Skin aging is related to intrinsic or chronological and extrinsic or environmental factors. Oxidative stress, with generation of Reactive Oxygen Species (ROS), occurs during oxidative cell metabolism, mostly on mitochondria, and it is aggravated by chronic exposition to Ultra-Violet (UV)B (short wavelength) and UVA (long wavelength) sun radiations. The consequences are transcription factors activation, lipid peroxidation, metalloproteinases (MMPs) expression, and DNA damage with strand breaks [1-6]. Visible light (especially blue light) also induces release of ROS, MMP-1, and interleukin (IL)-1 and deploys skin carotenoids. Infrared light produces heat and increases MMP-1 production [7]. Other factors have been considered as external aggressors, such as tobacco, pollution, and hot climate which cause activation of transient receptor potential channel, subfamily V, member 1 (TrpV1) [8-10]. Telomer shortening, gene loss, decreased DNA methylation, phosphorylation reaction, and decreased DNA repair are additional endogenous age-related processes [11]. Hormonal changes, such as estrogen decline after menopause, and testosterone decrease in both male and female, are also involved [12-15]. Aging may lead to dysregulation between oncogenes and tumor suppressor genes and development of cancer [16-18].

Intrinsic skin aging is characterized by generalized presence of fine wrinkles at rest, laxity, xerosis and benign neoplasms, such as seborrheic keratosis. The epidermis is thinning and the dermal-epidermal junction (DEJ) is flattened. There is general atrophy of the dermal extracellular matrix and reduced collagen production. On the other hand, extrinsic skin aging, also named photoaging, occurs in chronically sun-exposed areas, presenting coarse wrinkling at rest, roughness, hyperpigmentation, more severe laxity, superficial vascular abnormalities, pre-malignant actinic keratosis, and skin cancer. These signs are more evident and severe in photo-exposed areas of Caucasian population. The epidermis in these areas shows reduced or increased thickness, stratum corneum compaction and increased thickness of the granular layer. The DEJ is atrophic and there is an increased number of melanocytes. In the dermis, the mature collagen fibrils are degenerated and replaced by disorganized and fragmented collagen I and IV, characterizing the collagen basophilic degeneration; elastin is increased and occupies the areas previously inhabited by collagen fibers (solar elastosis). Endogenous antioxidant depletion, lipid peroxidation, and decomposition of sebaceous lipids are also processes related to skin aging [2,19,20]. Therefore, photoaging represents a superposition of intrinsic and extrinsic factors and is responsible for 85% of aged skin phenotype in exposed areas [21]. Additionally, forehead, eyes, and lip areas are affected by wrinkles related to muscular activity [22,23]. Altered skin barrier function, including reduced synthesis of stratum corneum lipids, with decrease of ceramides, is associated to skin aging [24-26]. In addition, compared to young epidermis, reduction in IL-1 receptor antagonist protein and deficiency in IL-1α receptor type 1 had been observed in aging skin, contributing to the delay of barrier recovery [27]. Furthermore, the lipid peroxidation of polyunsaturated fatty acids due to oxidative stress causes chromatin damage, impacting in the regulation of gene expression involved in inflammasomes, sebum production, cell survival and longevity [28,29].

The knowledge of skin aging mechanisms may be useful to understand the whole-body senescence and age-related diseases. The genomic aspects of skin aging are poorly investigated, despite the fact that skin is a useful organ to identify epigenomic changes and assess aging profile by transcriptomic analysis. The limitation of ex vivo studies like ours is the presence of multiple cell lines in the skin fragment obtained by biopsy. The transcriptomic analysis is not so precise as when an isolated cell line is used for in vitro analysis, particularly in skin cancer. The comparison of results between the two methodologies needs criticism. The majority of in vitro studies about the dysregulation of UV target genes and DNA damage aim to identify the genetic mechanisms of skin cancer. The most important study about UV-related genetic signature have used human keratinocytes from 4 donors which were exposed to UV radiation, between 290 and 340 nm, 3 days, 1 day and 4 hours prior to the collection of cells, corresponding to an acute exposition to UVB wavelength. Authors have observed a time-dependent transcriptomic changes in the Differentially Expressed Genes (DEGs) compared to controls. However, the results could not be compared with ours as we have analyzed skin chronically exposed to full solar spectrum in real life. In addition, they also have compared the transcriptome of squamous cell carcinoma (SCC) tissue and UV exposed adjacent normal skin. By using gene set enrichment analysis (GSEA) they have identified a significant enrichment between the two genomic signatures and proposed an UV biomarker panel. Considering that SCC develops after prolonged sun exposure we were able to identify similar genomic changes related to some pathways, such as: DNA replication and repair, cell cycle regulation, apoptosis and inflammation [30]. Another ex vivo study have used skin cells from healthy volunteers and rats to investigate the transcriptional profile after UVB irradiation related to inflammatory pain and detected many up-regulated genes for cytokines and chemokines [31]. There are some in vitro studies about the transcriptional responses caused by UV and ionizing radiation that have used variable cell lines, such as: lymphoblastoid [32], monocyte-derived dendritic cells [33], melanocytes [34], etc.

Our manuscript described a cross-sectional study about the skin structure and gene expression, by using skin punch biopsies, comparing exposed (face, with or without wrinkles at rest) and unexposed (gluteal) areas, in 15 menopausal women, aged from 55 to 65 years (mean 61), phototype III, according to Fitzpatrick classification [35]. The participants should present moderate periorbital dynamic wrinkles and signs of facial photoaging. We were able to confirm the described clinical and histological aspects of intrinsic and extrinsic aged skin, such as, thin, atrophic and rectified epidermis with fine and deep ridges, dermal elastosis and low density of collagen I and IV in upper dermis [19,20]. Gene expression differences were identified by transcriptome and RNA Seq methodology, followed by principal component analysis (PCA). The main genomic findings in exposed areas were: down-regulation of genes related to DNA replication and repair, cell cycle regulation and apoptosis. In our opinion, a relevant observation was the modulation of pathways related to lipid and aminoacids metabolism. The increased expression of lipid-related genes could be explained by the inflammatory process, as already reported [29]. The cellular damage in the epidermis and the generation of ROS oxidizes lipids, leading to inflammation by activation of the macrophage’s migration. These cells, in turn, release cytokines and ROS, amplifying inflammation and the dermal matrix degradation [36]. Therefore, some genes may have increased expression to compensate lipids damaged [37]. The referred study allowed to postulate that photoaging in menopausal women seemed to be particularly related to inflammatory and keratinization pathways and increased lipid and amino acids metabolism as a defense response to restore cutaneous barrier. The UV-decreased synthesis of free fatty acids and triglycerides in the epidermis which contributes to skin photoaging had already been reported [38]. A Chinese study have analyzed the transcriptome of sun-exposed pre-auricular and sun-protected post-auricular skin from 21 healthy females, aged from 34 to 55 yo. Up-regulation of TGF-β signaling pathways and cell cycle/related process (DNA replication) in sun-exposed skin was observed. Nevertheless, in contrast to our findings authors have identified down-regulation of lipid and amino acid metabolism, among various metabolic processes [39].

The limitation of ex vivo studies is the difficulty to separate epidermis and dermis to provide conclusive insights into intrinsic and extrinsic aging related gene expression in different skin layers and cells. It is extremely difficult to compare results from ex vivo genomic studies as multiple parameters may lead to differences in diverse populations [40]. Despite that, the findings of our study may provide additional information about the role of sun exposure on skin aging and can shed light into the development of future new targets and strategies for skin care and health, promoting aging prevention and control, as well as, lifestyle changes.

In contrast to relevant results from molecular and genomic studies, it is remarkable that the majority of clinical trials about the use of cosmetics and cosmeceuticals for skin aging control presents poor methodologic quality, including findings measured by subjective parameters [41-43]. The cosmeceuticals are a category of products not well defined and considered between drug and cosmetic. In other words, they are cosmetics with pharmaceutical bioactive ingredients. They represent a great interest and a huge market for cosmetic industry [44]. A review about a new and growing tendency in the cosmetic ingredient’s world, which is the use of seaweeds, have searched in the scientific literature for evidence about safety and efficacy. Despite the huge number of products in the market, they found only 8 placebo-controlled studies. They also observed small study populations and efficacy evaluation based in non-invasive instrumental measures for hydration, elasticity and thickness, which are technical-dependent and subjective. Results were expressed as reduction in skin spots appearance, improvement in wrinkles and brightness, without statistical analysis [45].

A recent commentary concluded that the industries use non-scientific concepts and in vitro findings, but take advantages of the cosmetic regulatory requirements that are distinct in different countries. On the other hand, their claims point out some of the drug’s effects (hair disorders, wrinkles, acne, aging, pigmentation, etc) with no or limited evidence base [46]. Cosmetics cannot penetrate the stratum corneum and their action is essentially the hydration improvement with benefits in epidermal barrier [46,47]. Nevertheless, an intriguing aspect is the great progress in formulations in the last years, such as the use of nanotechnology and new delivering systems. The objective is to enhance skin penetration and increase the activity of these compounds [48-53]. However, this is a very controversial issue since, by increasing the penetration in order to reach the deep epidermis and dermis, the final product will no longer be a cosmetic or even a dermocosmetic, that is, it will be considered a drug like tretinoin for aging control. Therefore, it is necessary to establish regulations and guidelines for nanotechnology based-products, and development of well-designed clinical trials to evaluate efficacy, safety and toxicity [54]. Observing the progress of the cosmetic market and the complexity of randomized and controlled clinical trials, it is evident that there is little interest on the part of companies in this direction. This is a dilemma and a great challenge. In our opinion, a new class of topical drugs should be defined with specific regulation, including the need of evaluation through randomized and controlled trials in respect to efficacy and safety as any other drug.   

Currently, skin care depends on adequate cleansing and hydration, aiming to maintain the integrity of cutaneous barrier. Skin moisturization can be achieved by a variety of cosmetic ingredients [41,44,53,54]. Ceramides are widely used and are also very useful for dermatosis presenting keratinization defects [25]. As aged skin exhibits reduced levels of hyaluronic acid, this compound, like the ceramides, associated or not to supposed bioactive ingredients, can target the age-related lipid deficiency and stratum corneum dysfunction [26,27]. Topical hyaluronic acid stimulates keratinocyte differentiation and lipid production, leading to improvement of epidermal barrier function in both young and aged skin [55]. Hyaluronic acid is one of the most efficient and safe ingredients used in moisturizes and anti-aging dermocosmetics [54,55]. The upregulation or administration of IL-1α and aquaporin 3 could also improve epidermal barrier function [27,55].

Photoprotection measures, mainly represented by daily and continuous use of broad-spectrum sunscreens, are the key approach for photoaging prevention. Beyond aesthetic benefits, in exposed areas, it is possible to prevent the development of pre-neoplastic lesions and non-melanoma skin cancer [56,57]. The gold standard treatment for photoaging still is topical all-trans retinoic acid or tretinoin, with high level of evidence based in several in vitro and in vivo studies [58-61]. Adapalene 0.3% gel is also effective on photoaging treatment [62]. Both are regulated and approved drugs from retinoid class, and their effectiveness is associated to the presence of specific nuclear receptors in skin cells. By this mechanism of action, they are capable to regulate gene expression and interfere in a variety of cellular processes.

A possible control of extrinsic skin aging, through dermocosmetics, depends on: development of novel testing methods; prevention by sunscreens that protect from UV, but also from visible light and infra-red related damage; protection and reversion of skin damage induced by other environmental factors; and boosting cell metabolism and cell renewal to restore skin mechanical properties and improve the appearance [42].

Nowadays, there is an increased demand for naturally-derived ingredients, possibly influenced by internet and social-media. Botanicals (polyphenols, flavonoids) and niacinamide are largely used in dermocosmetic formulations due to their antioxidant and anti-inflammatory properties. They really are potent antioxidants for the plants. In vitro studies, with good methodologic quality, demonstrated their effects, modulating skin biological functions, such as: prevention of UV-induced damage, DNA protection, activation or inactivation of enzymes, reduction of inflammation, inhibition of MMPs production and increase of collagen synthesis [63-69]. A variety of nonprescription dermocosmetics, based in botanicals, have been continuously introduced to the market. The analysis of 103 antiaging cosmetic products in 2018, detected 96 botanical species [47]. The efficacy of one final product depends on its whole formulation, including ingredients’ concentration and release, stability, interaction with other substances, and presence of penetration enhancers or skin delivery systems. Nevertheless, despite the widespread usage of botanical ingredients in antiaging cosmetics, few preparations have been well studied to prove their real effectiveness. Additionally, studies with adequate methodology may show disappointing results. As an example, a prospective, randomized and controlled trial compared a combination of hyaluronic acid serum and antioxidant cream to placebo for treatment of neck aging, by using subjective outcomes. The results showed that both active and placebo cream and serum improved the wrinkles, laxity, pigmentation, erythema, dryness, and texture of the skin. The products were well-tolerated and a high patient satisfaction score was obtained. Probably, the positive benefits were associated to increase in skin hydration and not to a real antioxidant and anti-inflammatory effect [54].

Another recent approach is the incorporation of macroalgae extracts in cosmeceuticals. Despite potential bioactivity showed by in vitro studies future clinical research is necessary to determine the optimal concentration, ideal formulation, long-term safety and efficacy [43].

Considering the lack of sufficient evidence of efficacy, despite considered safe, cosmeceuticals are still considered adjuvants to medical treatment, acting as effective moisturizers.

It is also an attractive strategy for aging control, mainly skin photoaging, the ingestion of food-derived functional components, acting as antioxidants [70]. However, the redox balance and signaling are mandatory for proper cellular functioning; so, the maintenance of physiological level of endogenous oxidants, as well as, enzymatic and non-enzymatic antioxidants mechanisms is essential for health. If that balance between free radicals and antioxidants favors the former, the oxidative stress will elicit pathological processes [71]. Taking into account the possible benefit of the so-called nutraceuticals or dietary supplements, also using botanicals, is offered in a huge number of products in the market. As discussed for the dermocosmetics, it is important to highlight that there is no evidence-based and approved oral therapy for prevention and control of skin aging. In general, the efficacy is not investigated by randomized and controlled clinical trials with adequate methodology [72-74]. In addition to oral antioxidants, low-dose oral isotretinoin had been proposed for the treatment of photoaging. The comparison of its use to topical 0.05% tretinoin, during 6 months, showed the same efficacy [60]. Considering the teratogenicity and possible side effects in adult population, there is no sense to indicate that drug just for aesthetic purposes. Its prescription should be considered for the control of field of cancerization, particularly in immunosuppressed patients [61].

In conclusion, molecular and genomic in vitro and ex vivo studies are relevant to better understand the pathways involved in skin aging, demonstrating new targets for therapeutic interventions. Meanwhile, the findings of these studies are not accompanied by the production of new effective final products for clinical use.