Date Published: March 16, 2017
Publisher: Public Library of Science
Author(s): Hui-Wen Chiu, Cheng-Hsien Chen, Yi-Jie Chen, Yung-Ho Hsu, Ying-Jan Wang.
Ultraviolet (UV) induces skin photoaging, which is characterized by thickening, wrinkling, pigmentation, and dryness. Collagen, which is one of the main building blocks of human skin, is regulated by collagen synthesis and collagen breakdown. Autophagy was found to block the epidermal hyperproliferative response to UVB and may play a crucial role in preventing skin photoaging. In the present study, we investigated whether far-infrared (FIR) therapy can inhibit skin photoaging via UVB irradiation in NIH 3T3 mouse embryonic fibroblasts and SKH-1 hairless mice. We found that FIR treatment significantly increased procollagen type I through the induction of the TGF-β/Smad axis. Furthermore, UVB significantly enhanced the expression of matrix metalloproteinase-1 (MMP-1) and MMP-9. FIR inhibited UVB-induced MMP-1 and MMP-9. Treatment with FIR reversed UVB-decreased type I collagen. In addition, FIR induced autophagy by inhibiting the Akt/mTOR signaling pathway. In UVB-induced skin photoaging in a hairless mouse model, FIR treatment resulted in decreased skin thickness in UVB irradiated mice and inhibited the degradation of collagen fibers. Moreover, FIR can increase procollagen type I via the inhibition of MMP-9 and induction of TGF-β in skin tissues. Therefore, our study provides evidence for the beneficial effects of FIR exposure in a model of skin photoaging.
Chronic exposure to ultraviolet (UV) irradiation is the major cause of skin damage that leads to premature aging of the skin, which is called photoaging. Photoaging describes the clinical signs including coarse wrinkles, roughness, laxity and pigmentation [1, 2]. Collagen, which is one of the main building blocks of human skin, is derived from fibroblasts that are regulated by both transforming growth factor-β (TGF-β), a cytokine that promotes collagen production, and activator protein-1 (AP-1), a transcription factor that promotes collagen breakdown by up regulating enzymes called matrix metalloproteinases (MMPs) . The standard fibrillar collagen molecule is characterized by amino- and carboxy-terminal propeptide sequences. These form the central triple helical structure of procollagen and collagen. Three α-chains are intracellularly assembled into the triple helix following initiation of this process by the C-terminal domain. Procollagen is secreted by cells into the extracellular space and converted into collagen by the removal of the N- and C-propeptides via enzymes . TGF-β induced Smad2 phosphorylation and the TGF-β/Smad axis is the main signaling pathway for collagen synthesis in dermal fibroblasts . UV irradiation generates increased reactive oxygen species (ROS) levels in the skin and amplifies signals, which lead to the activation of mitogen-activated protein kinases (MAPKs) and phosphatidylinositol-3-kinase (PI3K)/Akt [6, 7]. These kinases ultimately stimulate MMPs expression and can then cause collagen degradation .
Recently, data accumulated by us and others have revealed that FIR can be investigated as a potential therapeutic strategy in various diseases [20, 21, 30, 31]. However, the detailed mechanism is unknown. UV is the primary external stress that leads to oxidative stress, which is initiated by ROS and eventually results in premature skin aging . Leung et al. found that ceramic-emitted FIR (cFIR) significantly inhibits intracellular peroxide levels and LPS-induced peroxide production by macrophages. Furthermore, cFIR blocks ROS-mediated cytotoxicity . Additionally, UV is known to induce the expression of MMPs, which are the key enzymes that degrade collagen . The degradation of collagen by MMPs is part of the dermal remodeling that results from skin exposure to UV. Thus, MMPs and collagen type I are attractive targets for antiphotoaging research. In the present study, FIR could suppress the UVB-induced expression of MMP-1 and MMP-9 (Fig 2). In our in vivo study, skin tissues from hairless mice treated with UVB showed higher MMP-9 levels than did untreated mice. Furthermore, significant decreases in the expression of MMP-9 were observed in the FIR treatment group compared with the UVB group (Fig 5B and 5D). It has been reported that the TGF-β/Smad pathway acts as a potent stimulator of the synthesis of type I collagen . FIR has previously been reported to mediate therapeutic effects on skin wound healing by stimulating the secretion of TGF-β or by activating of fibroblasts . Our research showed that TGF-β and phosphorylation of Smad2 protein expression increased in NIH3T3 cells following FIR treatment (Fig 1B and 1C). We also found that the expression of TGF-β and procollagen type I were increased in the FIR treatment group compared with the UVB treatment group in a model of UVB-induced skin photoaging in hairless mice (Fig 5D). Collagen fibers by Masson’s trichrome stains were increased significantly in FIR-exposed mice compared with UVB-irradiated mice (Fig 5A). Previous studies have demonstrated that procollagen is translocated into the lumen of the endoplasmic reticulum (ER), in which a number of molecular chaperones and enzymes assist its folding and trimerization. Then, procollagen is secreted by cells into the extracellular space [4, 35]. In our study, we found that UVB can suppress the secretion of collagen and that FIR can significantly reverse the UVB-inhibited collagen secretion (Figs 1D and 2C). Therefore, our results showed that FIR increased collagen by inhibiting collagen breakdown and inducing collagen production. In addition, our previous study found that FIR ameliorated the burn-induced epidermal thickening . Another recent study concluded that FIR pretreatment attenuates apoptosis and cell death in dehydration-stressed cultured keratinocytes through the PI3K/Akt pathway . In the present study, FIR-treated mice had thinner epidermal layers than mice that were exposed to UVB alone (Fig 4B and 4C). Therefore, FIR may affect not only fibroblasts but also keratinocytes.