Transforming growth factor-β (TGF-β) superfamily members play an important role in growth, differentiation, adhesion, apoptosis, and development in many species from insects and worms to vertebrates. Recently, TGF-β signaling has been demonstrated to be negatively regulated by microtubules (MTs), which anchor endogenous Smad2/3 to cytosol and also directly interact with connexin43 (Cx43), and the activity of TGF-β is mediated by Cx43. However, the mechanism underlying the intracellular regulation of TGF-β activity by Cx43 remains unknown. Here, we found that the functional link between TGF-β activation and Cx43 is mediated by interactions among Smad2/3, MTs, and Cx43. We confirmed that Cx43 competes with Smad2/3 for binding to MTs, which Cx43 specifically induces release of Smad2/3 from MTs and increases phospho-Smad2 and which, as a result, Smad2/3 and Smad4 are accumulated in the nucleus, leading to activation of the transcription of target genes. Consistently, knockdown of the endogenous Cx43 activity with double-strand RNA (dsRNA) in HL1 cardiomyocytes and Cx43 knockout mice cardiomyocytes consistently show the opposite effect. Our findings demonstrate a novel mechanism for Cx43 positive regulation of TGF-β function.

Recent studies have revealed that a combination of chemical compounds enables direct reprogramming from one somatic cell type into another without the use of transgenes by regulating cellular signaling pathways and epigenetic modifications. The generation of induced pluripotent stem (iPS) cells generally requires virus vector-mediated expression of multiple transcription factors, which might disrupt genomic integrity and proper cell functions. The direct reprogramming is a promising alternative to rapidly prepare different cell types by bypassing the pluripotent state. Because the strategy also depends on forced expression of exogenous lineage-specific transcription factors, the direct reprogramming in a chemical compound-based manner is an ideal approach to further reduce the risk for tumorigenesis. So far, a number of reported research efforts have revealed that combinations of chemical compounds and cell-type specific medium transdifferentiate somatic cells into desired cell types including neuronal cells, glial cells, neural stem cells, brown adipocytes, cardiomyocytes, somatic progenitor cells, and pluripotent stem cells. These desired cells rapidly converted from patient-derived autologous fibroblasts can be applied for their own transplantation therapy to avoid immune rejection. However, complete chemical compound-induced conversions remain challenging particularly in adult human-derived fibroblasts compared with mouse embryonic fibroblasts (MEFs). This review summarizes up-to-date progress in each specific cell type and discusses prospects for future clinical application toward cell transplantation therapy.

Brown fats specialize in thermogenesis by increasing the utilization of circulating blood glucose and fatty acids. Emerging evidence suggests that brown adipose tissue (BAT) prevents the incidence of obesity-associated metabolic diseases and several types of cancers in humans. Mitochondrial energy metabolism in brown/beige adipocytes regulates both uncoupling protein 1 (UCP1)-dependent and -independent thermogenesis for cold adaptation and the utilization of excess nutrients and energy. Many studies on the quantification of human BAT indicate that mass and activity are inversely correlated with the body mass index (BMI) and visceral adiposity. Repression is caused by obesity-associated positive and negative factors that control adipocyte browning, de novo adipogenesis, mitochondrial energy metabolism, UCP1 expression and activity, and noradrenergic response. Systemic and local factors whose levels vary between lean and obese conditions include growth factors, inflammatory cytokines, neurotransmitters, and metal ions such as selenium and iron. Modulation of obesity-associated repression in human brown fats is a promising strategy to counteract obesity and related metabolic diseases through the activation of thermogenic capacity. In this review, we highlight recent advances in mitochondrial metabolism, thermogenic regulation of brown fats, and human metabolic diseases.

Direct conversion of mammalian fibroblasts into induced neuronal (iN) cells has been attained by forced expression of pro-neural transcriptional factors, or by combining defined factors with either microRNAs or small molecules. Here, we show that neuronal cells can be converted from postnatal human fibroblasts into cell populations with neuronal purities of up to >80% using a combination of six chemical compounds. The chemical compound-induced neuronal cells (CiNCs) express neuron-specific proteins and functional neuron markers. The efficiency of CiNCs is unaffected by either the donor's age or cellular senescence (passage number). We propose this chemical direct converting strategy as a potential approach for highly efficient generation of neuronal cells from human fibroblasts for such uses as in neural disease modeling and regenerative medicine.

Brown adipocytes play an important role in human energy metabolism and prevention of obesity and diabetes. Induced pluripotent stem cells (iPSCs) represent a promising source for brown adipocytes; however, exogenous gene induction is generally required for iPSCs generation, which might cause undesired effects particularly in long-term treatment after transplantation. We have previously reported a cocktail of six small chemical compounds that enables a conversion of human fibroblasts into chemical compound-induced neuronal cells (CiNCs). Here, we report that modified combinations of the chemical compounds and rosiglitazone, a PPARγ agonist, afforded direct conversion of human fibroblasts into brown adipocytes. The chemical compound-induced brown adipocytes (ciBAs) exhibit induction of human brown adipocyte-specific genes such as Ucp1, Ckmt1, Cited1 and other adipocyte-specific genes such as Fabp4, AdipoQ, and Pparγ. Treatment with either isoproterenol or Forskolin further induced the expression of Ucp1, suggesting that β adrenergic receptor signalling in ciBAs could be functional for induction of thermogenic genes. Moreover, oxygen consumption rates were elevated in ciBAs along with increase of cellular mitochondria. Our findings might provide an easily accessible approach for generating human brown adipocytes from fibroblasts and offer therapeutic potential for the management of obesity, diabetes, and related metabolic disorders.

Human brown fat is a potential therapeutic target for preventing obesity and related metabolic diseases by dissipating energy as heat through uncoupling protein 1 (UCP1). We have previously reported a method to obtain chemical compound-induced brown adipocytes (ciBAs) converted from human dermal fibroblasts under serum-free conditions. However, pharmacological responses to bioactive molecules have been poorly characterised in ciBAs. This study showed that the treatment with Capsaicin, an agonist of transient receptor potential vanilloid 1, directly activated adipocyte browning such as UCP1 expression, mitochondrial biogenesis, energy consumption rates, and glycerol recycling in ciBAs. Furthermore, genome-wide transcriptome analysis indicated that Capsaicin activated a broad range of metabolic genes including glycerol kinase and glycerol 3-phosphate dehydrogenase 1, which could be associated with the activation of glycerol recycling and triglyceride synthesis. Capsaicin also activated UCP1 expression in immortalised human brown adipocytes but inhibited its expression in mesenchymal stem cell-derived adipocytes. Altogether, ciBAs successfully reflected the direct effects of Capsaicin on adipocyte browning. These findings suggested that ciBAs could serve as a promising cell model for screening of small molecules and dietary bioactive compounds targeting human brown adipocytes.