ABSTRACT

Damage to the skin extracellular matrix (ECM) is the hallmark of long-term exposure to solar UV radiation. The aim of our study was to investigate the changes induced in unexposed human skin in vivo after single or repeated (five times a week for 6 weeks) exposure to 1 minimal erythemal dose (MED) of UV solar-simulated radiation. Morphological and biochemical analyses were used to evaluate the structural ECM components and the balance between the degrading enzymes and their physiologic inhibitors. A three-fold increase in matrix metalloproteinase 2 messenger RNA (mRNA) (P

Abbreviations: DEJ, dermal-epidermal junction; ECM, extracellular matrix; IL, interleukin; ITA, individual typological angle; MED, minimal erythemal dose; MMP, matrix metalloproteinase; mRNA, messenger RNA; PAI, plasminogen activator inhibitor; PBS, phosphate-buffered saline; rRNA, ribosomal RNA; RT-PCR, reverse transcription-polymerase chain reaction; sRNA, synthetic RNA; SSR, solar-simulated radiation; uPA, urinary-type plasminogen activator; UVR, ultraviolet radiation; VEGF, vascular endothelial growth factor.

INTRODUCTION

Aging of the skin encompasses two clinically and biologically independent processes that occur simultaneously. Chronological or intrinsic aging affects skin by slow and irreversible tissue changes, whereas extrinsic aging or photoaging that results from exposure to environmental factors including primarily ultraviolet radiations (UVR) leads to major visible alterations. In areas exposed to sun, skin damages called photoaging are superimposed on tissue defects resulting from chronological aging. Photoaging induces marked cutaneous alterations clinically characterized by wrinkles, roughness, sallowness, mottled dyspigmentation, telangectasia and a variety of benign or malignant neoplasms (1). The two types of aging processes evolve slowly over several decades probably as a result of cumulative minute changes. The ultimate stage of the processes is mainly found in the dermal connective tissue, as observed by histological and ultrastructural investigations (2,3). This is characterized by rarefaction and disorganization of collagen fibers framework and deposits of elastotic material. It is now recognized that both UV-A and UV-B range of wavelengths and daily cumulative exposure to solar UVR contribute to chronic photodamage of human skin (4).

The extracellular matrix (ECM) in the dermis is composed primarily of Type-I collagen, associated with a lower amount of Type-III collagen, elastin and associated microfibrils, proteoglycans and fibronectin. Sun-exposed sites usually display a loss of mature Type-I collagen fibers and an increased collagen III-collagen I ratio. A significant correlation was found between the reduced level of Type-I collagen and the severity of photodamage in human skin (5). The histopathological hallmark of photoaging is a massive accumulation of elastotic material in the upper and mid dermis (6). This process, known as solar elastosis, has been ascribed to changes in elastin network, the principal component of elastic fibers. Alterations in the microfibrillar components of the elastic fibers, in particular fibrillin 1, have also been reported (7). The accumulation of elastotic material is accompanied by degeneration of the surrounding collagen meshwork (5). Matrix-degrading metalloproteinases (MMP), a family of proteolytic enzymes that collegially degrade all matrix proteins forming the dermal connective tissue, are critical for matrix remodeling during development and wound healing (8). The activity of MMP is tightly controlled by several transcriptional and posttranscriptional mechanisms regulating their expression and activation. Their proteolytic activity is also modulated by the presence of specific inhibitors, the tissue inhibitor of matrix metalloproteinase (TIMP) (9). MMP are also involved in many pathological conditions, such as tumoral progression and metastasis, rheumatoid arthritis, aortic aneurysms and plaque instability in vascular occlusive diseases. They are obviously implicated in UV-induced photodamage in vivo (10,11). Their modulation by UV light has been extensively investigated in skin cells in vitro (10).

Our study was designed to evaluate on a comparative basis the regulation of the main ECM macromolecules and their degrading enzymes, activators and inhibitors, as well as inflammatory and angiogenic mediators in human skin in vivo, at one time point after exposure to environmental doses of UV solar-simulated radiation (SSR). The main goal was to investigate differences in expression of these proteins by measuring their messenger RNA (mRNA) levels using a quantitative reverse transcription-polymerase chain reaction (RT-PCR) procedure after single (acute SSR) or repeated (chronic SSR) exposure to 1 minimal erythemal dose (MED) of the previously unexposed skin of young volunteers. These measurements were completed by immunomorphological analyses.