Ultraviolet (UV) irradiation from the sun adversely impacts skin health through complex, multiple molecular pathways. Premature skin aging (photoaging) is among the most widely appreciated harmful effects of chronic exposure to solar UV irradiation. Extensive damage to the dermal connective tissue is a hallmark of photoaged skin. Disruption of the normal architecture of skin connective tissue impairs skin function and causes it to look aged. UV irradiation induces expression of certain members of the matrix metalloproteinase (MMP) family, which degrade collagen and other extracellular matrix (ECM) proteins that comprise the dermal connective tissue. Although the critical role of MMPs in photoaging process is undeniable, important questions remain. This article summarizes our current understanding regarding the role of MMPs in the photoaging process and presents new data that 1) describe expression and regulation by UV irradiation of all members of the MMP family in human skin in vivo, and 2) quantify the relative contributions of the epidermis and dermis to expression of UV irradiation-induced MMPs in human skin in vivo.

The dermal extracellular matrix (ECM) provides strength and resiliency to skin. The ECM consists mostly of type I collagen fibrils, which are produced by fibroblasts. Binding of fibroblasts to collagen fibrils generates mechanical forces, which regulate cellular morphology and function. With aging, collagen fragmentation reduces fibroblast-ECM binding and mechanical forces, resulting in fibroblast shrinkage and reduced function including collagen production. Here, we report that these age-related alterations are largely reversed by enhancing structural support of the ECM. Injection of dermal filler, cross-linked hyaluronic acid, into the skin of persons over seventy years-old stimulates fibroblasts to produce type I collagen. This stimulation is associated with localized increased of mechanical forces, indicated by fibroblast elongation/spreading, and mediated by up-regulation of type II TGF-β receptor and connective tissue growth factor. Interestingly, enhanced mechanical support of the ECM also stimulates fibroblast proliferation, expands vasculature, and increases epidermal thickness. Consistent with our observations in human skin, injection of filler into dermal equivalent cultures causes elongation of fibroblasts, coupled with type I collagen synthesis, which is dependent on the TGF-β signaling pathway. Thus, fibroblasts in aged human skin retain their capacity for functional activation, which is restored by enhancing structural support of the ECM.

The role of angiogenesis during mechanically induced bone formation is incompletely understood. The relationship between the mechanical environment, angiogenesis, and bone formation was determined in a rat distraction osteogenesis model. Disruption of either the mechanical environment or endothelial cell proliferation blocked angiogenesis and bone formation. This study further defines the role of the mechanical environment and angiogenesis during distraction osteogenesis.Introduction: Whereas successful fracture repair requires a coordinated and complex transcriptional program that integrates mechanotransductive signaling, angiogenesis, and osteogenesis, the interdependence of these processes is not fully understood. In this study, we use a system of bony regeneration known as mandibular distraction osteogenesis (DO) in which a controlled mechanical stimulus promotes bone induction after an osteotomy and gradual separation of the osteotomy edges to examine the relationship between the mechanical environment, angiogenesis, and osteogenesis. Materials and Methods: Adult Sprague-Dawley rats were treated with gradual distraction, gradual distraction plus the angiogenic inhibitor TNP-470, or acute distraction (a model of failed bony regeneration). Animals were killed at the end of distraction (day 13) or at the end of consolidation (day 41) and examined with CT, histology, and immunohistochemistry for angiogenesis and bone formation (n ‫ס‬ 4 per time-point per group). An additional group of animals (n ‫ס‬ 6 per time-point per group) was processed for microarray analysis at days 5, 9, 13, 21, and 41. Results and Conclusions: Either TNP-470 administration or disruption of the mechanical environment prevented normal osteogenesis and resulted in a fibrous nonunion. Subsequent analysis of the regenerate showed an absence of angiogenesis by gross histology and immunohistochemical localization of platelet endothelial cell adhesion molecule in the groups that failed to heal. Microarray analysis revealed distinct patterns of expression of genes associated with osteogenesis, angiogenesis, and hypoxia in each of the three groups. Our findings confirm the interdependence of the mechanical environment, angiogenesis, and osteogenesis during DO, and suggest that induction of proangiogenic genes and the proper mechanical environment are both necessary to support new vasculature for bone induction in DO.

Keloids are proliferative dermal growths representing a pathologic wound healing response. We have previously demonstrated that coculture of fibroblasts derived from either keloid or normal skin have an elevated proliferation rate when cocultured with keloid-derived keratinocytes vs. normal keratinocytes. In these studies, we examined the contribution of transforming growth factor-beta (TGF-beta) to this phenomenon using a two-chamber coculture system. Fibroblast proliferation in coculture was slower with the addition of a pan-TGF-beta neutralizing antibody. Keloid keratinocytes in coculture expressed more TGF-beta1, -beta3, and TGF-beta receptor 1 than normal keratinocytes. Keloid fibroblasts cocultured with keloid keratinocytes expressed more mRNA for TGF-beta1, -beta2, TGF-beta receptor 1, and Smad2. Keloid fibroblasts also produced more type I collagen, connective tissue growth factor, and insulin-like growth factor-II/mannose-6-phosphate receptor when cocultured with keloid keratinocytes vs. normal keratinocytes. Levels of total and activated TGF-beta activity increased when fibroblasts were cocultured with keratinocytes, correlating with the changes in transcriptional activity of TGF-beta. In conclusion, we find a complex paracrine interaction regulates TGF-beta mRNA expression and activation between keratinocytes and fibroblasts. These data suggest that keloid pathogenesis may result from both an increased TGF-beta production and activation by the keloid keratinocyte, and elevated TGF-beta expression, utilization, and signaling in keloid fibroblasts.