Intrinsic ageing and reactive oxygen species

During intrinsic skin ageing, reactive oxygen species (ROS) levels rise, while antioxidant activity declines11. It is hypothesised that the ROS activate a number of phosphorylase mediated kinases leading to signal transduction throughout the epidermis, which results in the formation of activator protein 1 (AP-1). In turn, this up-regulates expression of matrix metalloproteinase (MMP) genes for MMP-1 (collagenase), MMP-3 (stromelysin) and MMP-9 (gelatinase), leading to the degradation of collagen12,13.

Intrinsic ageing and the sex steroids

The sex steroids play a significant role in intrinsic skin ageing as they are involved in supporting skin structure and function14. With age, dehydroepiandrosterone and its sulphate analogue (DHEAS) decrease both in males and females, but their concentration is always less in females15, 16. Androgens decline slowly with age in men, mostly compromising wound healing. Following the menopause, the concentration of plasma oestradiol drops considerably, resulting in thinner, drier skin with reduced elasticity as a result of lower collagen content. The plasma concentration of oestradiol in men does not decrease significantly17, 18. In post-menopausal women, the decline in skin thickness and collagen content was not correlated with the chronological age, but with the duration of oestrogen deficiency, with a decrease of 2.1% in collagen content for the first 15–18 post-menopausal years19.

Intrinsic ageing and advanced glycation end-products

Plasma levels of advanced glycation end-products (AGEs) are genetically determined20. AGEs have been studied mostly in association with diabetes, but in recent years they are also being discussed in skin ageing. AGEs are formed in higher amounts in diabetes and in lower amounts in normal metabolic processes21. Accumulation of AGEs in the skin could be detected in both diabetic ageing and in chronological ageing22–24. Glycated collagen first appears at the age of 20 years, with an accumulation of 3.7% each year, reaching a 30–50% increase at 80 years24, 25. When collagen becomes glycated, it loses its flexibility, and its susceptibility to mechanical stimuli is increased26. Its replacement by a new and functional collagen is inhibited because modified collagen resists degradation by MMPs27. MMPs not only affect the skin by glycation of proteins, but also when binding to their receptors (RAGE), they initiate a signalling cascade influencing gene expression, inflammation, cell cycle, cell proliferation, and extracellular matrix synthesis28. RAGE have been shown to decrease skin cell proliferation, increase MMP production and induce apoptosis, with most of these effects involving nuclear factor-κB (NFκB) signalling29.

Pathomechanisms of extrinsic skin ageing or photoageing

While chronological skin ageing comprises smooth, pale and finely wrinkled skin, extrinsic ageing is characterised by coarse, deep wrinkles with telangiectasia and dyspigmentation30. The main culprit of extrinsic skin ageing is UV radiation, which causes 80% of facial photoageing31. Other factors include ionising radiation, infrared radiation, thermal heat32, severe physical and psychological stress33, poor nutrition, overeating, alcohol intake, tobacco34, and air pollution35.

Expression of matrix metalloproteinases and signal transduction pathways

In older people, sun-exposed skin shows higher expression of MMPs than chronological aged skin in the same individuals36. In vivo, UVA radiation induces the expression of MMP-1 by dermal fibroblasts37, and in vitro the expression of MMP-2 and MMP-338. In vivo, UVB induces the expression of MMP-1, -3 and -937. The main cause for activating these MMPs is AP-1, which is up-regulated by signal transduction pathways caused by UV exposure. These pathways include p38, c-jun N-terminal kinases (JNKs), and mitogen-activated protein kinases (MAP)37. Increased AP-1 production also leads to down-regulation of types I and III pro-collagen by inhibiting transforming growth factor-β (TGF-β), and down-regulation of type II TGF-β receptors13.

Figure 1

Collagen degradation is further increased by the UV-induced transcription of the iron-dependent NF-κB pathway39. This leads to the expression of pro-inflammatory cytokines such as interleukin (IL)-6, IL-8, and tumour necrosis factor (TNF). Neutrophil collagenase and neutrophil elastase are stimulated by these pro-inflammatory cytokines, leading to more damage in the ECM39. In addition, these pro-inflammatory cytokines bind to their cell surface receptors, leading to further AP-1 and NF-κB production39.

Usually, MMPs are regulated by their tissue inhibitor of matrix metalloproteinases (TIMPs), but UV radiation disrupts this inhibition. Eventually, ECM degradation leads to a loss of mechanical tension on fibroblasts, resulting in their collapse40. The loss of mechanical tension results in a positive feedback loop for ROS production40.

Mutations in DNA and telomeres

UV radiation has direct and indirect damaging effects on DNA. UVB radiation causes the direct dimerisation of contiguous pyrimidines on the DNA. This results in the formation of cyclobutane pyrimidine dimers (CPD) or 6-4 photoproducts formed between pyrimidine bases41. DNA can be damaged indirectly by the production of ROS — mostly as a result of UVA radiation, and less a result of UVB42. ROS lead to mutations involving trans-urocanic acid, which results in singlet oxygen production and a nick in the DNA43, 44. As the most endogenous source of ROS production is from the mitochondria, most of the mutations occur there. Mitochondria also have inefficient repair mechanisms45, and therefore they are responsible for most of the functional changes seen with skin ageing46.

Likewise, UV radiation also damages telomeres as it targets dithymidine residues (TT) and G-bases on the chromosome. A telomere is a region of repetitive DNA found at the end of a chromosome8. It does not encode any gene products and forms a 3-dimensional structure called a t-loop. This t-loop is required for telomere capping and it is inserted in the 3’ overhang of the DNA8. UV radiation destabilises the t-loop configuration, exposing the 3’ overhang and leading to DNA-damage response, which can lead to apoptosis47, 48.

Disruption of the immune system and antioxidant levels

The increase in ROS increases the production of IL-1049, 50, which together with DNA damage51, results in local and systemic immunosuppression52. UV radiation leads to functional and morphological changes in Langerhans cells. The reduction in Langerhans cells results in a decrease in hypersensitivity reactions and a delayed type hypersensitivity reaction after UV exposure53–55.

Furthermore, UV radiation has a negative impact on the endogenous enzymatic and non-enzymatic antioxidant levels of the skin56, 57. Non-enzymatic antioxidants include ascorbate, carotenoids, vitamin E and coenzyme Q10 (CoQ10)57. Enzymatic antioxidants include superoxide dismutase (SOD), glutathione peroxidase (GPx), glutathione reductase (GR), and catalase56. At low doses of UV radiation, most of the antioxidant components are destroyed, while at high doses all components are damaged56. Vitamin C is rapidly depleted at doses of 0–5 J/cm², but less impacted at 5–25 J/cm² 56. SOD is significantly decreased at 25 J/cm², while GPx and GR are affected much less at this dose. Catalase is severely destroyed at doses above 5 J/cm², and the reason for this is because UV radiation damages it directly, while on the others it has a more indirect effect56.

Vitamin A disruption

The retinoic (RA) family includes vitamin A/all-trans-retinol and retinoids, which are the natural and synthetic derivatives of vitamin A58. RA maintains normal skin homeostasis and negatively regulates AP-159, 60. The effect of RA is exerted through retinoic acid receptors (RARs) and retinoid X receptors (RXRs). UV exposure in vivo decreases RAR-γ and RXR-α, which are two important receptors in the human skin58, causing a decrease in the cutaneous functional role of vitamin A61. This increases the activity of AP-1 pathway leading to an increase in MMPs.

Increase in advanced glycation end-product production

UV radiation increases AGEs in sun-exposed skin of young individuals24, 62. Furthermore, UV radiation also stimulates glycation of elastin in the presence of sugars resulting in the assembly of large and irregular structures63. In vitro studies have shown that the exposure of AGEs to UVA radiation results in the formation of ROS, such as hydrogen peroxide and superoxide anion64. Smoking and fried food also accelerate the deposition of AGEs in the skin65, 66.