Glycosaminoglycan and proteoglycan in skin aging

Glycosaminoglycans (GAGs) and proteoglycans (PGs) are abundant structural components of the extracellular matrix in addition to collagen fibers. Hyaluronic acid (HA), one of GAGs, forms proteoglycan aggregates, which are large complexes of HA and HA-binding PGs. Their crosslinking to other matrix proteins such as the collagen network results in the formation of supermolecular structures and functions to increase tissue stiffness. Skin aging can be classified as intrinsic aging and photoaging based on the phenotypes and putative mechanism. While intrinsic aging is characterized by a thinned epidermis and fine wrinkles caused by advancing age, photoaging is characterized by deep wrinkles, skin laxity, telangiectasias, and appearance of lentigines and is mainly caused by chronic sun exposure. The major molecular mechanism governing skin aging processes has been attributed to the loss of mature collagen and increased matrix metalloproteinase expression. However, various strategies focusing on collagen turnover remain unsatisfactory for the reversal or prevention of skin aging. Although the expression of GAGs and PGs in the skin and their regulatory mechanisms are not fully understood, we and others have elucidated various changes in GAGs and PGs in aged skin, suggesting that these molecules are important contributors to skin aging. In this review, we focus on skin-abundant GAGs and PGs and their changes in human skin during the skin aging process.