Mark P Brincat
University of Malta, Malta
Mark P. Brincat, Dean, Faculty of Medicine and Surgery, Head, Department of Obstetrics and Gynaecology, University of Malta Medical School, Malta; E-mail: thedean@keyworld.net.
Summary
Oestrogen loss at the menopause has a profound influence on skin. Oestrogen treatment in post-menopausal women has repeatedly been shown to increase collagen content, dermal thickness and elasticity. Data on the effect of oestrogen on skin water content are also promising. Further, physiological studies on oestrogen and wound healing suggest that hormone replacement therapy (HRT) may play a beneficial role in cutaneous injury repair. Results of the effect of HRT on other physiological characteristics of skin, such as elastin content, sebaceous secretions, wrinkling and blood flow, are discordant. Given the responsiveness of skin to oestrogen, the effects of HRT on ageing skin require further examination and careful molecular studies will probably clarify oestrogen’s effects at the cellular level.
Keywords: collagen, HRT, menopause, oestrogen, skin
Introduction
Due to an increased life expectancy, women in the West can now expect to spend more than one-third of their lifetime after menopause,[1,2] leading to increased concern regarding post-menopausal health care. The hypo- oestrogenism that accompanies menopause is known to induce vasomotor symptoms and vaginal atrophy, as well as exacerbate bone loss; studies have documented that post-menopausal hormone therapy (oestrogen only or oestrogen opposed by a progestin) can reduce the symptoms of menopause and prevent early bone loss. However, other organs that are also intimately dependent on oestrogen have not garnered comparable attention with regard to post-menopausal treatment. For example, oestrogen has a profound influence on skin. Despite skin being the largest organ of the body, and the primary barrier against pathogen invasion, dehydration, and elemental damage, the effects of post-menopausal not well documented. Further work on the impact of exogenous hormones on women’s skin is still in its infancy.
The aim of this paper is to review the clinical literature regarding the physiological effect of oestrogen on structural and physical characteristics of skin. In particular, studies that have examined post-menopausal hormone therapy and the role it may play in maintaining skin integrity and preventing age-related deleterious changes will be discussed. Given the importance of skin for general health and the intriguing information available on a role for oestrogen in skin, further examination of mechanistic and clinical effects of oestrogen on skin parameters is warranted.
Skin physiology and the role of oestrogen
The skin is the largest organ of the body. Anatomically, skin is comprised of two main layers: the epidermis forms the thin outer layer and is made primarily of keratinocytes and melanocytes; the dermis is the deeper layer that comprises the main bulk of skin. The dermis is predominantly made up of connective tissue and blood vessels. The fibres present in dermal connective tissue consist of two main types of fibrous proteins, collagen and elastin.
The collagen fibres (mainly types I and III) are produced by fibroblasts, are arranged parallel to the skin surface, and are responsible for the main mass and tensile strength of skin. In contrast, elastin fibres are arranged as a thinly distributed subepidermal network and provide the skin with elasticity and resilience.[3] Dermal connective tissue also contains nerve fibres, sensory receptors, and the supportive glycosaminoglycans (GAGs).[4]
Skin quality deteriorates with age due to the synergistic effects of chronological ageing, photoaging, environmental factors, and hormonal deficiency.[4] Hormonal ageing of skin due to oestrogen loss at menopause is thought to include atrophy; decreased collagen content, water content, and sebaceous secretions; loss of elasticity; and manifestations of hyperandrogenism. Further, the cumulative effect of oestrogen deficiency on skin is thought to contribute to the poor wound healing that accompanies ageing.
An integral role for oestrogen in skin integrity was substantiated with the discovery of oestrogen receptors in dermal fibroblasts and epidermal keratinocytes.[5] Further, a study aimed at identifying specific oestrogen- sensitive structures examined human skin for the ability to bind an antibody against a protein, p29, typically found in oestrogen-responsive cells.[6] Strong and specific binding to the p29 antibody was seen in discrete structures within skin.[6] However, there is a lack of information regarding the effects of oestrogen on these target cells. Expression of both aromatase and 17P-hydroxysteroid dehydrogenase type I - two enzymes involved in the formation of potent oestrogens - has also been demonstrated in skin.[7]
Effects of oestrogen on structural components of skin
Collagen content
In normal human dermis, collagen is a relatively stable molecule, synthesized from procollagen molecules by the action of specific enzymes and degraded by collagenases. Although anabolic steroids have been shown to increase collagen synthesis in human dermal fibroblasts,[8] oestro- gens have no stimulatory effect on procollagen synthesis in cultured human cells.[9] Using a rat model, oestrogen has been shown to inhibit collagen degradation.[10] Given the complex involvement of regulatory factors, receptors, and enzymes in maintaining collagen balance, the exact mechanism for the role of oestrogen on collagen integrity is not known.
A reduction in collagen is traditionally considered the principal factor in the pathogenesis of skin atrophy. Although Castelo-Branco et al.[11] found a closer correlation between collagen loss and chronological age than between collagen loss and time since menopause, others report a stronger correlation between skin collagen loss and oestrogen deficiency due to menopause.[12-14] The finding of Castelo-Branco et al.[11] may be explained by the fact that the individuals in that study were between 40 and 55 years of age, had recently undergone surgical menopause, and had therefore not been oestrogen deficient for a long period.[15] It has been suggested that as much as 30% of skin collagen (both type I and III) is lost in the first 5 years after the menopause.[14,16] Brincat et al.[12,13] found that total collagen declines by an average of 2.1% per post-menopausal year over a period of 15 years, and this decline can be prevented in women receiving oestrogen therapy.[17] Consistent with the presence of type I collagen in bone, the decrease in skin collagen content has been shown to correlate with age- related decreases in bone mineral density.[12,13,18]
Most clinical studies demonstrate a beneficial effect of subcutaneous,[12,13,16-19] topical,[12,13,20,21] or oral[11,22] oestrogen treatment on the collagen content of skin (Table 1). The extent of the reported oestrogen-induced increase has varied, depending upon the dose, route of administration, and duration of hormone treatment. For example, a 3-month study in post-menopausal women found that treatment with topical oestradiol resulted in a 38% increase in total collagen,[21] whereas another study examining both oral and transdermal oestrogen treatment for 12 months found that collagen levels increased significantly, but by only 2-5%, depending on the treatment regimen.[11] In addition, results between studies are often not comparable, due to differences in the methods employed to assess collagen levels.
Table 1. Oestrogen therapy and collagen content.
The increase in collagen with oestrogen is proportionate to baseline collagen levels.[12,13] To date, only one clinical study has failed to show a beneficial effect of oestrogen therapy on collagen levels. Haapasaari et al.[23] observed no change in collagen levels of post-menopausal women with 1 year of ERT or HRT. However, that study examined the effects of oestrogen on very early menopausal women, on average within the first year of menopause, and the authors recognize that given the short time since menopause the amount and synthetic rate of collagen may have been at an optimum level.[24] Based on observations by Brincat et al.,[18] that demonstrate a delay in the collagen decrease after the onset of menopause, HRT would not be expected to have an effect on collagen levels in such early menopausal women. Cumulatively, these studies suggest that oestrogen treatment may be prophylactic for women with high skin collagen levels and both prophylactic and therapeutic for women with low collagen content.[12,13]
Elastin fibres
Accelerated degenerative changes in dermal elastic fibres have been observed in young women with premature menopause,[4] and histological studies demonstrate that topical oestrogen can increase the number and thickness of elastic fibres in skin.[25] However, reports from several recent clinical trials that examined the effects of HRT on elastin fibre content in skin demonstrate no observable improvement from baseline with systemic oestrogen therapy.[22,23,26] Importantly, two of these studies treated a small sample (n < 21) of women for only 6 months,[22,26] and the third involved very early menopausal women.23
One of the most common dermatological conditions in older women is dry skin.[27] Healthy skin requires substantial water content, which is determined by both cutaneous evaporation rate and epidermal hydration. It has been shown that the transepidermal water flux, or evaporation, varies during the menstrual cycle[28] and decreases with age.[29] A sensitive measure of functional changes in water-holding capacity of the skin is the plastic occlusion stress test (POST). One small clinical study on 15 healthy menopausal women used the POST method to show that transdermal oestrogen therapy can lead to a significant increase in the water-holding capacity of the stratum corneum.[30]
Notable alterations in epidermal hydration result in changes in the capacitance of the stratum corneum.
Several small trials that have measured skin capacitance have demonstrated that HRT can improve the water content of skin[20,30-32] when compared with a control group, although some differences did not reach statistical significance.[20,32] Other results demonstrated no effect of oestrogen on skin capacitance.[23,33]
In the First National Health and Nutrition Examination Survey (NHANES I),[34] standardized dermatological assessment of 3875 post-menopausal women showed that oestrogen use was associated with a statistically significant decrease in the likelihood of senile dry skin. Given the limits inherent to epidemiological studies, corroboration of a role for oestrogen in skin water content will best be addressed with large double-blind, randomized, controlled studies.[35]
Positive effects of oestrogen on the water content of skin may be related to oestrogen-stimulated increases in mucopolysaccharides and hyaluronic acid levels in skin,[36-39] which correlate to an increased dermal water content,[37] and to increases in skin thickness, which subsequently elevate natural moisturizing factors.[20] An improvement in water-holding capacity of the skin enhances the barrier function of the epidermis and subsequently results in less frequent development of dermatoses.[30]
Sebaceous secretions
The activity of cutaneous sebaceous glands is regulated by levels of circulating hormones; oestrogen can reduce the size and number of sebaceous glands, as well as the production of sebum, whereas androgens oppose this action, thereby stimulating secretory activity.[4] Indeed, clinical studies have shown that sebaceous secretions decrease with age[40] and oestrogen replacement alone has a sebum-suppressive action,[31] but the addition of a progestin results in significant increases in skin surface lipids.[31,33] In a study by Callens et al.[33-49] post-menopausal women taking oestradiol with progesterone were compared with a control group; a 38% increase in sebum production was observed with hormone treatment.
Effects of oestrogen on physical characteristics of skin
Skin thickness
Skin thickness increases up to the age of 35-49 years, followed by an age-related thinning. During the menopausal years, the decrease in skin thickness accelerates with as much as a 1.13% annual decline for the initial 15-18 post-menopausal years.[12,13] Decreases in collagen, water, and GAG content all contribute to the thinning effect. Consistent with early studies that note a high incidence of thin skin in osteoporotic women,[41-44] a strong correlation between skin thickness, collagen content, and bone mineral density has been observed in post-menopausal women.[12,13,45]
Attempts have been made to slow skin thinning with oestrogen therapy for several decades. The majority of clinical trials have demonstrated that post-menopausal women who take HRT have greater skin thickness than non-users.[11,26,31,33,45-49] An HRT-induced increase in thickness is detected in the dermis[26,46,49] through increases in dermal connective tissue, but not the epidermis.[22,26,46,49]
An early randomized, double-blind, placebo-controlled study examined the use of conjugated equine oestrogen (CEE) in 60 post-menopausal nuns for 1 year and found that CEE use for 1 year was associated with a 30% increase in dermal thickness on skin biopsies when compared with the placebo.[49] A recent crosssectional observational study used diagnostic ultrasound to compare skin thickness between HRT users, non-users, and pre-menopausal women.[50] Chen et al.[50] found that HRT-treated women had 10% greater skin thickness than non-users and achieved thickness levels comparable to pre-menopausal individuals. Interestingly, the benefit with HRT was similar for women taking therapy from 6 months to 6 years, suggesting a therapeutic role for HRT during the initial period of treatment and a prophylactic role thereafter. The absolute changes in thickness measured in these studies are notably small (in the sub-mm range) and may not necessarily be clinically relevant. However, these studies suggest that hormonal status influences skin thickness and that skin atrophy, specifically dermal atrophy, can be ameliorated with postmenopausal hormone therapy.
Two studies examining early menopausal women[23,51] found no significant change in skin thickness with oestrogen treatment. Similar to oestrogen’s effects on collagen levels, observable benefits of oestrogen on skin thickness is inversely proportional to initial thickness values, and therefore it is not surprising that treatment of early menopausal women did not result in significant changes.
Elasticity and distensibility
The mechanical properties of skin can be defined and quantified by extensibility and elasticity measurements using computerized devices. Ageing in skin, in particular in the face, is associated with a progressive increase in extensibility and a reduction in skin elasticity. The influence of climacteric ageing on rheological qualities of skin was quantified using a non-invasive computerized suction device in a recent study by Pierard-Franchimont and colleagues.[51] These authors monitored changes in the tensile properties of skin on the upper part of the cheeks of 140 early menopausal women over 5 years. They found that in the absence of HRT, distensibility in the facial skin increased by 1.1% per year and elasticity decreased by 1.5% per year, in agreement with other reports.[52,53] The women enrolled in the study who were given HRT for 5 years experienced little change in skin extensibility and elasticity, in support of earlier findings that HRT can mitigate age-related changes in tensile properties.[52,53] Recently, a small ad hoc study involving women aged 45-68 years (mean age, 54.9 years) followed the effect of HRT on rheological measures for 6 months.[31] Significant increases in elasticity were observed for both transdermal and oral oestrogen preparations. Through its ability to limit age-related increases in cutaneous extensibility and to improve elasticity, HRT has been shown to exert a preventative effect on skin slackness commonly associated with ageing.[53]
Wrinkles
The normal age-associated loss of connective tissue in skin results in increased distensibility, and loss of tonicity is accompanied by a progressive deepening of facial creases and wrinkling. The documented association between HRT and increases in collagen content and elasticity suggest HRT would be expected to decrease wrinkling. However, due to technical challenges in quantifying a visual endpoint such as facial wrinkles, few clinical studies have specifically examined HRT and facial wrinkling.
Several observational studies have reported that post-menopausal women treated with oestrogen are less wrinkled than untreated women,[34,54] although in one study, improvement with HRT was only observed in non-smokers.[54] In NHANES I, a cross-sectional analysis of a national probability cohort, the effects of oestrogen use on wrinkling were ascertained in close to 4000 post-menopausal women aged 40 years and older at baseline.[34] Among all women in the NHANES I cohort, after adjustment for age, body mass index and sunlight exposure, the odds of wrinkling were substantially lower in oestrogen users (odds ratio, 0.68, 95% CI 0.52-0.89) Results from clinical trials are inconclusive. Optical profilometry and computerized image analysis have been used to demonstrate that topical HRT on the face can improve fine wrinkling[47] and significantly decrease wrinkle depth[20] within 2 months of treatment. However, others have used similar techniques and failed to demonstrate changes in facial wrinkling with HRT.[33,52]
Blood flow
Healthy skin requires integrity in both the structure and function of capillary blood vessels, and cutaneous circulation is important in humans in maintaining core temperature homeostasis. The effect of oestrogen on the cutaneous circulation of women has not been well studied. Consistent with the formation of pre-menstruation oedema in women, cutaneous blood flow has been shown to vary over the course of the menstrual cycle.[28] In addition, peripheral microcirculation at the level of the nail-fold capillaries has been shown to decrease significantly with the menopause.[55]
Oestrogens are known to substantially improve both endothelium-dependent and -independent vascular reactivity in the cutaneous microcirculation of postmenopausal women.[56,57] Studies measuring the effect of HRT on blood flow rates, however, are somewhat inconsistent. Although some have demonstrated that 6-12 months of HRT use increases capillary blood flow in the nail-fold by as much as 20% to 30%,[55] others have reported that long-term oestrogen therapy (> 2 years) does not increase cutaneous vascular flow when compared with untreated women.[58]
Effect of oestrogen on wound healing
The effects of intrinsic ageing on cutaneous wound healing are profound. Age-related skin changes include increased susceptibility to trauma, resulting in fragile skin that tears and bruises easily. In addition, chronic wounds commonly suffered by the elderly - venous ulcers and pressure sores - inflict significant suffering and cost, and they impose a burden for physicians and patients alike.[59] The role of oestrogen and/or progesterone in age-related decreases in wound healing is poorly understood. Historically, the vast majority of research in wound healing has used animal models and has produced inconclusive results. More recently, limited data from molecular and human studies appear promising.
Preclinical studies on oestrogen and wound healing
The initial phases of cutaneous wound healing involve vascularization, granulation, collagen deposition, and re-epithelialization.[60] An early study by Lindhe and colleagues[61] found oestrogen injections did not affect the vascularization of wound areas in oophorectomized rabbits. These data are in agreement with studies in various other animal models that show that oestradiol alone has no effect on vascularization;[62,63] by contrast a large study in oophorectomized rats that used similar microangiographic techniques observed a significant suppression in vascularization in oestrogen-treated animals.[64] Examination of wound vascularization after treatment with oestrogen in combination with a progesterone has also produced inconclusive results. Whereas two studies reported that oestradiol given in conjunction with progesterone decreases vascular exudation during the inflammatory phase of wound healing,[62,63] others have found no effect.[64,65] Recent molecular research on the influence of oestrogen on the cells and vasculature involved in the inflammatory phase of wound repair suggests that oestrogen may reduce the cellular activation of blood platelets and may affect phagocytic activity of neutrophils.[60] A direct association with post-menopausal oestrogen treatment and the inflammatory phase of wound healing remains to be investigated.
The majority of studies on oestrogen’s effect on wound collagen deposition and strength have also utilized animal models. Oestrogen treatment has been shown to increase collagen deposition in wounds of oophorect- omized rats[66-68] and rabbits;[66] however, others have found that systemic oestrogen exerts either no effect[63,69,70] or decreases collagen deposition,[69,70] depending on dose and time since wounding. Similarly, oestrogen given in conjunction with a progesterone has been shown to either have no effect[70] or to decrease collagen in wound granulomas;[63,70] the effect again can depend on the time since wounding.[70] Consistent with the findings for collagen deposition, measures of wound tensile strength in animals have produced varied results.[66,70,71]
There is very limited information from animal studies on how any effects of hormone treatment translate into wound healing rates. One early study in young and senescent rats found that oestrogen injected at low doses (0.33-1 μg/kg) improved wound healing time, but higher doses (3.3-10 μg/kg) prolonged healing time.[72] Further, a study in oophorectomized rats showed that prolonged loss of ovarian hormones is associated with slow wound contraction,[73] but these authors did not examine the effect of systemic oestrogen treatment. In general, investigations with animals demonstrate contradictory findings as to the effect of oestrogen levels on the stages of wound healing. These discrepancies are probably due to differences in species, duration of treatment, and methodologies employed.[60]
Clinical and molecular studies on oestrogen and wound healing
Some of the more recent work on wound healing has examined the molecular role of oestrogen on the cells and metabolic processes involved in wound repair. For example, age-related delays in wound healing have been partially attributed to low levels of transforming growth factor-beta 1 (TGF-β1), decreased collagen synthesis, and increased presence of proteases, specifically elastase. The presence of the oestrogen receptor on the major cell type involved in wound repair (i.e. fibroblasts) suggests that oestrogen may directly modulate the function of these cells. Indeed, a study using wound biopsy specimens from healthy females suggested that oestrogen exerts a positive influence on healing, not by increasing fibroblast proliferation within a wound, but rather by inducing TGF- β1 secretion by dermal fibroblasts.[74] In addition, recent clinical studies by Ashcroft and colleagues have demonstrated that oestrogen has a beneficial effect on wound healing by increasing collagen content[59,74] and reducing collagenolysis.[59] In one small study, 10 post-menopausal women taking HRT were compared with age-matched controls.[74] The women underwent two 4-mm biopsies from the upper inner arm; healing was monitored. Hormone replacement therapy use was associated with significantly accelerated wound healing. In addition, a randomized double-blind study of elderly males and females demonstrated that topical oestrogen reduces activity of the protease elastase in cutaneous wounds when compared with placebo.[59] Decreased elastase would stimulate matrix deposition and allow for faster wound healing. Indeed, that same study observed that topical oestrogen applied to normal elderly skin prior to wounding and 24 h post-wounding significantly accelerated wound healing in both males and females.[59]
Data on wound healing in humans are very limited but promising, and further work is necessary before valid conclusions can be drawn regarding the use of HRT and wound-healing properties of skin. Data have demonstrated that oestrogens may prevent the age-related decline in skin collagen, may increase the water-holding capacity, enhance the epidermal barrier function and decrease alterations in elasticity of the skin. Cumulatively, these results suggest that older women who use HRT may have healthier skin than non-users and may be at lower risk for the development of dermatoses,[30] such as ecchymosis.
Discussion
The technology now exists to further explore the molecular basis of oestrogen’s role in maintaining skin integrity. Using gene-chip microarray techniques, oestrogen-responsive genes in skin can be identified, characterized, and monitored under varying hormonal conditions. Determining the oestrogen sensitivity of enzymes responsible for the production and/or degradation of structural components of the skin, such as GAGs or collagen and elastin fibres, will substantially enhance our understanding of the mechanistic role of oestrogen in skin.
When biochemical and histological parameters for oestrogen-sensitive structures in skin have been established at the molecular level, a correlation between these markers, oestrogen status and physical attributes of skin need to be carefully investigated and quantified in large, randomized placebo-controlled trials. In addition, larger, controlled clinical trials of HRT use and visible measures of skin, such as the assessment of wrinkles, are necessary. Large trials involving HRT and skin should take into consideration patients’ evaluation of changes related to their skin appearance, and they should factor patient satisfaction into any perceived benefit of treatment.
Preclinical and clinical data on the effect of oestrogen on skin water content are also promising. Physiological studies on oestrogen and wound healing suggest that HRT may play a beneficial role in cutaneous injury repair; however, molecular studies have yet to articulate the mechanisms. Differences on other structural and physiological characteristics of skin with HRT are limited, and results are somewhat discordant due to differences in protocol, hormone regimen, and assessment measures.
Careful examination of the molecular effects of oestrogen on skin parameters is necessary. In addition, research is needed to identify any correlation between quantitative measures of skin characteristics and observed manifestations in skin appearance. Further large-scale clinical trials are also required in order for physicians to make informed recommendations regarding post-menopausal oestrogen use and its potential application to skin care. Indeed, whereas the less visible benefits of RT have been stressed by health professionals, potential effects of oestrogen on the skin have not been emphasized and may prove meaningful to the target population of post-menopausal women.
Acknowledgements
Oestrogen loss profoundly affects the skin. Oestrogen treatment of post-menopausal woman increases collagen content, dermal thickness and elasticity.
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