Mitochondrial dynamics shoot an arrow at skin aging
524
Presented by: Yushi Katsuyama
Introduction
However, some questions remain, such as what situations decrease MITOL and how decreased MITOL causes the formation of insufficient dermal fibers. Thus, this study was conducted to elucidate those questions.
Methods
MITOL expression in normal human dermal fibroblasts (NHDFs)
NHDFs were irradiated with UVA, then treated with hydrogen peroxide (H2O2) or with buthionine sulfoximine (BSO), which is an inhibitor of glutathione synthesis, and their mRNA expression levels of MITOL were quantified using real-time PCR.
ER stress in MITOL-KD and in tunicamycin-treated NHDFs
The mRNA expression level of spliced X-box binding protein 1 (sXBP1), which is an ER stress marker regarding inositol-requiring enzyme 1 α (IRE1), in NHDFs was quantified.
MMP-1 expression by NHDFs
mRNA and protein levels of MMP-1 were evaluated using real-time PCR and western blotting, respectively.
Results
MITOL expression in NHDFs
The mRNA expression level of MITOL in NHDFs was decreased by UVA, by H2O2 and by BSO.
ER stress in NHDFs
The mRNA expression level of sXBP1 in MITOL-KD and in tunicamycin-treated NHDFs was increased.
MMP-1 secretion
Secreted amounts of MMP-1 from MITOL-KD and from tunicamycin-treated NHDFs were significantly increased.
Discussion and Conclusion
MITOL was down-regulated by internal and external oxidative stresses. The results indicate that oxidative stress must be one trigger for the decrease of MITOL and results in lower dynamics of MT likely to cause ER stress. In aged dermis, fibroblasts are continuously exposed to oxidative stress because they are surrounded by oxidative proteins such as CPs and AGEs. Thus, it is anticipated that fibroblasts in aged dermis cause ER stress due to the lower quality of MT resulting from the decrease of MITOL.
Meanwhile, the dermal matrix is regenerated by the degradation of fibers by MMPs and the reconstruction by resynthesized fiber proteins. The excessive decomposition of fibers such as collagen also gives stress to fibroblasts like excessive oxidation. In fact, ER stress triggered by decreased MITOL or by treatment with tunicamycin which is an inducer of ER stress, increased the secretion of MMP-1. Gathering these results, we conclude that alterations of the dermal matrix caused by oxidative stress expose fibroblasts to continuous oxidative stress, and result in accelerating skin aging due to the decreased dynamics of MT triggered by the decrease of MITOL.
Thus, based on this study, we propose a skin aging mechanism in which MT dynamics regulated by MITOL plays a central role. To prevent and improve skin aging, we should not only focus on some downstream events like enhancing the reconstruction of dermal fibers but also by maintaining fundamental cellular functions like MT.
Skin conditions are continuously affected by the internal and external environments, and stresses that occur in response to those conditions promote skin aging. In particular, chronic sun-exposure containing ultraviolet (UV) radiation alters the skin structure and eventually leads to photoaged skin. Photoaged dermis is characterized by a decrease of collagen fibers and a loss of oxytalan fibers. One reason for those phenomena is the dysfunction of fibroblasts in matrix reconstruction following UV radiation. That dysfunction of fibroblasts is caused by alterations of their characteristics. In fact, carbonylated proteins (CPs) and glycation proteins (AGEs), which are types of oxidized proteins, are found at higher levels in the dermis at sun-exposed sites. We have found that CPs that accumulate in the dermis reduce the ability of fibroblasts for matrix reconstruction. Thus, it is important to understand the mechanism of the cellular dysfunction in matrix reconstruction in order to propose solutions to prevent skin aging.
Mitochondria (MT) are essential intracellular organelles that interact with the endoplasmic reticulum (ER) and peroxisomes in order to maintain each other’s functions through their dynamics. MT might be responsible for the progression of photo-aged skin because there is often a decreased quality of MT in photo-aged skin. In general, the quality of MT is maintained by their dynamics such as the balance between fission and fusion, which is regulated by mitochondrial ubiquitin ligase (MITOL). A previous study revealed that knock-down (KD) of MITOL in fibroblasts reduced the formation of dermal fibers according to the decreased quality of MT. Those results indicate that a decrease of MITOL causes the characteristics of aged skin.However, some questions remain, such as what situations decrease MITOL and how decreased MITOL causes the formation of insufficient dermal fibers. Thus, this study was conducted to elucidate those questions.
Methods
MITOL expression in normal human dermal fibroblasts (NHDFs)
NHDFs were irradiated with UVA, then treated with hydrogen peroxide (H2O2) or with buthionine sulfoximine (BSO), which is an inhibitor of glutathione synthesis, and their mRNA expression levels of MITOL were quantified using real-time PCR.
ER stress in MITOL-KD and in tunicamycin-treated NHDFs
The mRNA expression level of spliced X-box binding protein 1 (sXBP1), which is an ER stress marker regarding inositol-requiring enzyme 1 α (IRE1), in NHDFs was quantified.
MMP-1 expression by NHDFs
mRNA and protein levels of MMP-1 were evaluated using real-time PCR and western blotting, respectively.
Results
MITOL expression in NHDFs
The mRNA expression level of MITOL in NHDFs was decreased by UVA, by H2O2 and by BSO.
ER stress in NHDFs
The mRNA expression level of sXBP1 in MITOL-KD and in tunicamycin-treated NHDFs was increased.
MMP-1 secretion
Secreted amounts of MMP-1 from MITOL-KD and from tunicamycin-treated NHDFs were significantly increased.
Discussion and Conclusion
MITOL was down-regulated by internal and external oxidative stresses. The results indicate that oxidative stress must be one trigger for the decrease of MITOL and results in lower dynamics of MT likely to cause ER stress. In aged dermis, fibroblasts are continuously exposed to oxidative stress because they are surrounded by oxidative proteins such as CPs and AGEs. Thus, it is anticipated that fibroblasts in aged dermis cause ER stress due to the lower quality of MT resulting from the decrease of MITOL.
Meanwhile, the dermal matrix is regenerated by the degradation of fibers by MMPs and the reconstruction by resynthesized fiber proteins. The excessive decomposition of fibers such as collagen also gives stress to fibroblasts like excessive oxidation. In fact, ER stress triggered by decreased MITOL or by treatment with tunicamycin which is an inducer of ER stress, increased the secretion of MMP-1. Gathering these results, we conclude that alterations of the dermal matrix caused by oxidative stress expose fibroblasts to continuous oxidative stress, and result in accelerating skin aging due to the decreased dynamics of MT triggered by the decrease of MITOL.
Thus, based on this study, we propose a skin aging mechanism in which MT dynamics regulated by MITOL plays a central role. To prevent and improve skin aging, we should not only focus on some downstream events like enhancing the reconstruction of dermal fibers but also by maintaining fundamental cellular functions like MT.