A developed serum-free medium and an optimized chemical cocktail for direct conversion of human dermal fibroblasts into brown adipocytes Yukimasa takeda & ping Dai * Brown adipocytes coordinate systemic energy metabolism associated with the pathogenesis of obesity and related metabolic diseases including type 2 diabetes. We have previously reported chemical compound-induced brown adipocytes (ciBAs) converted from human dermal fibroblasts without using transgenes. In this study, to reveal a precise molecular mechanism underlying the direct conversion and human adipocyte browning, we developed serum-free brown adipogenic medium (SFBAM) with an optimized chemical cocktail consisting of Rosiglitazone, Forskolin, and BMP7. During the direct conversion, treatment with BMP7 enhanced Ucp1 expression rather than the conversion efficiency in the absence of BMP signalling inhibitors. Moreover, treatment with a TGF-β signalling pathway inhibitor was no longer required in the serum-free medium, likely because the TGF-β pathway was already suppressed. SFBAM and the chemical cocktail efficiently converted human dermal fibroblasts into ciBAs within four weeks. The ciBAs exhibited increased mitochondrial levels, elevated oxygen consumption rate, and a response to β-adrenergic receptor agonists. Thus the ciBAs converted by the serum-free medium and the chemical cocktail provide a novel model of human brown (beige) adipocytes applicable for basic research, drug screening, and clinical applications.Direct lineage reprogramming is an attractive approach to rapidly prepare desired cell types from patient-derived autologous somatic cells for transplantation therapy bypassing a pluripotent state 1,2 . So far human fibroblasts have been directly converted into myoblasts, cardiomyocytes, hepatocytes, and neurons by overexpression of a specific set of transcription factors 3-6 . The fibroblasts could also be converted to brown adipocytes by overexpression of two transcription factors, C/EBP-β and C-Myc 7 . However, in general direct reprogramming with virus vector-mediated overexpression of multiple transcription factors requires complicated procedures and also increases the risk of genomic instability and subsequent tumor formation 8 . Accumulating evidence has suggested that small molecules enable efficient cellular differentiation and direct reprogramming by modulating activity of target transcription factors as well as major signalling pathways 9,10 . We have previously reported that neuronal cells and brown adipocyte-like cells were converted from human fibroblasts by small molecules only without the use of transgene 11,12 . In addition, combinations of small molecules only also enable conversion of human fibroblasts into astrocytes, Schwann cells, osteoblasts, and cardiomyocytes [13][14][15][16] . Such simple and reproducible techniques for direct reprogramming are promising for the preparation of target cells with a reduced risk of mutations and tumorigenesis for transplantation therapy and other clinical applications 1 .Two types o...