**5. Photoaging and skin cells**

#### **5.1. Photoaging**

keratinocytes. The mechanism of transfer has long been a mystery, however a recent study summarizes the transfer mechanism in three steps: (i) the melanocyte's dendrite attaches to the surface of the keratinocyte, (ii) a portion of the melanocyte's dendrite sheds in order to deposit the melanosomes on the surface of the keratinocyte, (ii) and finally, the keratinocyte

Since a number of naturally derived bioactive molecules have been found to reduce skin pigmentation, the cosmetic industry has focused on the development of these compounds for use in their products. These compounds inhibit the production of melanin at different stages of melanogenesis. There are several compounds that specifically inhibit tyrosinase. For example, terrein, a bioactive fungal metabolite agent isolated from the *Penicilllium* spe‐ cies, has been shown to strongly decrease the production of tyrosinase by down-regulating MITF through dual pathways[22]. One pathway of these pathways may involve the activa‐ tion of extracellular signal-regulated kinase (ERK) which has been reported to reduce mela‐ nin synthesis[23]. A second pathway recently identified may involve the ubiquitination and

Flavonoids, in particular, have received a great deal attention for their potential pigment-re‐ ducing activity because they are seen as "natural" and are associated with less side-effects than conventional medications and synthetically-designed compounds. Flavonoids are poly‐ phenolic compounds typically found in plants. These compounds are ideal candidates for cosmetic purposes because of their potent bio-activities and low toxicity. Aloesin, a com‐ pound isolated from aloe extracts, has been shown to successfully reduce tyrosinase *in vitro* by inhibiting the hydroxylase activities[25], [26]. Resveratrol, a compound found in red wine, appears to have an affinity for tyrosinase and, by an unknown mechanism, reduces tyrosinase activity and MITF expression[27] although some studies have questioned its abili‐ ty to reduce tyrosinase expression when used alone[28]. Flavonoids isolated from licorice are able to inhibit mushroom tyrosinase[29] and a skin lightening agent that uses licorice ex‐ tract in its formulation (comprised of many bioactive ingredients) has been shown to reduce pigmentation in patients[30]. There are many reports of flavonoids from various sources in‐ terfering with or reducing the activity of tyrosinase, and although they all show some effica‐ cy at reducing the biochemical pathways that lead to pigmentation, it is still unclear whether targeting these pathways in human skin would necessarily lead to reduced melanin forma‐

Melanin formation is a complex process and there are compounds that may inhibit melano‐ genesis without necessarily inhibiting tyrosinase function. Anti-oxidants such as fermented rice bran and vanillic acid can reduce expression of MITF, MC1R, and associated biochemi‐ cal pathways, without affecting tyrosinase activity,[31], [32] possibly due to their ability to impair peroxidase activity. Anti-oxidants, in general, may potentiate the MC1R pathway which itself is able to reduce downstream levels of hydrogen peroxide and promote nucleo‐

engulfs the melanosome package[21].

subsequent breakdown of tyrosinase itself[24].

tion *in vivo*.

**4. Natural ingredients that reduce pigmentation**

262 Using Old Solutions to New Problems - Natural Drug Discovery in the 21st Century

As skin ages, a number of changes occur in the structure of the connective tissues. Healthy human dermis consists primarily of an extracellular matrix and a fibrous component formed by collagen and elastic fibers. The complex network of collagen accounts for approximately 80 percent of the dry weight of the skin and it is the component that provides the resistance to deformation. Elastic fibers comprise 2 to 4 percent of the dry weight of the skin and pro‐ vide elasticity. Glycosaminoglycans (complex sugars, including hyaluronic acid) and water provide the viscous component of the deformation itself. As skin undergoes normal aging, a general reduction in the production of collagen fibers and proteins that make up the extrac‐ ellular matrix contributes the formation of wrinkles and flaccid skin. Similarly an increased rate of degeneration of elastic fibers leads to the loss of elasticity.

Many of the changes that occur in skin structure as a result of aging are expressed prema‐ turely in photodamaged skin, a phenomenon known as photoaging. Photoaging refers to the various cellular processes associated with senescence that result from photodamage. These processes are characterized by two events referred to as intrinsic (chronoaging) and extrinsic (photoaging) ageing. Leveque et al. first described the effects of chronic sun exposure on the mechanical properties of the skin and showed that long term sun exposure caused a de‐ crease in skin extensibility and elastic recovery[35]. The skin also appeared thicker, more rigid and less susceptible to deformation. Berardesca and Maibach confirmed these findings using controlled ultraviolet (UVA) treatment and showed that even short term, low UVA and UVB exposure can impair skin elasticity and that inflammation and edema are charac‐ teristic of this process[36]. Other clinical features of photoaging include wrinkles, dyspig‐ mentation, yellow hue, laxity, vascular ectasia, and malignancies. Adult populations in European and North American countries experience a prevalence of photoaging as high as 80 to 90 percent.

#### **6. Photodamage and impact on skin cells**

#### **6.1. Photodamage**

Photodamage refers to the damage of human skin by ultraviolet light, specifically UVA and UVB, and although the detailed mechanisms of photodamage are not fully understood, some general features have been described and are used as a standard by which topical treatments are judged. Physiological consequences of UVA radiation include thickening of the stratum corneum, epidermal hyperplasia, depletion of Langerhans cells, and dermal in‐ flammation. Although it may seem as if these changes are random and entirely dependent upon UV exposure, photodamage is, in fact, a tightly controlled and regulated process that is facilitated by specialized skin cells. The ways in which these cells interact with each other and their extracellular matrix ultimately determines the morphological and histological con‐ sequences of light exposure in the skin. The cells and proteins involved in this process are numerous and the connections between them are complex. However, exposure of the skin to UVR initiates oxidative stress and triggers the synthesis of matrix metalloproteases (MMP) [37]. The production of MMP results in the degradation of dermal collagen and other matrix molecules thus reducing the elasticity and integrity of the skin[38].

#### **6.2. Keratinocytes**

Since keratinocytes in the upper layers of the epidermis are dead and do not underdo mito‐ sis or metabolisis, they are relatively resistant to UVR induced damage. Younger, keratino‐ cytes located deeper in the epidermis are much more susceptible to UVR damage and repeated UVR exposure results in accumulated DNA mutations that can lead to epidermal malignancies. Keratinocytes play a central role in elaborating innate responses that lead to inflammation and influence the generation of adaptive immune responses in skin.

Apart from the minor cellular constituents of the epidermis, specifically Langerhans cells and melanocytes, keratinocytes are the major source of cytokines. UV radiation exposure stimulates keratinocytes to secrete abundant pro-inflammatory IL-1-family proteins, IL-1α, IL-1β, IL-18, and IL-33. Normal skin contains only low levels of inactive precursor forms of IL-1β and IL-18, which require caspase 1-mediated proteolysis for their maturation and se‐ cretion. However, caspase-1 activation is not constitutive, but dependents on the UV-in‐ duced formation of an active inflammasome complex. IL-1 family cytokines can induce a secondary cascade of mediators and cytokines from keratinocytes and other cells resulting in wide range of innate processes including infiltration of inflammatory leukocytes, induc‐ tion of immunosuppression, DNA repair or apoptosis. Thus, the ability of keratinocytes to produce a wide repertoire of proinflammatory cytokines can influence the immune response locally as well as systematically, and alter the host response to photodamaged cells.

#### **6.3. Fibroblasts and langerhans cells**

The most common type of cell found in the connective tissue of skin is fibroblast. They assist in producing and organizing the extracellular matrix of the dermis and also communicate with each other and other cells in the regulation of skin physiology. Dermal fibroblasts are a heterogeneous population and are identified by their location within the dermis. Fibroblasts that exist in the papillary dermis are generally in a more active state and divide at faster rates than those that exist in the reticular layer of the dermis[39]. Fibroblasts produce and secrete precursors of the extracellular matrix such as collagen, glycoproteins, and other mol‐ ecules that support the structure of the skin.

Studies have shown that when exposed to UVA, fibroblasts proliferative activity is slowed or stopped due to morphological and functional changes. When exposed to sublethal UVA radiation does *in vitro*, researchers observed mutations in the mitochondrial DNA (mtDNA) in fibroblasts[40]. Subsequent studies have shown that the initial mutations in the mtDNA induced by UVR render the mitochondria in the fibroblasts more susceptible to ROS dam‐ age due to an increase in common deletions and a persistence of the mutations event after the termination of UVR exposure[41].

Langerhans cells are a subset of dendritic cells and are found in the epidermis of the skin. Langerhans cells play a role in the initiation and regulation of immune response by acquir‐ ing antigens in the skin and migrating to lymph nodes where immune responses are initiat‐ ed. When human skin is exposed to UVR it results in a depletion of Langerhans cells and reduces the expression of immunophenotypic markers associated with antigen presenta‐ tion[42]-[44]. The lack of functional Langerhans cells ultimately results in an immunosup‐ pressive state in the skin[45] which also involves regulatory T cells and cytokine production[46].

#### **6.4. Mast cells**

some general features have been described and are used as a standard by which topical treatments are judged. Physiological consequences of UVA radiation include thickening of the stratum corneum, epidermal hyperplasia, depletion of Langerhans cells, and dermal in‐ flammation. Although it may seem as if these changes are random and entirely dependent upon UV exposure, photodamage is, in fact, a tightly controlled and regulated process that is facilitated by specialized skin cells. The ways in which these cells interact with each other and their extracellular matrix ultimately determines the morphological and histological con‐ sequences of light exposure in the skin. The cells and proteins involved in this process are numerous and the connections between them are complex. However, exposure of the skin to UVR initiates oxidative stress and triggers the synthesis of matrix metalloproteases (MMP) [37]. The production of MMP results in the degradation of dermal collagen and other matrix

Since keratinocytes in the upper layers of the epidermis are dead and do not underdo mito‐ sis or metabolisis, they are relatively resistant to UVR induced damage. Younger, keratino‐ cytes located deeper in the epidermis are much more susceptible to UVR damage and repeated UVR exposure results in accumulated DNA mutations that can lead to epidermal malignancies. Keratinocytes play a central role in elaborating innate responses that lead to

Apart from the minor cellular constituents of the epidermis, specifically Langerhans cells and melanocytes, keratinocytes are the major source of cytokines. UV radiation exposure stimulates keratinocytes to secrete abundant pro-inflammatory IL-1-family proteins, IL-1α, IL-1β, IL-18, and IL-33. Normal skin contains only low levels of inactive precursor forms of IL-1β and IL-18, which require caspase 1-mediated proteolysis for their maturation and se‐ cretion. However, caspase-1 activation is not constitutive, but dependents on the UV-in‐ duced formation of an active inflammasome complex. IL-1 family cytokines can induce a secondary cascade of mediators and cytokines from keratinocytes and other cells resulting in wide range of innate processes including infiltration of inflammatory leukocytes, induc‐ tion of immunosuppression, DNA repair or apoptosis. Thus, the ability of keratinocytes to produce a wide repertoire of proinflammatory cytokines can influence the immune response

inflammation and influence the generation of adaptive immune responses in skin.

locally as well as systematically, and alter the host response to photodamaged cells.

The most common type of cell found in the connective tissue of skin is fibroblast. They assist in producing and organizing the extracellular matrix of the dermis and also communicate with each other and other cells in the regulation of skin physiology. Dermal fibroblasts are a heterogeneous population and are identified by their location within the dermis. Fibroblasts that exist in the papillary dermis are generally in a more active state and divide at faster rates than those that exist in the reticular layer of the dermis[39]. Fibroblasts produce and secrete precursors of the extracellular matrix such as collagen, glycoproteins, and other mol‐

molecules thus reducing the elasticity and integrity of the skin[38].

264 Using Old Solutions to New Problems - Natural Drug Discovery in the 21st Century

**6.2. Keratinocytes**

**6.3. Fibroblasts and langerhans cells**

ecules that support the structure of the skin.

It has been recognized that photoaging and normal chronological aging differ in several morphological and biological aspects, one of which is distinct alterations in elastic fibers and microvasculature. Previous studies have shown that one of the main contributors to this phenomenon is activation of immune cells in the epidermis, one of which is the mast cell. Studies comparing photo-exposed and sun-protected skin from the same individual shows that sun-exposed skin is characterized by the presence of higher numbers of infiltrating leu‐ cocytes and mast cells than is found in sun-protected skin[47]. Mast cells are cutaneous cells that contain pro-fibrotic and pro-inflammatory mediators and can contribute to skin inflam‐ mation and disease. Recent evidence has suggested that mast cells maintain tissue homeo‐ stasis and that many of the responses observed in photoaging are an attempt of the mast cell to repair damaged tissue systems. Mast cell activation can initiate or exacerbate many of the alterations associated with chronically sun-exposed skin such as massive accumulation of abnormal elastic fibers, loss of collagen, increase in glycosaminoglycans and telangiectatic vessels[47]. For example, mast cells produce fibroblast growth factors that can activate fibro‐ blasts to produce abnormal matrix molecules. Mast cells also produce matrix-degrading en‐ zymes and can activate the production of matrix proteinases by other skin cells such as keratinocytes. Mast cell products can recruit and activate infiltrating inflammatory cells such as neutrophils, macrophages and lymphocytes that can further damage the local tissue (see Figure 4).

Although an increased number of mast cells have been associated with photoaging, very lit‐ tle is known about their role in this process. The interaction between mast cells and other skin cells such as keratinocytes in photoaging is poorly understood. In fact, the precise mechanisms of mast cell activation in photodamaged skin is not known although it does not appear to be through the more common pathways such as the high affinity IgE receptor (FcεRI) or pathogen-associated pattern recognition receptors. In photoaging, mast cell acti‐

**Figure 4.** Role of skin cells in photoaging.

vation appears to be unique but our laboratory has shown that G protein coupled receptors (GPCR) may be responsible. Chronic exposure to UV causes sensory nerve fibers to release a series of neuropeptides including substance P (SP) and calcitonin gene-related peptide (CGRP)[48], both of which can activate mast cell degranulation and pro-inflammatory medi‐ ator release[49]. Furthermore, opiates that modify nerve function can also activate skin mast cells[50]. Interestingly, SP causes mast cells to downregulate their expression of allergen-ac‐ tivated receptors (FcεRI)[51] and primes them to respond to pathogen-activated (TLR) stim‐ uli[52]. In skin that is chronically exposed to UV, mast cells may modulate a chronic inflammatory state, thus causing further damage to skin's structural integrity. Furthermore, mast cells appear to be important in the immunosuppressive environment of photodamaged skin since UVB causes a mast cell-dependent suppression of contact hypersensitivity re‐ sponses[53]. This finding supports our hypothesis that human mast cells exposed to UV may undergo a phenotypic change from an allergic inflammatory effector cell to a more pathogen/structural response cell capable orchestrating structural changes in the skin. How‐ ever, it is also possible that mast cell activation is a protective phenomenon and that further potentiating mast cell activation in photodamaged skin may, in fact, inhibit or reverse some of the immunosuppression associated with photodamage.

#### **6.5. Inflammation during photodamage**

vation appears to be unique but our laboratory has shown that G protein coupled receptors (GPCR) may be responsible. Chronic exposure to UV causes sensory nerve fibers to release a series of neuropeptides including substance P (SP) and calcitonin gene-related peptide (CGRP)[48], both of which can activate mast cell degranulation and pro-inflammatory medi‐ ator release[49]. Furthermore, opiates that modify nerve function can also activate skin mast cells[50]. Interestingly, SP causes mast cells to downregulate their expression of allergen-ac‐ tivated receptors (FcεRI)[51] and primes them to respond to pathogen-activated (TLR) stim‐ uli[52]. In skin that is chronically exposed to UV, mast cells may modulate a chronic inflammatory state, thus causing further damage to skin's structural integrity. Furthermore, mast cells appear to be important in the immunosuppressive environment of photodamaged skin since UVB causes a mast cell-dependent suppression of contact hypersensitivity re‐ sponses[53]. This finding supports our hypothesis that human mast cells exposed to UV may undergo a phenotypic change from an allergic inflammatory effector cell to a more pathogen/structural response cell capable orchestrating structural changes in the skin. How‐ ever, it is also possible that mast cell activation is a protective phenomenon and that further

**Figure 4.** Role of skin cells in photoaging.

266 Using Old Solutions to New Problems - Natural Drug Discovery in the 21st Century

Traditionally, photodamage has been characterized by the mechanical damage caused to the skin's constituents – i.e. the degradation of the proteins and structures that hold the skin to‐ gether and give its integrity. However, recent evidence has shown that at least some of the damage associated with UV exposure is due to the initiation of low but chronic inflamma‐ tion that ultimately results in impaired cellular responses. This has some major implications because it suggests that the skin is not only losing its ability to form a physical barrier but is also losing some of its immune and regenerative abilities. Recent studies have shown that photodamage reduces immune responses by inhibiting the pathways that activate immune cells and causes changes in biochemical processes that regulate wound healing.

Using histological analysis, Bosset et al showed that sun-exposed skin had an increase in in‐ filtrating mononuclear cells than sun-protected skin[54]-[56]. Immunohistochemical analysis also showed an increased number of cells that one would normally associate with a Th2 in‐ flammatory response – namely, mast cells, macrophages and CD4+ CD45RO+ T cells -- in sun-exposed dermis as well as a higher number of CD1a+ dendritic cells in sun-exposed epi‐ dermis, compared with the sun-protected samples. Although Bosset et al. found that photo‐ aging displayed histological features of chronic skin inflammation they found a decreased expression of interleukin-1beta mRNA in sun-exposed compared with sun-protected sug‐ gesting that other pro-inflammatory mediators were mediating these effects. This hypothe‐ sis seems likely since these patients' skin looked clinically normal and would suggest an inflammatory pathology that is very different from those normally associated with acute Th2 inflammatory reactions such as atopic dermatitis or urticaria.

Inflammation is a complex process that has many different points of initiation and can be perpetuated by a diverse array of mediators. However, a study by Yan et al.[57] suggests that leukotrienes, potent lipid inflammatory mediators, may be responsible for this low but chronic inflammatory process associated with UV exposure. In their study, these investiga‐ tors evaluated the protective effects of a nonsedative histamine H1-receptor antagonist, miz‐ olastine, on UVB-exposed skin dermal fibroblasts. They found that relatively low levels of mizolastine (10 nM) inhibited UVB-induced LTB4 production by skin fibroblasts. This de‐ crease in LTB4 production was associated with a down-regulation of 5-lipooxygenase ( 5- LO) mRNA expression and inhibition of 5-LO translocation suggesting that UV-induced leukotriene synthesis was dependent upon 5-LO expression.

The idea that chronic inflammation due to photodamage is a Th2 mediated response is pos‐ sible, but some evidence suggests that it may be, in fact, a Th1 mediated event. UVB expo‐ sure of the skin results in a profound upregulation of the anti-inflammatory cytokine IL-10 and suppression of contact hypersensitivity (CHS). Furthermore, using an IL-10 transgenic mouse model (IL-10tg), Ma et al.[58] have shown that unexposed IL-10tg animals showed a diminished CHS response compared to wild-type. Yet when IL-10tg animals were exposed to UVB, the CHS response was not further suppressed, but rather was restored to the level observed in unexposed wild-type animals suggesting a normalization of the hypersensitivi‐ ty response. If one believes that Th1 and Th2 responses counter-balance one another, this data would suggest that chronic inflammation associated with photodamage is mediated by a Th1 controlled inflammatory response.

## **7. Natural ingredients as treatments for photodamage and photodamage**

#### **7.1. Targeting the skin's immune response**

Natural products that are used to treat the symptoms associated with photoaging and pho‐ todamage are designed to either stimulate new skin cell formation or inhibit the biochemical processes that cause skin damage. Sometimes, compounds are promoted as having biologi‐ cal activity in both of these contexts. Natural health products (NHPs) used to treat photo‐ damaged skin are often mixtures of many different molecules and the precise mechanism of their beneficial effects are not always easy to determine from the myriad of information (an‐ ecdotal and experimental) available. However, there are a few groups of compounds that are naturally-derived and that appear to have some efficacy in at least reducing some of the biochemical manifestations of photodamage.

Botanical extracts are the most popular sources of naturally-sourced bioactives in skin health clinics. Most often, these are mixed into topical creams and applied to the epidermis. These extracts can even provide some small measure of sun protective activity (SPF) as well as reducing the biochemical and cellular consequences of photodamage described previous‐ ly[59]. Topical application of green and white tea extracts, can reduce some of the detrimen‐ tal immunomodulatory effects of photodamage[60]. Topical administration of Vitamin A and its analogues inhibit the expression of MMP and stimulate collagen synthesis in both photodamaged and photoprotected aged skin[61]. The list of these compounds is extensive and there are many comprehensive reviews on these compounds and their effects on human skin. However, these compounds all share the ability to modify the cellular responses in the dermis, whether through the induction of regenerative processes associated with keratino‐ cyte renewal or inhibition of enzymes that break down the structural integrity of the skin. These compounds are all tested for their direct effect on these systems in bioassays designed to tease out the mechanisms on a single or small group of skin cells.

Some of these extracts, however, have efficacy when taken orally so their mechanisms of much more difficult to determine. For example, taking a specific *Polypodium leucotomos* ex‐ tract (Fernblock, *Cantabria Farmaceutica*) 7.5 mg/kg daily for two doses before UVA treatment can modestly reduce erythema, edema, and signs of epidermal damage and may decrease subsequent hyperpigmentation in some patients[62]. In fact, it has been suggested that a diet rich in green tea polyphenols, grape seed proanthocyanidins and silymarin may act as che‐ mopreventative agents to protect skin from photocarcinogenesis[63]. Taking what we know about the skin immune system and the possible molecular targets of these compounds we can hypothesize that this process may involve the induction of interleukin-12 (IL-12), DNA repair mechanisms, protection of Langerhans cell depletion and stimulation of cytotoxic T cells. However, many questions remain. How much of these chemoprotective agents must we eat and how often? Are some people (i.e. different skin tones) better protected by these dietary compounds than others? What are the best sources of these compounds in our diet? These are all relevant questions that are difficult to answer experimentally.

observed in unexposed wild-type animals suggesting a normalization of the hypersensitivi‐ ty response. If one believes that Th1 and Th2 responses counter-balance one another, this data would suggest that chronic inflammation associated with photodamage is mediated by

**7. Natural ingredients as treatments for photodamage and photodamage**

Natural products that are used to treat the symptoms associated with photoaging and pho‐ todamage are designed to either stimulate new skin cell formation or inhibit the biochemical processes that cause skin damage. Sometimes, compounds are promoted as having biologi‐ cal activity in both of these contexts. Natural health products (NHPs) used to treat photo‐ damaged skin are often mixtures of many different molecules and the precise mechanism of their beneficial effects are not always easy to determine from the myriad of information (an‐ ecdotal and experimental) available. However, there are a few groups of compounds that are naturally-derived and that appear to have some efficacy in at least reducing some of the

Botanical extracts are the most popular sources of naturally-sourced bioactives in skin health clinics. Most often, these are mixed into topical creams and applied to the epidermis. These extracts can even provide some small measure of sun protective activity (SPF) as well as reducing the biochemical and cellular consequences of photodamage described previous‐ ly[59]. Topical application of green and white tea extracts, can reduce some of the detrimen‐ tal immunomodulatory effects of photodamage[60]. Topical administration of Vitamin A and its analogues inhibit the expression of MMP and stimulate collagen synthesis in both photodamaged and photoprotected aged skin[61]. The list of these compounds is extensive and there are many comprehensive reviews on these compounds and their effects on human skin. However, these compounds all share the ability to modify the cellular responses in the dermis, whether through the induction of regenerative processes associated with keratino‐ cyte renewal or inhibition of enzymes that break down the structural integrity of the skin. These compounds are all tested for their direct effect on these systems in bioassays designed

Some of these extracts, however, have efficacy when taken orally so their mechanisms of much more difficult to determine. For example, taking a specific *Polypodium leucotomos* ex‐ tract (Fernblock, *Cantabria Farmaceutica*) 7.5 mg/kg daily for two doses before UVA treatment can modestly reduce erythema, edema, and signs of epidermal damage and may decrease subsequent hyperpigmentation in some patients[62]. In fact, it has been suggested that a diet rich in green tea polyphenols, grape seed proanthocyanidins and silymarin may act as che‐ mopreventative agents to protect skin from photocarcinogenesis[63]. Taking what we know about the skin immune system and the possible molecular targets of these compounds we can hypothesize that this process may involve the induction of interleukin-12 (IL-12), DNA repair mechanisms, protection of Langerhans cell depletion and stimulation of cytotoxic T

a Th1 controlled inflammatory response.

268 Using Old Solutions to New Problems - Natural Drug Discovery in the 21st Century

**7.1. Targeting the skin's immune response**

biochemical manifestations of photodamage.

to tease out the mechanisms on a single or small group of skin cells.

In addition to prophylactic treatments to alleviate the symptoms of photoaging, there is a strong market for compounds that can either stop or delay the sigs of photoaging. Anti-ag‐ ing products are used to reduce the various cellular processes associated with senescence – both intrinsic (chronoaging) and extrinsic (photoaging) ageing. Younger individuals seek to prevent or reduce the manifestations of the extrinsic aging process which is responsible for approximately 80 percent of facial aging. Older individuals are often concerned with pre‐ venting or reversing the signs of both intrinsic and extrinsic aging processes. As such, treat‐ ments used to prevent photoaging target the underlying inflammatory and molecular responses associated with damage of skin integrity. Treatments used to reverse or stop the appearance of photoaging often target the biomechanical properties of skin.

There are numerous compounds currently on the market and in development and it is be‐ yond the scope of this chapter to cover all of them. However, alpha hydroxy acids serve as excellent example to illustrate their effects on the skin microenvironment. AHA include gly‐ colic, citric, lactic, malic, tartaric and lactic acids and are used extensively to promote skin rejuvenation and renewal. They are sometimes marketed as "fruit acids" because citric acid is found in citric fruits, malic acid is present in apples and tartaric acid is sometimes isolated from grapes. However, other foods also contain these acids (sugarcane contains glycolic acid and sour milk contains lactic acid). AHA work mainly by exfoliating the stratum corneum thus improving the smoothness and texture of skin. Although the exact mechanism of this effect is unknown, it is believed that AHA inhibit the enzymes that form ionic sulfate and phosphate bonds in the stratum corneum[64]. In the dermis, AHA induce desquamation and promoting cell renewal as well as stimulate collagen synthesis and induce dermal thick‐ ness[65].

AHA peels have recently been recognized as important adjunctive therapy in a variety of conditions including photodamage, actinic damage, melasma, hyperpigmentation disorders, acne, and rosacea. Since AHA are considered to have a reasonable level of safety and effica‐ cy in a variety of skin types, they have been used extensively in the cosmetic industry as well[66]. Although their exact mechanism of action is unknown, it has been suggested that AHA improve skin pathological disorders by thinning the stratum corneum, promoting epi‐ dermolysis, dispersing basal layer melanin, and increasing collagen synthesis within the dermis[67]. Specifically in patients with photodamaged skin, AHA peels and topical prod‐ ucts are often combined with retinoids and other antioxidants to reduce the thickness of the stratum corneum and smooth out the skin's surface. It has been suggested that AHA may help increase the skin's barrier function, making it more resilient to chemical damage[68].

A new class of hydroxy acids called polyhydroxy acids (PHAs), have similar effects as AHA but irritate the skin the same way as classical AHAs. PHAs cam be used on clinically sensi‐ tive skin, including rosacea and atopic dermatitis, and are compatible with cosmetic proce‐ dures[69]. Compared to AHA, PHA formulations are more moisturizing and can enhance stratum corneum barrier function. PHAs such as gluconolactone or lactobionic acid may be used in combination with other products, ingredients, or procedures such as laser and mi‐ crodermabrasion to provide additional benefits to therapy or to enhance the therapeutic ef‐ fect. PHAs used in combination with retinyl acetate (pro-vitamin A) in a cream base can have some anti-aging skin benefits such as skin smoothing and plumping although it is un‐ clear if these also involve biochemical or immunomodulatory effects. In some formulations, PHA and hydroquinone together have demonstrated some superficial improvement in mor‐ phological changes associated with photoaging and have resulted in a decrease in skin pig‐ mentation.

#### **8. Conclusions**

The skin is a complex and dynamic organ that is constantly bombarded with environmental trauma including UV radiation. UV light can cause major and often irreversible changes in the skin including damage to the three layers of skin. In the context of this constantly chang‐ ing and sometimes harsh environment, the skin must maintain homeostasis and protect a delicate network of biochemical and physiological systems. There are a number of processes that are designed to protect the skin from photodamage, including the stratum corneum, melanin and underlying immunological processes to control inflammation. However, it is becoming increasingly clear that, over time, the skin's protective mechanisms can become overwhelmed by the damage caused by UV radiation, resulting in irreversible structural damage, chronic inflammation and, in some cases, photocarcinogenesis. Understanding the role that the microenvironment plays in this complex process is important to developing in‐ novative new strategies for its treatment and possibly reversal.

### **Author details**

Marianna Kulka\*

National Research Council, Canada

#### **References**

[1] Wakamatsu K, Nakanishi Y, Miyazaki N, Kolbe L, Ito S. UVA-induced oxidative deg‐ radation of melanins: fission of indole moiety in eumelanin and conversion to benzo‐ thiazole moiety in pheomelanin. Pigment cell & melanoma research 2012; 25(4): 434-45.

[2] Curto EV, Kwong C, Hermersdorfer H, et al. Inhibitors of mammalian melanocyte tyrosinase: in vitro comparisons of alkyl esters of gentisic acid with other putative in‐ hibitors. Biochemical pharmacology 1999; 57(6): 663-72.

stratum corneum barrier function. PHAs such as gluconolactone or lactobionic acid may be used in combination with other products, ingredients, or procedures such as laser and mi‐ crodermabrasion to provide additional benefits to therapy or to enhance the therapeutic ef‐ fect. PHAs used in combination with retinyl acetate (pro-vitamin A) in a cream base can have some anti-aging skin benefits such as skin smoothing and plumping although it is un‐ clear if these also involve biochemical or immunomodulatory effects. In some formulations, PHA and hydroquinone together have demonstrated some superficial improvement in mor‐ phological changes associated with photoaging and have resulted in a decrease in skin pig‐

270 Using Old Solutions to New Problems - Natural Drug Discovery in the 21st Century

The skin is a complex and dynamic organ that is constantly bombarded with environmental trauma including UV radiation. UV light can cause major and often irreversible changes in the skin including damage to the three layers of skin. In the context of this constantly chang‐ ing and sometimes harsh environment, the skin must maintain homeostasis and protect a delicate network of biochemical and physiological systems. There are a number of processes that are designed to protect the skin from photodamage, including the stratum corneum, melanin and underlying immunological processes to control inflammation. However, it is becoming increasingly clear that, over time, the skin's protective mechanisms can become overwhelmed by the damage caused by UV radiation, resulting in irreversible structural damage, chronic inflammation and, in some cases, photocarcinogenesis. Understanding the role that the microenvironment plays in this complex process is important to developing in‐

[1] Wakamatsu K, Nakanishi Y, Miyazaki N, Kolbe L, Ito S. UVA-induced oxidative deg‐ radation of melanins: fission of indole moiety in eumelanin and conversion to benzo‐ thiazole moiety in pheomelanin. Pigment cell & melanoma research 2012; 25(4):

novative new strategies for its treatment and possibly reversal.

mentation.

**8. Conclusions**

**Author details**

Marianna Kulka\*

**References**

434-45.

National Research Council, Canada


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