Inhibition of type I procollagen synthesis by damaged collagen in photoaged skin and by collagenase-degraded collagen in vitro

Type I and type III procollagen are reduced in photodamaged human skin. Thi s reduction could result from increased degradation by metalloproteinases a nd/or from reduced procollagen synthesis. In the present study, we investig ated type I procollagen production in photodamaged and sun-protected human skin. Skin samples from severely sun-damaged forearm skin and matched sun-p rotected hip skin from the same individuals were assessed for type I procol lagen gene expression by in situ hybridization and for type I procollagen p rotein by immunostaining. Both mRNA and protein were reduced (similar to 65 and 57%, respectively) in photodamaged forearm skin compared to sun-protec ted hip skin. We next investigated whether reduced type I procollagen produ ction was because of inherently reduced capacity of skin fibroblasts in sev erely photodamaged forearm skin to synthesize procollagen, or whether conte xtual influences within photodamaged skin act to down-regulate type I proco llagen synthesis. For these studies, fibroblasts from photodamaged skin and matched sun-protected skin were established in culture. Equivalent numbers of fibroblasts were isolated from the two skin sites. Fibroblasts from the two sites had similar growth capacities and produced virtually identical a mounts of type I procollagen protein. These findings indicate that the lack of type I procollagen synthesis in sun-damaged skin is not because of irre versible damage to fibroblast collagen-synthetic capacity. It follows, ther efore, that factors within the severely photodamaged skin may act in some m anner to inhibit procollagen production by cells that are inherently capabl e of doing so. Interactions between fibroblasts and the collagenous extrace llular matrix regulate type I procollagen synthesis. In sun-protected skin, collagen fibrils exist as a highly organized matrix. Fibroblasts are found within the matrix, in close apposition with collagen fibers. In photodamag ed skin, collagen fibrils are shortened, thinned, and disorganized. The lev el of partially degraded collagen is similar to3.6-fold greater in photodam aged skin than in sun-protected skin, and some fibroblasts are surrounded b y debris. To model this situation, skin fibroblasts were cultured in vitro on intact collagen or on collagen that had been partially degraded by expos ure to collagenolytic enzymes. Collagen that had been partially degraded by exposure to collagenolytic enzymes from either bacteria or human skin unde rwent contraction in the presence of dermal fibroblasts, whereas intact col lagen did not. Fibroblasts cultured on collagen that had been exposed to ei ther source of collagenolytic enzyme demonstrated reduced proliferative cap acity (22 and 17% reduction on collagen degraded by bacterial collagenase o r human skin collagenase, respectively and synthesized less type I procolla gen (36 and 88% reduction, respectively, on a per cell basis). Taken togeth er, these findings indicate that 1) fibroblasts from photoaged and sun-prot ected skin are similar in their capacities for growth and type I procollage n production; and 2) the accumulation of partially degraded collagen observ ed in photodamaged skin may inhibit, by an as yet unidentified mechanism, t ype I procollagen synthesis.