Intracellular Degradation of Elastin by Cathepsin K in Skin Fibroblasts— A Possible Role in Photoaging Katerine A. Codriansky, 1 Maria J. Quintanilla-Dieck, 1 Stephanie Gan, 1 Michelle Keady, 1,2 Jag Bhawan 1,2 and Thomas M. Ru ¨ nger* 1 1 Department of Dermatology, Boston University School of Medicine, Boston, MA 2 Dermatopathology Section, Boston University School of Medicine, Boston, MA Received 29 January 2009, accepted 5 May 2009, DOI: 10.1111 ⁄ j.1751-1097.2009.00592.x ABSTRACT Solar elastosis is observed in the dermis of photoaged skin and is characterized by an accumulation of abnormal elastin in the extracellular space. Several proteases that degrade elastin in the extracellular space have been implicated in its formation. The lysosomal protease cathepsin K (catK) has recently been described to be highly expressed in skin fibroblasts under certain pathologic conditions. As cat K is one of the most potent mammalian elastases, we hypothesized that catK-mediated intracellular elastin degradation may play a role in the formation of solar elastosis. Immunostaining of cultured skin fibroblasts incubated with labeled elastin demonstrated internalization of extracellular elastin to lysosomes and its degradation by catK. Induction of catK expression in fibroblasts was observed both in vitro and in vivo after exposure to longwave UVA. In contrast to fibroblasts from young donors, cells from old donors failed to activate catK in response to UVA. These data suggest a role of intracellular elastin degradation by catK in the formation of solar elastosis. We propose that an age-related decline in catK activity, in particular after UV exposure, may promote the formation of actinic elastosis through a decline of orderly intracellular elastin degradation and subsequent accumulation of elastin in the extracellular space. INTRODUCTION Photoaging of the skin results from cumulative detrimental effects of prolonged and repetitive exposure to UV light. This extrinsic aging is superimposed on the age-associated degen- erative changes of the skin (intrinsic aging). While photoaging affects all compartments and cellular components of the skin, prominent and clinically visible features of photoaged skin include wrinkling and loss of elasticity. Those are caused by structural changes in the dermal extracellular matrix (ECM) (1–5). In addition to a reduced content of collagen fibers (6), chronically UV-exposed skin typically exhibits large deposits of fibrillary basophilic material in the upper and mid dermis, composed mainly of elastin and fibrillin. This material is commonly termed solar elastosis, as it is strongly positive with elastin stains, degraded by elastase and resistant to collagenase degradation. The accumulation of abnormal, misfolded, toxic or dam- aged proteins as a consequence of a declining elimination rate is a common feature of aging and age-related diseases in many organs (7–11). Solar elastosis epitomizes this view of the aging process in the skin. Although solar elastosis is well described, an understanding of the exact mechanisms of how solar elastosis is generated remains elusive. Several lines of evidence suggest that both increased elastin synthesis and elastin degradation contribute to the formation of solar elastosis (3,4). Several proteases with elastolytic activity have been described in skin. UV-induced elastin degradation is often attributed to the serine protease neutrophil elastase, as neutrophils can be found in the dermis after acute UV- exposure (12,13). A role of neutrophil elastase in solar elastosis was also suggested by Starcher et al. (14), who described that in neutrophil elastase-deficient mice, in contrast to normal mice, chronic UV exposure does not result in increased dermal elastin content. Fibroblasts have also been described as a source of elastase, suggesting that elastinolytic activity can also be maintained in the absence of inflammatory cells (15,16). Matrix metalloproteinases (MMPs) are a well characterized family of proteases well known to degrade ECM proteins. From the MMP family, the keratinocyte-derived matrilysin (MMP-7), the macrophage metalloelastase (MMP-12) and the gelatinases MMP-2 and MMP-9 are known to have elastolytic activities (17–19). All of these proteases with elastolytic activities, fibroblast-derived (fibroblast elastase, MMP-2, and MMP-9) or inflammatory cell-derived (neutrophil elastase and MMP-12) have in common that they are secreted to then mediate elastin degradation in the extracellular space. To our knowledge, alternative mechanisms of dermal elastin degrada- tion have not been proposed. We recently described high expression levels of cathepsin K (catK) in dermal fibroblasts of surgical scars, while it was not detectable in normal skin (20). As catK is a member of the cysteine protease family with strong collagenolytic and elast- olytic activity well established to mediate bone resorption by osteoclasts (21–27), we suggested that its proteolytic activities may contribute to ECM homeostasis also in skin (20). A similar function was suggested for catK expression in lung fibroblasts, as catK knock-out mice are predisposed to bleomycin-induced lung fibrosis (28). In osteoclasts, a short 15-amino acid sequence is cleaved from the full-length sequence of catK (pre-pro catK) to *Corresponding author email: truenger@bu.edu (Thomas M. Ru¨ nger) Ó 2009 The Authors. Journal Compilation. The American Society of Photobiology 0031-8655/09 Photochemistry and Photobiology, 2009, 85: 1356–1363 1356