a failure of spermatid alignment and sperm release and loss of germ cells into lumen, abnormalities that resembled those in vitamin A-deficient and RARα-knockout testes. Importantly, the induced sterility was reversible. Enhanced efficacy and a lengthened infertility period with full recovery of spermatogenesis were observed using systematically modified dosing regimens. Hematology, serum chemistry, and hormonal and pathological evaluations revealed no detectable abnormalities or adverse side effects except the distinct testicular pathology. Our results suggest that testes are exquisitely sensitive to disruption of retinoid signaling and that RAR antagonists may represent new lead molecules in developing nonsteroidal male contraceptives.

Epidermal Hyperplasia and Elevated HB-EGF are More Prominent in Retinoid Dermatitis Compared with Irritant Contact Dermatitis Induced by Benzalkonium Chloride

BACKGROUND

'Retinoid dermatitis' is a retinoid-induced irritant contact dermatitis (ICD). The mechanism of retinoid dermatitis may be different from that of other ICDs. However, it remains uncertain how topical retinoid induce ICD.

OBJECTIVE

We compared several aspects of contact dermatitis induced by topical retinol and benzalkonium chloride (BKC) on hairless mice skin.

METHODS

2% retinol or 2.5% BKC was applied to hairless mice and transepidermal water loss (TEWL), ear thickness, histologic and immunohistochemical findings were compared. We also compared mRNA expression of inflammatory cytokines, epidermal differential markers, cyclooxygenases (COXs) and heparin binding epidermal growth factor like growth factor (HB-EGF).

RESULTS

Topical application of 2% retinol and 2.5% BKC increased TEWL and ear thickness in similar intensity. Epidermal hyperplasia was more prominent in retinol treated skin. Proliferating cell nuclear antigen, involucrin and loricrin expression were higher in retinol-treated skin than in BKC-treated skin. Filaggrin, however, was more expressed in BKC-treated skin. The mRNA expression of IL-8, TNF-alpha, COX-2, involucrin, loricrin and filaggrin were increased in both retinol- and BKC-treated skin in similar intensity. HB-EGF was more significantly increased in retinol-treated skin.

CONCLUSION

Elevated HB-EGF and epidermal hyperplasia are more prominent features of retinoid dermatitis than in BKC-induced ICD.

Keratins 2 and 4/13 in reconstituted human skin are reciprocally regulated by retinoids binding to nuclear receptor RARalpha

Disorders of keratinization are often treated with vitamin A derivatives (retinoids) which affect keratinocyte differentiation, including keratin (KRT) gene expression. In vivo, suprabasal keratinocytes normally express only keratin (K) 1, K2 and K10, but after topical application of all-trans retinoic acid (ATRA), the granular cells will additionally express K4 and K13, i.e. keratins normally present in oral mucosa and in cultured epidermal keratinocytes. To learn more about the retinoid regulation of keratin expression under in vivo-like conditions, we cultured keratinocytes on de-epidermized dermis in only 0.5% serum. These cells produce a normal-looking epidermis that expresses high mRNA levels of KRT1, KRT2 and KRT10, but minimal amounts of KRT4 and KRT13. Addition of ATRA to the medium for 48 h caused a dose-dependent increase in KRT4/KRT13 and a down-regulation of KRT2 mRNA. An increase in K4 protein was also found. The response was greater than the up-regulation of another retinoid-regulated gene, CRABPII. By studying 10 retinoids with different affinities for the retinoic acid receptors (RAR) and retinoid X receptors (RXR) isoforms, the reciprocal expression of KRT2 and KRT4/KRT13 could be connected with agonists for RARalpha. Two of these agonists, CD336/Am580 and CD2081, altered the expression profile with similar potency as the pan-RAR agonists ATRA and CD367. Co-addition of a pan-RAR antagonist (CD3106/AGN193109) markedly inhibited the induction of KRT4/KRT13 expression, whereas the down-regulation of KRT2 was less affected. In conclusion, RARalpha agonists elicit a reciprocal modulation of KRT2 and KRT4/KRT13 expression in human epidermis, but whether or not the keratin genes also possess RARalpha-specific regulatory elements is still unclear.

Topical retinoids in acne vulgaris: update on efficacy and safety

Topical retinoids represent a mainstay of acne treatment because they expel mature comedones, reduce microcomedone formation, and exert anti-inflammatory effects. The first-generation retinoid tretinoin (all-trans retinoic acid) and the synthetic third-generation polyaromatics adapalene and tazarotene are approved for acne treatment by the US FDA, whereas topical tretinoin, isotretinoin (13-cis retinoic acid), and adapalene are accredited in Canada and Europe. Topical retinoids have a favorable safety profile distinct from the toxicity of their systemic counterparts. Local adverse effects, including erythema, dryness, itching, and stinging, occur frequently during the early treatment phase. Their impact varies with the vehicle formation, skin type, frequency and mode of application, use of moisturizers, and environmental factors such as sun exposure or temperature. The broad anti-acne activity and safety profile of topical retinoids justifies their use as first-line treatment in most types of non-inflammatory and inflammatory acne. They are also suitable as long-term medications, with no risk of inducing bacterial resistance, for maintenance of remission after cessation of initial combination therapy.

Retinoid receptors

"Retinoid" refers to the naturally occurring compounds with vitamin A activity and to synthetic analogues of retinol. Retinoids are key regulators of differentiation, proliferation, and inflammation. Their successful use in the treatment of various skin diseases and neoplasias has revolutionized the practice of dermatology as well as oncology. This article focuses on the retinoid receptors to elucidate our understanding of their complex biologic activity that is reflected in their therapeutic clinical effects as well as in their adverse reactions.

Marked improvement induced in photoaged skin of hairless mouse by ER36009, a novel RARgamma-specific retinoid, but not by ER35794, an RXR-selective agonist

BACKGROUND

Photoaging (premature skin aging) results largely from repeated exposure of the skin to ultraviolet (UV) radiation from the sun. Topical all-trans retinoic acid (RA), the only agent that has been approved for the treatment of photoaging, has been shown to reverse this process. In this study, we evaluated the pharmacologic effects of novel synthetic retinoids, ER36009 and ER35794, on murine wrinkles induced by UVB. ER36009 is a specific agonist of retinoic acid receptor (RAR)gamma, the most abundant RAR subtype in the skin, while ER35794 is a potent retinoid X receptor (RXR)-selective agonist.

METHOD

After a 10-week exposure to escalating doses of UVB irradiation, the animals were treated three times per week with ER36009 (0.0001%, 0.00025%, 0.0005%), ER35794 (0.025%, 0.05%, 0.1%), RA (0.05%) or acetone (control) for 3 weeks.

RESULTS

ER36009 exerted a dose-dependent wrinkle-effacing effect, and 0.0005% ER36009-treated skin was significantly different from the control. ER36009 also significantly and dose-dependently increased both epidermal thickness and the area of the dermal repair zone defined by newly synthesized collagen. The effect of 0.0005% ER36009 on photodamaged skin was superior to that of 0.05% RA. In contrast, ER35794 was inactive in this model, though this compound exhibited lower local toxicity than other retinoids.

CONCLUSIONS

These data indicate that RARgamma, but not RXR, plays an important role in the improvement of the signs of photoaging, and so a specific RARgamma agonist might be superior to an RAR pan-agonist for clinical treatment. We conclude that ER36009 is a candidate for a potent anti-skin-aging agent.

Comparison of topical retinoids in the treatment of acne

Topical retinoids are been used to successfully treat acne for almost 3 decades. At the beginning, a retinoid was a compound of similar structure and action to retinol (vitamin A).(1) Changes at the carboxylic end group, the polyene chain, and the aromatic ring can result in the modification of the original molecule. To date, three generations of retinoids have been developed: the nonaromatics (retinol, tretinoin, and isotretinoin), the monoaromatics (etretinate and acitretin), and the polyaromatics (arotinoid, adapalene, and tazarotene). The new synthetic retinoid molecules have little resemblance with retinol but nonetheless are included in this family because they have the ability to bind with or activate retinoid receptors. Therefore, retinoids are vitamins and also hormones.(3)

PADMA 28: A Multi-Component Herbal Preparation with Retinoid-Like Dermal Activity but Without Epidermal Effects

PADMA 28, a multi-component herbal mixture formulated according to an ancient Tibetan recipe, was assessed for effects on human dermal fibroblasts and epidermal keratinocytes in monolayer culture, and for effects on human skin in organ culture. PADMA 28 stimulated survival of fibroblasts in monolayer culture. In fibroblast monolayer culture and human skin organ culture, levels of matrix metalloproteinase-1 (MMP-1; interstitial collagenase) were reduced and type I procollagen production was increased. When keratinocytes were examined, there was no evidence of growth stimulation over a wide range of PADMA 28 concentrations. At high concentration, PADMA 28 inhibited keratinocyte proliferation. When organ cultures of human skin were treated with PADMA 28, there was no evidence of hyperplastic growth in the epidermis. Topical treatment of rhino mice with PADMA 28 failed to induce epidermal hyperplasia and was completely non-irritating. The ability to stimulate collagen production and inhibit the major collagen-degrading enzyme in skin without inducing a hyperplastic response in the epidermis may provide a basis for development of the herbal preparation as a "skin-repair" agent.

Topical pretreatment of diabetic rats with all-trans retinoic acid improves healing of subsequently induced abrasion wounds

In the current study, rats were made diabetic with streptozotocin (STZ) and maintained for 8 weeks, during which time they were treated topically on alternative days with a solution of 0.1% all-trans retinoic acid in a vehicle of 70:30% ethanol/propylene glycol. STZ-induced diabetic rats treated with vehicle served as controls. Additional nondiabetic rats were treated with all-trans retinoic acid or vehicle in parallel. At the end of the 8-week period, rats from all four treatment groups were subjected to abrasion wound formation. Wounds healed more rapidly in vehicle-treated nondiabetic skin than in vehicle-treated diabetic skin (96% of the wound surface area closed in nondiabetic rats within 6 days vs. 41% closed in diabetic rats). Wounds in all-trans retinoic acid-treated diabetic skin healed more rapidly than wounds in vehicle-treated diabetic skin (85% of the wound surface area closed in all-trans retinoic acid-treated diabetic rats vs. 41% closed in vehicle-treated diabetic rats). At the histological level, recently healed skin from vehicle-treated diabetic rats was shown to contain a thin, wispy provisional matrix in which many of the embedded cells were rounded and some were pycnotic. In contrast, a much denser provisional matrix with large numbers of embedded spindle-shaped cells was observed in healed wounds from diabetic skin that had been pretreated with all-trans retinoic acid. The all-trans retinoic acid-treated diabetic skin was histologically similar to vehicle-treated (or all-trans retinoic acid-treated) skin from nondiabetic animals. In light of these findings, we suggest that prophylactic use of retinoid-containing preparations might be useful in preventing the development of nonhealing skin ulcers resultant from minor traumas in at-risk skin.

Retinoids in nephrology: Promises and pitfalls

BACKGROUND

Retinoids, a family of vitamin A metabolites or analogs, play an important role in regulating cell proliferation, differentiation, and apoptosis.

METHODS

The biological importance of retinoids in the kidney and the potential of retinoids in the treatment of renal diseases are reviewed.

RESULTS

Vitamin A deficiency and mutations of retinoid nuclear receptors cause abnormalities in fetal kidneys, which might predispose to adult diseases such as hypertension. Further, the therapeutic value of retinoids in animal models of kidney diseases, such as lupus nephritis, puromycin aminonucleoside nephrosis, anti-glomerular basement membrane nephritis, mesangioproliferative nephritis, and acute renal allograft rejection has been unveiled recently. Retinoids target mesangial cells, podocytes, tubular epithelial cells, interstitial fibroblasts, as well as lymphocytes and macrophages. The anti-inflammation, anti-coagulation effects, and the proliferation- and immunity-modulating actions of retinoids, have been widely appreciated. Our recent in vitro data revealed a direct antifibrotic effect and a cytoprotective effect of retinoids in various renal cell types. In animal studies, the adverse effects of retinoids are generally minimal; however, the clinical use of retinoids in other diseases points to some major side effects. In addition, in vitro, retinoids can induce lipid accumulation in smooth muscle cells and macrophages and increase expression of some proinflammatory molecules, indicating that their clinical toxicity profile in the setting of renal diseases needs to be better understood.

CONCLUSION

Retinoids not only are important in renal development, but also show promise as a new generation of renal medication and deserve to be tested in clinical trials to clarify their full potential.

Retinoid-induced epidermal hyperplasia in human skin organ culture: inhibition with soy extract and soy isoflavones

Organ cultures of human skin were incubated for 8 days with 1 microg/ml 14-all trans retinoic acid (14-all trans RA) and concomitantly treated with varying concentrations of soy extract. The epidermis of organ cultures treated with 14-all trans RA alone underwent a hyperplastic response. In cultures treated with a combination of 14-all trans RA and soy extract (4-40 microg/ml), hyperplasia was reduced by 16-41%. The same concentrations of soy extract that reduced epidermal hyperplasia in organ culture also suppressed proliferation of rapidly growing keratinocytes in monolayer culture (approximately 25% reduction at 20 and 40 microg/ml). On the other hand, soy extract did not further inhibit proliferation of quiescent keratinocytes; rather, it stimulated growth (50-52% increase relative to control). When dermal fibroblasts were examined for a response to soy extract (i.e., proliferation and synthesis of type I procollagen), both responses were stimulated (proliferation: 75% increase and collagen production 114% increase relative to control). Genistein, the major isoflavone in extracts of soy also inhibited epidermal hyperplasia in organ culture (34-40% reduction relative to control). The same concentrations that reduced epidermal hyperplasia (0.5-1.0 microg/ml) also inhibited keratinocyte proliferation in monolayer culture but had little effect on fibroblast growth. Two other isoflavones (daidzein and glycetein) were also inhibitory, but were less effective than genistein. Taken together, these data suggest that use of soy extract or its constituent isoflavones in conjunction with 14-all trans RA may provide a way to mitigate unwanted epidermal effects of topical retinoid therapy without compromising beneficial retinoid effects in the dermis.

Separation of retinoid-induced epidermal and dermal thickening from skin irritation

The ability of the synthetic retinoid MDI-301, in which the carboxylic acid of 9- cis-retinoic acid (9-cis-RA) is replaced with an ester linkage, to induce epidermal and dermal thickening and skin irritation (erythema and flaking) in hairless (rhino) mice following its topical application was investigated in comparison with that of 14-all- trans-retinoic acid (14-all-trans-RA) and 9-cis-RA. MDI-301 induced epidermal proliferation leading to a thickened epidermis. Treated animals also demonstrated a prominent band of organized connective tissue immediately below the epidermis. In its ability to induce epidermal thickening, MDI-301 was quantitatively similar to 14-all-trans-RA and 9-cis-RA. However, unlike 14-all-trans-RA and 9-cis-RA, which produced skin irritation associated with a perivascular influx of mononuclear leukocytes into the dermis, there was no evidence of irritation with MDI-301 and little leukocyte infiltration. Intraperitoneal injection of either 14-all-trans-RA or MDI-301 also resulted in epidermal and dermal thickening. Irritation of skin was not observed in these animals but splenomegaly was prominent in animals treated with either agent.

Changing paradigms in dermatology: nuclear hormone receptors

Nuclear hormone receptors (NHRs) are a family of proteins that function similarly as nuclear transcription factors. The NHR family includes glucocorticoid receptors, retinoic acid and retinoid receptors, vitamin D receptors, thyroxin receptors, and peroxisome proliferator activated receptors. These proteins are targets for some of the most commonly prescribed medications in dermatology, including corticosteroids, retinoids, and vitamin D analogues, all of which have limiting side effects. Advances in this field have led to better understanding of the mechanisms of NHR therapeutic and toxic effects, receptor subtypes, tissue distribution, and interaction with other molecules. New generations of more specific NHR ligands designed to increase therapeutic efficacy and limit adverse effects have dramatically expanded the clinical application of NHR-targeting drugs. The current understanding of NHRs and future directions for NHR ligands in dermatology are discussed.

C-H...O hydrogen bonds in the nuclear receptor RARgamma--a potential tool for drug selectivity

Hydrogen bonds between polarized atoms play a crucial role in protein interactions and are often used in drug design, which usually neglects the potential of C-H...O hydrogen bonds. The 1.4 A resolution crystal structure of the ligand binding domain of the retinoic acid receptor RARgamma complexed with the retinoid SR11254 reveals several types of C-H...O hydrogen bonds. A striking example is the hydroxyl group of the ligand that acts as an H bond donor and acceptor, leading to a synergy between classical and C-H...O hydrogen bonds. This interaction introduces both specificity and affinity within the hydrophobic ligand pocket. The similarity of intraprotein and protein-ligand C-H...O interactions suggests that such bonds should be considered in rational drug design approaches.

Physiological and retinoid-induced proliferations of epidermis basal keratinocytes are differently controlled

To investigate the roles of retinoic acid (RA) receptors (RARs) in the physiology of epidermis that does not express RAR beta, conditional spatio-temporally controlled somatic mutagenesis was used to selectively ablate RAR alpha in keratinocytes of RAR gamma-null mice. Keratinocyte proliferation was maintained in adult mouse epidermis lacking both RAR alpha and RAR gamma, as well as in RAR beta-null mice. All RAR-mediated signalling pathways are therefore dispensable in epidermis for homeostatic keratinocyte renewal. However, topical treatment of mouse skin with selective retinoids indicated that RXR/RAR gamma heterodimers, in which RXR transcriptional activity was subordinated to that of its RAR gamma partner, were required for retinoid-induced epidermal hyperplasia, whereas RXR homodimers and RXR/RAR alpha heterodimers were not involved. RA-induced keratinocyte proliferation was studied in mutant mice in which RXR alpha, RXR alpha and RAR alpha, RAR gamma, or RXR alpha and RAR gamma genes were specifically disrupted in either basal or suprabasal keratinocytes. We demonstrate that the topical retinoid signal is transduced by RXR alpha/RAR gamma heterodimers in suprabasal keratinocytes, which, in turn, stimulate proliferation of basal keratinocytes via a paracrine signal that may be heparin-binding EGF-like growth factor.

Developments in topical retinoid therapy for acne

Topical retinoic acid was introduced for acne treatment three decades ago. Since that time, researchers have discovered thousands of retinoids, originally defined as chemical analogs of vitamin A. After the identification of nuclear retinoid receptors in 1987, the definition of this class expanded to include molecules that bind to and activate such receptors. The receptor-selective retinoid agents, adapalene and tazarotene, were developed in the 1990s. Other innovations of the past decade include retinoid formulations and methods aimed at limiting retinoid absorption. Cutaneous irritation may be reduced without losing retinoid efficacy by inhibiting retinoid penetration into the deep epidermis and dermis. Examples include tretinoin in slow-release vehicles and the short-contact method of tazarotene gel therapy. Only trace amounts of adapalene are absorbed after topical application, perhaps explaining its relatively low irritancy. New formulations of existing agents, such as additional concentrations of tretinoin in microsphere gel and cream formulations of tazarotene, are now under investigation for acne. Current research focused on receptor selectivity holds the promise of yielding new retinoid molecules with improved benefits and safety.

Enantiomer discrimination illustrated by high-resolution crystal structures of the human nuclear receptor hRARgamma

The human retinoic acid receptor (hRAR) is a member of the nuclear receptor superfamily that regulates the transcription of target genes in a ligand-dependent manner. The three hRAR isotypes are targets for retinoids that are used in the treatment of various diseases, including breast cancer and skin diseases. Drug efficiency and safety depend on the pharmacological activity of enantiomers, which can differ because of the chiral environment generated by the target. We report the crystal structures of the hRARgamma ligand-binding domain bound to two enantiomers, the active BMS270394 and the inactive BMS270395, solved at 1.6 A and 1.7 A resolution, respectively. The crystal structures reveal that in both enantiomers, the hydroxyl moiety attached to the chiral center forms a hydrogen bond to the Met-272 sulfur atom, thus imposing a conformation of BMS270395 that differs significantly from that observed for BMS270394 and other known retinoids. BMS270395 adopts an energetically unfavorable conformation, accounting for its inactivity; in contrast, the conformation of BMS270394 is close to an energy minimum. Our high-resolution data allow rationalization of enantiomer discrimination by the receptor and provide a model system for the pharmacological properties of enantiomeric pairs.

Suprabasal expression of epidermal alpha 2 beta 1 and alpha 3 beta 1 integrins in skin treated with topical retinoic acid

In normal adult human skin, expression of epidermal integrins is confined to keratinocytes in the basal layer. However, suprabasal expression of alpha 2, alpha 3 and beta 1 integrin subunits is noted in hyperproliferative epidermis in wound repair and psoriasis. In this study, we examined the effect of topical all-trans-retinoic acid (RA), known to induce epidermal hyperplasia, on expression of integrins in human epidermis. Immunostaining of vehicle-treated skin revealed expression of alpha 2, alpha 3 and beta 1, as well as alpha 6 and beta 4 integrin subunits entirely on basal keratinocytes. Topical application of RA (0.1%) for 2 weeks resulted in marked suprabasal expression of alpha 2, alpha 3 and beta 1 integrin subunits, whereas alpha 6 and beta 4 staining remained on basal keratinocytes. Staining for putative ligands of alpha 2 beta 1 and alpha 3 beta 1 integrins, i.e. type IV collagen, laminin-5 and fibronectin, was not detected in the epidermal layer in RA- or vehicle-treated skin. Treatment of HaCaT keratinocytes in culture with RA (1 mumol/L) enhanced alpha 2 and beta 1 mRNA abundance. Furthermore, RA slightly up-regulated the expression of alpha 2, alpha 3 and beta 1 integrin subunits on primary epidermal keratinocytes and HaCaT cells in culture with no effect on cell proliferation. These results provide evidence that RA-elicited epidermal hyperplasia is associated with aberrant suprabasal expression of alpha 2 beta 1 and alpha 3 beta 1 integrins, and that this also involves direct stimulation of keratinocyte integrin expression by RA.

Lack of involvement of retinoic acid receptor alpha in retinoid-induced skin irritation in hairless mice

It has been proposed that RAR gamma, the major retinoic acid receptor (RAR) subtype in skin, mediates retinoid-induced skin irritation. However, RAR alpha is also found in skin, and its role in retinoid-induced skin irritation has not been tested. In this study, RAR subtype-specific agonists and antagonists were used to test the possible contribution of RAR alpha to retinoid-induced skin irritation. Female hairless mice were treated topically on the dorsal skin for 5 days with various retinoids over a 2-log dose range, and cutaneous toxicity was scored by semiquantitative visual observations of skin flaking and abrasions daily up to 3 days post-treatment. Three RAR alpha-selective agonists were > or = 100-fold less potent as skin irritants than the structurally-related RAR pan-agonist, TTNPB. Skin irritation potency decreased in the following order: TTNPB > > Am580 > AGN 193835 > > 193836 and correlated with RAR beta and/or RAR gamma binding affinity rather than RAR alpha binding affinity. TTNPB-induced skin irritation was blocked in a dose-dependent fashion by co-treatment with the RAR pan-antagonist AGN 193109 but was not blocked by co-treatment with the RAR alpha-specific antagonist AGN 194301. In contrast, skin irritation induced by the RAR alpha-selective agonist AGN 193835 was almost completely blocked by co-treatment with AGN 193644, an RAR beta/gamma-selective antagonist. These data demonstrate that RAR alpha is not significantly involved in mediating retinoid-induced skin irritation in mice and suggest that RAR alpha-selective agonists may have reduced mucocutaneous side effects relative to other retinoids.

Topical all-trans retinoic acid (RA) induces an early, coordinated increase in RA-inducible skin-specific gene/psoriasin and cellular RA-binding protein II mRNA levels which precedes skin erythema

Separation of specific and nonspecific "irritant" effects of topical all-trans retinoic acid (RA) is a key to understanding the mechanism of retinoid action in skin. Cellular RA-binding protein (CRABP) II has been found to be a marker of RA activity in human skin. We have also previously identified a skin-specific gene (RIS-1/psoriasin) which is rapidly induced in human skin treated with RA. Here we compared the kinetics and time-course of RIS-1 and CRABP II gene activation by RA, sodium lauryl sulfate (SLS), a classical irritant, and their vehicle (VH), using a quantitative reverse transcription polymerase chain reaction. RIS-1 and CRABP II were both expressed at very low levels in untreated normal human skin, and in RA-treated skin the kinetics and time course of RIS-1 and CRABP II mRNA induction were similar. Relative to VH-treated skin, RA induced RIS-1 mRNA levels within 6 h, which further increased to 6.4-fold by 24 h (n = 4). Similarly, CRABP II mRNA levels increased from 2.6-fold at 6 h to 7.8-fold after 24 h. At 48 h the relative mRNA levels for both genes decreased towards the steady-state levels. Relative to SLS-treated skin, RIS-1 mRNA increased by 3.2-fold after 6 h and by 5.1-fold after 12 h (n = 3). Also, a 2.6-fold higher CRABP II mRNA observed after 6 h increased to 6-fold after 12 h. After 24 and 48 h RA treatment the relative mRNA levels for both genes decreased towards the steady-state levels. RA-induced skin erythema was not obvious until 24 to 48 h. We conclude, therefore, that induction of RIS-1 and CRABP II mRNA levels by topical RA in human skin are early, coordinated molecular events which precede the clinical cutaneous erythematous response to RA.

Pattern of retinoid-induced teratogenic effects: possible relationship with relative selectivity for nuclear retinoid receptors RAR alpha, RAR beta, and RAR gamma

Retinoic acid, an oxidative metabolite of vitamin A, is involved in the control of many biological processes including embryonic development. Excess as well as deficiency of retinoids were found to be teratogenic. The effects of retinoids in normal as well as abnormal development may be mediated by two members of retinoid receptors, the RAR's and RXR's, which exhibit a specific temporal and spatial expression during development. The significance of the retinoid receptors was investigated here by studying the teratogenic effects of retinoid ligands with relative selectivity for binding and transactivation of the retinoic acid receptors RAR alpha, RAR beta and RAR gamma. Pregnant NMRI mice were administered 5 or 15 mg/kg of CD 336 (Am 580) (alpha-ligand), CD 2019 (beta-ligand), CD 437 (gamma-ligand) or 37.5 mg/kg all-trans-retinoic acid in 25% Cremophor EL on day 8.25 or day 11 of gestation by gastric intubation. External, visceral and skeletal malformations were observed on day 18 of gestation. The order of teratogenic potency was: alpha-ligand > beta-ligand > gamma-ligand. In addition, these retinoids also produced a different spectrum of defects. The alpha-ligand induced the most varied defects including severe ear, mandible, and limb malformations. The beta-ligand induced defects of the urinary system and liver in greater frequency than expected from its relative potency. The gamma-ligand preferentially induced ossification deficiencies and defects of the sternebrae and vertebral body. Our results show that these three retinoids, which were previously demonstrated to exhibit retinoid-like activities in several systems, exert differing teratogenic activities, in regard to both potency and regioselectivity: we hypothesize that the relative selectivity for binding and transactivation of the three retinoic acid receptors could possibly be related to the differences of teratogenic effects observed in this study. The low potency of the gamma-ligand may lead the way to interesting new retinoids with improved therapeutic ratio.