Decorin is an extracellular matrix proteoglycan belonging to the small leucine-rich proteoglycan (SLRP) family with a protein core containing leucine repeats with a glycosaminoglycan (GAG) chain of either chondroitin sulfate (CS) or dermatan sulfate (DS). Decorin, together with versican (bind to elastic fibers) are the most abundant proteoglycan found in the connective tissue of dermis. Decorin binds to type I collagen fibrils – the dominant collagen type (another collagen in skin dermis is type III collagen) forming the network of collagen fibers in the skin dermis. Biological functions of decorin in the skin is to regulate matrix assembly and collagen fibrillogenesis – the development of fine fibrils (fine fiber of ~ 1 nm in diameter) normally present in collagen fibers of connective tissue. Collagen fibrils (1 nm) are formed via cross-linking of its precursor procollagen or tropocollagen. Procollagen or tropocollagen is a triple helix protein with 3 peptide chains and is synthesized by fibroblast and then secreted. The formation of collagen fibril (the polymer of tropocollagen) from tropocollagen or procollagen is known as collagen fibrillogenesis. Because collagen fibers and elastin fibers form the structural bone of skin dermis layer, providing skin its strength and elasticity. The disrupt and dysfunction of collagen fibrillogenesis and extracellular matrix assembly will adversely affect the overall health of skin. Decorin were discovered in the epidermal cells (keratinocytes) suggesting that this molecule plays an important role in the regulation of skin’s homeostasis as well.
Studies have shown that decorin level decreases in photo aged skin which may account for a progressive disorganization of collagen fibers in aged skin. The decorin alteration in chronological aged skin appear to be contradictory to that of photodamaged skin. Decorin amount was increased by 50% secreted from the senescent fibroblast culture in vitro. In a study of skin samples extracted from matured and aged skin showed an significant increase of decorin with a concomitant decrease of versican in senescent skin. Not only the amount of decorin changes with skin aging, the structure of the GAG chain of decorin changes with skin aging as well. Decorin is distributed along collagen fibrils with the core protein and the decorin GAG chain controls the distance between the collagen fibrils. Reducing the length of the decorin GAG chain reduces the distance between the collagen fibrils. Age-related changes in decorin are apparent in the length of GAG chain and sulfate location but not in the core protein. Structural changes in the decorin GAG chain may be related to changes in collagen matrix assembly during the aging process. The increase of decorin and decrease of collagen Type I, III and the decreased of collagen to decorin ratio in aged skin may be responsible for the decreased collagen fiber bundle diameters in aged skin which might affect their tensile strength.
In addition, the study showed the presence of a truncated form of decorin in its core protein component in aged skin which is a catabolic fragment of decorin known as decorunt that is absent in fetal skin. Analysis of collagen binding by surface plasmon resonance indicates that the affinity of decorunt for type I collagen is 100-fold less than that of decorin. This observation correlates with the structural analysis of decorunt, in that it lacks regions of decorin previously shown to be important for interaction with type I collagen. The detection of a catabolic fragment of decorin suggests the existence of a specific catabolic pathway for this proteoglycan. However, the role of this catabolic pathway and decorunt in regulating collagen fibrillogenesis and matrix assembly is not clear. In addition, decorunt also lack the domain for binding to epidermal growth factor receptor (EGFR) and transforming growth factor-beta (TGF-beta) which may induce a defect of skin cell stimulation and renewal.
One peptide anti-aging ingredient used in cosmetic products has been shown to increase decorin – the Copper TriPeptide GHK-Cu. This peptide has been scientifically proven to tighten collagen in aging skin, tighten loose skin, reduce wrinkles and improve skin elasticity.