Here, we demonstrated that UVA irradiation led to significant reductions in GPDH activity and the triglyceride content of adipocytes. Members of the Kruppel-like family of transcription factors have been shown to play important roles in cellular differentiation and growth (24). In addition, reduced adipogenesis by UVA was recovered upon the treatment with anti-MIF antibodies. AMP-activated protein kinase phosphorylation and up-regulation of Kruppel-like factor 2 (KLF2) were induced by UVA. Taken together, these findings suggest that the inhibition of adipogenic differentiation of human adipose tissue-derived mesenchymal stem cells by UVA occurs primarily through the reduced expression ofPPAR, which is mediated by up-regulation ofKLF2via the activation of MIF-AMP-activated protein kinase signaling. Keywords:Adipocyte, Adipose Tissue, AMP-activated Kinase (AMPK), AP-1 Transcription Factor, C/EBP Transcription Factor, Cell Differentiation, Gene Expression, Gene Regulation, Obesity, PPAR == Introduction == Obesity is associated with a number of pathological disorders, including non-insulin-dependent diabetes, hypertension, hyperlipidemia, and cardiovascular diseases (1). Adipose tissue biology studies have led to improved understanding of the mechanisms that link obesity to metabolic syndrome and other complications. In addition, recent advances in adipocyte biology have established that adipose tissue serves as a means of energy storage in the form of triglycerides as well as exerts secretory/endocrine gland functions, producing various secretory molecules such as hormones and cytokines (2). In view of the preventive and therapeutic aspects of the diseases mentioned above, most intensive clinical interventions have primarily been directed at decreasing excessive amounts of adipose tissue by changing the balance between intake and the expenditure of energy (3). Such RIPK1-IN-7 changes are typically induced via daily exercise and diet control (4). Mechanical stimuli such as stretching and rubbing of fat and skeletal muscle have also been found to decrease obesity. Indeed, Tanabeet al.(5) reported that mechanical stretching inhibits adipocyte differentiation through extracellular signal-regulated kinase (ERK)-mediated down-regulation of proliferator-activated receptor (PPAR)22. Adipocytes are the major cellular component in adipose tissue, and their excessive growth, differentiation, and hypertrophy are fundamental processes involved in obesity. Maturation of adipocytes can occur among cells from a pre-existing pool of adipocyte progenitor cells that are present, irrespective of age (6). Therefore, from a pathophysiological point of view, both the proliferation and differentiation of preadipocytes into mature adipocytes remain important issues. Ultraviolet (UV) irradiation is a major environmental factor responsible for a high incidence of skin aging, which is referred to as photoaging, as well as skin cancer and melanoma. UVA (320380 nm) irradiation represents 90% of the solar UV light that reaches the surface of the earth; therefore, its contribution to human life may be significant. The UVA component of sunlight has oxidizing properties that may be deleterious to skin cells and tissue but that can also lead to strong up-regulation of the heme-catabolizing enzyme, heme oxygenase-1. This enzyme has well established antioxidant actions in cells as well as anti-inflammatory properties in mammals. There is also evidence from rodent models that this enzyme is responsible for the UVA-mediated protection against UVB-induced immunosuppression that occurs in skin. Rabbit Polyclonal to ZADH2 The relevance of these findings to RIPK1-IN-7 the acute and chronic effects of sunlight, including skin carcinogenesis, is currently under investigation as RIPK1-IN-7 are the potential implications for sunlight protection in humans (7,8). A range of mammalian cells such as fibroblasts (9), keratinocytes (10), melanocytes (11), cardiomyocytes (12), vascular endothelial cells (13), smooth muscle cells (14), and osteoblasts (15) can respond to UVA irradiation. To the best of our knowledge, no studies have been conducted to evaluate the direct effects of UVA irradiation on adipocyte differentiation. Here, we demonstrate that UVA irradiation inhibited the differentiation of human adipose tissue-derived mesenchymal stem cells (hAMSCs) into adipocytes. This effect was mostly due to the reduced expression of PPAR , an adipocyte-specific nuclear hormone receptor/adipogenic transcription factor (16). This reduced expression was mediated by the activation of migration inhibitory factor (MIF)-AMP-activated protein kinase (AMPK)-Kruppel-like factor (KLF) 2 signaling. The modulation of adipocyte differentiation in response to UVA irradiation might.