JNK1 and JNK2 regulate α‐SMA in hepatic stellate cells during CCl4‐induced fibrosis in the rat liver

Following liver injuries, hepatic stellate cells (HSCs) express α‐SMA. Mitogen activated protein kinase (MAPK) signaling pathways mediate α‐SMA expression in distinct cell types. However, the regulation of α‐SMA expression by MAPKs in HSCs has been rarely studied. We aimed to study the role of MAPKs in the activation of HSCs during liver fibrosis. Liver fibrosis of rats was induced by carbon tetrachloride. HSC‐T6 cells, murine embryonic fibroblasts, JNK1^?/? and JNK2^?/? cells were used for in vitro studies. Immunohistochemistry and immunoblot analysis were used. We have found that the expression of JNK and α‐SMA co‐localized in HSCs during liver fibrosis, but ERK and p38 expressed in macrophages. The expression of α‐SMA was up‐regulated by JNK1 and JNK2 in non‐stress condition. Under TGF‐β stimulation, however, the level α‐SMA expression was increased by only JNK1, but not significantly changed by JNK2. We suggest that JNKs are responsible for α‐SMA regulation, and especially JNK1 has a major role in up‐regulation of α‐SMA expression in HSCs under stress condition induced by TGF‐β during liver fibrosis.     

Mechanism of interleukin‐1β‐induced proliferation in rat hepatic stellate cells from different levels of signal transduction

Hepatic stellate cells (HSCs) are the major producers of collagen in the liver. Their conversion from resting cells to proliferative, contractile, and activated cells is a critical step leading to liver fibrosis that is characterized by the deposition of excessive extracellular matrix. Interleukin‐1 (IL‐1) may play a role in maintaining HSC in a proliferative state that is responsible for hepatic fibrogenesis. The aim of this study was to study the roles of the IL‐1 type I receptor (IL‐1R1), c‐Jun N‐terminal kinase (JNK), and activation protein‐1 (AP‐1) in IL‐1β–mediated proliferation in rat HSCs. We showed that IL‐1β can upregulate proliferation in rat HSCs; however, inhibition of the JNK pathway could inhibit HSCs proliferation. Furthermore, IL‐1β activated IL‐1R1 expression, the JNK signaling pathway, and AP‐1 activity in a time‐dependent manner in rat HSCs. These data demonstrate that IL‐1β could promote the proliferation of rat HSCs and that the IL‐1R1, JNK, and AP‐1 pathways were involved in this process. In summary, IL‐1β‐induced proliferation is possibly mediated by the IL‐1R1, JNK, and AP‐1 pathways in rat HSCs. Therefore, drugs that block these pathways may inhibit the proliferation of HSCs and suppress liver fibrosis.     

Reversible phospho‐Smad3 signalling between tumour suppression and fibrocarcinogenesis in chronic hepatitis B infection

Transforming growth factor (TGF)‐β, type I receptor (TβRI) and c‐Jun N‐terminal kinases (JNK) phosphorylate Smad3 differentially to create 2 isoforms phosphorylated (p) at the COOH‐terminus (C) or at the linker region (L) and regulate hepatocytic fibrocarcinogenesis. This study aimed to compare the differences between how hepatitis B virus (HBV) infection affected hepatocytic Smad3 phosphorylated isoforms before and after anti‐viral therapy. To clarify the relationship between Smad3 phosphorylation and liver disease progression, we studied 10 random patients in each stage of HBV‐related fibrotic liver disease (F1–4) and also 10 patients with HBV‐associated HCC. To examine changes in phosphorylated Smad3 signalling before and after anti‐HBV therapies, we chose 27 patients with chronic hepatitis B who underwent baseline and follow‐up biopsies at 52 weeks from the start of nucleoside analogue treatments (Lamivudine 100 mg daily or Telbivudine 600 mg daily). Fibrosis stage, inflammatory activity and phosphorylated Smad3 positivity in the paired biopsy samples were compared. Hepatocytic pSmad3C signalling shifted to fibrocarcinogenic pSmad3L signalling as the livers progressed from chronic hepatitis B infection to HCC. After nucleoside analogue treatment, serum alanine aminotransferase (ALT) and HBV‐DNA levels in 27 patients with HBV‐related chronic liver diseases were decreased dramatically. Decrease in HBV‐DNA restored pSmad3C signalling in hepatocytes, while eliminating prior fibrocarcinogenic pSmad3L signalling. Oral nucleoside analogue therapies can suppress fibrosis and reduce HCC incidence by successfully reversing phosphorylated Smad3 signalling; even liver disease progressed to cirrhosis in chronic hepatitis B patients.     

Over‐expression of prothymosin‐α antagonizes TGFβ signalling to promote the development of emphysema

Emphysema, a major consequence of chronic obstructive pulmonary disease (COPD), is characterized by the permanent airflow restriction resulting from enlargement of alveolar airspace and loss of lung elasticity. Transforming growth factor‐β (TGFβ) signalling regulates the balance of matrix metalloproteinase (MMP)/tissue inhibitor of matrix metalloproteinase (TIMP) to control matrix homeostasis. Patients with COPD have dysregulated TGFβ signalling and reduced histone deacetylase (HDAC) activity through epigenetic up‐regulation of histone acetylation in the promoters of pro‐inflammatory genes. However, the potential link between decreased HDAC activity and dysregulated TGFβ signalling in emphysema pathogenesis remains to be determined. Prothymosin α (ProT), a highly conserved acidic nuclear protein, plays a role in the acetylation of histone and non‐histone proteins. The aim of this study was to test the hypothesis that ProT inhibits TGFβ–Smad signalling through Smad7, thereby contributing to emphysema pathogenesis. We show that ProT enhances Smad7 acetylation by decreasing its association with HDAC and thereby down‐regulates TGFβ–Smad signalling. ProT caused an imbalance between MMP and TIMP through acetylated Smad7 in favour of MMP expression. In addition to interfering with R‐Smad activation and targeting receptors for degradation in the cytoplasm, acetylated Smad7 potentiated by ProT competitively antagonized binding of the pSmad2/3–Smad4 complex to the TIMP‐3 promoter, resulting in reduced TIMP‐3 expression. These effects were detected in ProT‐over‐expressing cells, lungs of ProT transgenic mice displaying an emphysema phenotype and in emphysema patients. Importantly, increased Smad7 and reduced TIMP‐3 were found in the lungs of emphysema patients and mice with cigarette smoke extract (CSE)‐induced emphysema. Such effects could be abrogated by silencing endogenous ProT expression. Collectively, our results uncover acetylated Smad7 regulated by ProT as an important determinant in dysregulated TGFβ signalling that contributes to emphysema pathogenesis. Copyright © 2015 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

大鼠肝纤维化过程中4种肝脏细胞对纤溶系统的调节

目的:探讨在大鼠肝纤维化发展过程中,肝细胞、肝星状细胞(HSCs)、Kupffer细胞和内皮细胞(ECs)对纤溶系统的调节作用。方法:采用1周2次皮下注射50%CCl4、持续12周的方法制备大鼠慢性肝纤维化模型;将肝纤维化不同时期的4种肝脏细胞进行分离;采用Northern印迹法及Western印迹法分别测定大鼠各类肝脏细胞中尿激酶型纤溶酶原激活物(uPA)、以及它的抑制物(PAI-1)和受体(uPAR)的mRNA和蛋白表达。结果:Northern印迹法及Western印迹法分析均显示,在大鼠肝纤维化过程中,HSCs和ECs均为PAI-1、uPAR的主要产生细胞,相比之下,HSCs产生更多。结论:在大鼠肝纤维化发生发展过程中,HSCs和ECs对纤溶系统成分的产生起着重要的调节作用,而HSCs则是PAI-1、uPAR最主要的产生细胞。     

Elevated TGFβ–Smad signalling in experimental Pkd1 models and human patients with polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a common inherited renal disease characterized by many fluid‐filled cysts and interstitial fibrosis in the kidneys, leading to chronic renal failure. During cystogenesis the renal tubules undergo extensive structural alterations that are accompanied by altered cellular signalling, directly and/or indirectly regulated by the PKD1 and PKD2 proteins. Since transforming growth factor (TGF)‐β signalling modulates cell proliferation, differentiation, apoptosis, adhesion and migration of various cell types, we studied the activation of this signalling pathway in Pkd1‐mutant mouse models at different stages of the disease. Therefore, we analysed expression of the TGFβ–Smad signalling pathway and its target genes in different Pkd1 mutant mouse models in various stages of polycystic disease. Nuclear accumulation of P‐Smad2 in cyst lining epithelial cells was not observed in the initiation phase but was observed at mild and more advanced stages of PKD. This coincides with mild fibrosis and increased mRNA levels of TGFβ target genes, such as fibronectin, collagen type I, plasminogen activator inhibitor 1 and matrix metalloproteinase‐2. At this stage many interstitial fibroblasts were found around cysts, which also showed nuclear localization for P‐Smad2. However, bone morphogenetic protein (BMP) signalling, which can antagonize TGFβ signalling, is not affected, since nuclear expression of P‐Smad1/5/8 and expression of the BMP target gene, inhibitor of DNA binding/differential‐1 (ID‐1) is not altered compared to wild‐type controls. Also, human kidneys with progressive ADPKD showed increased nuclear localization of P‐Smad2, while in general expression of P‐Smad1/5/8 was weak. These results exclude TGFβ signalling at the initiation of cystogenesis, but indicate an important role during cyst progression and in fibrogenesis of progressive ADPKD. Copyright © 2010 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

Role of c-Jun N-terminal kinase and p38/activation protein-1 in interleukin-1β-mediated type I collagen synthesis in rat hepatic stellate cells

Interleukin-1 (IL-1) may play a role in maintaining hepatic stellate cell (HSC) in activated state that is responsible for hepatic fibrogenesis. However, the signal transduction pathway that is stimulated by IL-1 in HSC remains to be fully elucidated. The aims of this study were to investigate the role of c-Jun N-terminal kinase (JNK) and p38/activation protein (AP-1) in IL-1β-mediated type I collagen synthesis in rat HSCs. Here, we show that IL-1β could activate JNK and p38 in a time-dependent manner, and that inhibition of the JNK pathway could increase collagen synthesis; however, inhibition of the p38 pathway could inhibit collagen synthesis. Furthermore, IL-1β activated AP-1 in a time-dependent manner in rat HSCs. These data demonstrate that L-1β could promote the synthesis of type I collagen in rat HSCs, and the JNK and p38/AP-1 pathways were involved in this process. In summary, IL-1β-induced collagen synthesis is possibly mediated by cytoplasmic JNK and p38/AP-1 pathways. Therefore, drugs that block the p38/AP-1 pathway may inhibit liver extracellular matrix synthesis and suppress liver fibrosis.     

CHOP‐mediated hepcidin suppression modulates hepatic iron load

The liver is the central regulator of iron metabolism and accordingly, chronic liver diseases often lead to systemic iron overload due to diminished expression of the iron‐regulatory hormone hepcidin. To study the largely unknown regulation of iron metabolism in the context of hepatic disease, we used two established models of chronic liver injury, ie repeated carbon tetrachloride (CCl₄) or thioacetamide (TAA) injections. To determine the impact of CCAAT/enhancer‐binding protein (C/EBP)‐homologous protein (CHOP) on hepcidin production, the effect of a single TAA injection was determined in wild‐type and CHOP knockout mice. Furthermore, CHOP and hepcidin expression was assessed in control subjects and patients with alcoholic liver disease. Both chronic injury models developed a distinct iron overload in macrophages. TAA‐, but not CCl₄‐ injected mice displayed additional iron accumulation in hepatocytes, resulting in a significant hepatic and systemic iron overload which was due to suppressed hepcidin levels. C/EBPα signalling, a known hepcidin inducer, was markedly inhibited in TAA mice, due to lower C/EBPα levels and overexpression of CHOP, a C/EBPα inhibitor. A single TAA injection resulted in a long‐lasting (> 6 days) suppression of hepcidin levels and CHOP knockouts (compared to wild‐types) displayed significantly attenuated hepcidin down‐regulation in response to acute TAA administration. CHOP mRNA levels increased 5‐fold in alcoholic liver disease patients versus controls (p < 0.005) and negatively correlated with hepcidin expression. Our results establish CHOP as an important regulator of hepatic hepcidin expression in chronic liver disease. The differences in iron metabolism between the two widely used fibrosis models likely reflect the differential regulation of hepcidin expression in human liver disease. Copyright © 2013 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

AMPKα2 reduces renal epithelial transdifferentiation and inflammation after injury through interaction with CK2β

TGFβ1/Smad, Wnt/β‐catenin and snail1 are preferentially activated in renal tubular epithelia after injury, leading to epithelial–mesenchymal transition (EMT). The stress response is coupled to EMT and kidney injury; however, the underlying mechanism of the stress response in EMT remains elusive. AMP‐activated protein kinase (AMPK) signalling is responsive to stress and regulates cell energy balance and differentiation. We found that knockdown of AMPKα, especially AMPKα2, enhanced EMT by up‐regulating β‐catenin and Smad3 in vitro. AMPKα2 deficiency enhanced EMT and fibrosis in a murine unilateral ureteral obstruction (UUO) model. AMPKα2 deficiency also increased the expression of chemokines KC and MCP‐1, along with enhanced infiltration of inflammatory cells into the kidney after UUO. CK2β interacted physically with AMPKα and enhanced AMPKα Thr172 phosphorylation and its catalytic activity. Thus, activated AMPKα signalling suppresses EMT and secretion of chemokines in renal tubular epithelia through interaction with CK2β to attenuate renal injury. Copyright © 2015 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

TGFβ pathway deregulation and abnormal phospho‐SMAD2/3 staining in hereditary cerebral hemorrhage with amyloidosis‐Dutch type

Hereditary cerebral hemorrhage with amyloidosis‐Dutch type (HCHWA‐D) is an early onset hereditary form of cerebral amyloid angiopathy (CAA) pathology, caused by the E22Q mutation in the amyloid β (Aβ) peptide. Transforming growth factor β1 (TGFβ1) is a key player in vascular fibrosis and in the formation of angiopathic vessels in transgenic mice. Therefore, we investigated whether the TGFβ pathway is involved in HCHWA‐D pathogenesis in human postmortem brain tissue from frontal and occipital lobes. Components of the TGFβ pathway were analyzed with quantitative RT‐PCR. TGFβ1 and TGFβ Receptor 2 (TGFBR2) gene expression levels were significantly increased in HCHWA‐D in comparison to the controls, in both frontal and occipital lobes. TGFβ‐induced pro‐fibrotic target genes were also upregulated. We further assessed pathway activation by detecting phospho‐SMAD2/3 (pSMAD2/3), a direct TGFβ down‐stream signaling mediator, using immunohistochemistry. We found abnormal pSMAD2/3 granular deposits specifically on HCHWA‐D angiopathic frontal and occipital vessels. We graded pSMAD2/3 accumulation in angiopathic vessels and found a positive correlation with the CAA load independent of the brain area. We also observed pSMAD2/3 granules in a halo surrounding occipital vessels, which was specific for HCHWA‐D. The result of this study indicates an upregulation of TGFβ1 in HCHWA‐D, as was found previously in AD with CAA pathology. We discuss the possible origins and implications of the TGFβ pathway deregulation in the microvasculature in HCHWA‐D. These findings identify the TGFβ pathway as a potential biomarker of disease progression and a possible target of therapeutic intervention in HCHWA‐D.     

Ligand-independent transforming growth factor-β type I receptor signalling mediates type I collagen-induced epithelial-mesenchymal transition

Evidence suggests epithelial-mesenchymal transition (EMT) as one potential source of fibroblasts in idiopathic pulmonary fibrosis. To assess the contribution of alveolar epithelial cell (AEC) EMT to fibroblast accumulation in vivo following lung injury and the influence of extracellular matrix on AEC phenotype in vitro, Nkx2.1-Cre;mT/mG mice were generated in which AECs permanently express green fluorescent protein (GFP). On days 17-21 following intratracheal bleomycin administration, ~4% of GFP-positive epithelial-derived cells expressed vimentin or α-smooth muscle actin (α-SMA). Primary AECs from Nkx2.1-Cre;mT/mG mice cultured on laminin-5 or fibronectin maintained an epithelial phenotype. In contrast, on type I collagen, cells of epithelial origin displayed nuclear localization of Smad3, acquired spindle-shaped morphology, expressed α-SMA and phospho-Smad3, consistent with activation of the transforming growth factor-β (TGFβ) signalling pathway and EMT. α-SMA induction and Smad3 nuclear localization were blocked by the TGFβ type I receptor (TβRI, otherwise known as Alk5) inhibitor SB431542, while AEC derived from Nkx2.1-Cre;Alk5(flox/KO) mice did not undergo EMT on collagen, consistent with a requirement for signalling via Alk5 in collagen-induced EMT. Inability of a pan-specific TGFβ neutralizing antibody to inhibit effects of collagen together with absence of active TGFβ in culture supernatants is consistent with TGFβ ligand-independent activation of Smad signalling. These results support the notion that AECs can acquire a mesenchymal phenotype following injury in vivo and implicate type I collagen as a key regulator of EMT in AECs through signalling via Alk5, likely in a TGFβ ligand-independent manner.     

TGF‐β indirectly favors the development of human Th17 cells by inhibiting Th1 cells

Human Th17 clones and circulating Th17 cells showed lower susceptibility to the anti‐proliferative effect of TGF‐β than Th1 and Th2 clones or circulating Th1‐oriented T cells, respectively. Accordingly, human Th17 cells exhibited lower expression of clusterin, and higher Bcl‐2 expression and reduced apoptosis in the presence of TGF‐β, in comparison with Th1 cells. Umbilical cord blood naïve CD161⁺CD4⁺ T cells, which contain the precursors of human Th17 cells, differentiated into IL‐17A‐producing cells only in response to IL‐1β plus IL‐23, even in serum‐free cultures. TGF‐β had no effect on constitutive RORγt expression by umbilical cord blood CD161⁺ T cells but it increased the relative proportions of CD161⁺ T cells differentiating into Th17 cells in response to IL‐1β plus IL‐23, whereas under the same conditions it inhibited both T‐bet expression and Th1 development. These data suggest that TGF‐β is not critical for the differentiation of human Th17 cells, but indirectly favors their expansion because Th17 cells are poorly susceptible to its suppressive effects.     

The small heat‐shock protein αB‐crystallin is essential for the nuclear localization of Smad4: impact on pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a devastating disease characterized by the proliferation of myofibroblasts and the accumulation of extracellular matrix (ECM) in the lungs. TGF‐β1 is the major profibrotic cytokine involved in IPF and is responsible for myofibroblast proliferation and differentiation and ECM synthesis. αB‐crystallin is constitutively expressed in the lungs and is inducible by stress, acts as a chaperone and is known to play a role in cell cytoskeleton architecture homeostasis. The role of αB‐crystallin in fibrogenesis remains unknown. The principal signalling pathway involved in this process is the Smad‐dependent pathway. We demonstrate here that αB‐crystallin is strongly expressed in fibrotic lung tissue from IPF patients and in vivo rodent models of pulmonary fibrosis. We also show that αB‐crystallin‐deficient mice are protected from bleomycin‐induced fibrosis. Similar protection from fibrosis was observed in αB‐crystallin KO mice after transient adenoviral‐mediated over‐expression of IL‐1β or TGF‐β1. We show in vitro in primary epithelial cells and fibroblasts that αB‐crystallin increases the nuclear localization of Smad4, thereby enhancing the TGF‐β1–Smad pathway and the consequent activation of TGF‐β1 downstream genes. αB‐crystallin over‐expression disrupts Smad4 mono‐ubiquitination by interacting with its E3–ubiquitin ligase, TIF1γ, thus limiting its nuclear export. Conversely, in the absence of αB‐crystallin, TIF1γ can freely interact with Smad4. Consequently, Smad4 mono‐ubiquitination and nuclear export are favoured and thus TGF‐β1–Smad4 pro‐fibrotic activity is inhibited. This study demonstrates that αB‐crystallin may be a key target for the development of specific drugs in the treatment of IPF or other fibrotic diseases. Copyright © 2013 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

Loss of miR‐101 expression promotes Wnt/β‐catenin signalling pathway activation and malignancy in colon cancer cells

Colorectal cancer (CRC) is the second leading cause of cancer‐related mortality in Western countries. Although the aberrant expression of several microRNAs (oncomiRs) is associated with CRC progression, the molecular mechanisms of this phenomenon are still under investigation. Here we show that miR‐101 expression is differentially impaired in CRC specimens, depending on tumour grade. miR‐101 re‐expression suppresses cell growth in 3D, hypoxic survival and invasive potential in CRC cells showing low levels of miR‐101. Additionally, we provide molecular evidence of a bidirectional regulatory mechanism between miR‐101 expression and important CRC pro‐malignant features, such as inflammation, activation of the Wnt/β‐catenin signalling pathway and epithelial–mesenchymal transition (EMT). We then propose that up‐regulated miR‐101 may function as a tumour suppressor in CRC and that its pharmacological restoration might hamper the aggressive behaviour of CRC in vivo. MiR‐101 expression may also represent a cancer biomarker for CRC diagnosis and prognosis. Copyright © 2012 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

MicroRNA‐214 exerts a Cardio‐protective effect by inhibition of fibrosis

The miRNAs play important roles in regulating myocardial fibrosis. The purpose of this study was to determine the potential roles of microRNA‐214 (miR‐214) in cardiac fibrosis in vitro and in vivo. In vitro experiment, Ang II‐induced cardiac fibroblasts (CFBs) are transfected with pre‐miR‐214, anti‐miR‐214 and their oligo controls. Gene expression was checked by Quantitative realtime‐PCR (qRT‐PCR) and western blotting. In the present experiment, compared with controls, expressions of collagen type I (COL I), collagen type III (COL III), transforming growth factor (TGF)‐β1, and tissue inhibitors of metalloproteinase (TIMP)‐1 were all increased, but matrix metalloproteinase (MMP)‐1 was reduced in CFB by Ang II treatment at both mRNA and protein levels, and these alterations were found reversed by miR‐214 transfection. In vivo, an anterior transmural acute myocardial infarction (AMI) was created by occlusion of the left anterior descending coronary artery after Ad‐pre‐miR‐214, Ad‐anti‐miR‐214 or Ad‐GFP was delivered separately. Four weeks after AMI, protein contents of COL I, COL III and TGF‐β1 in tissue from border area were found increased after AMI, but impaired by overexpression of miR‐214. While the expression of MMP‐1 was increased by miR‐214 stimulation but decreased by miR‐214 inhibition. These results suggested that miR‐214 exerts cardio‐protective effects by inhibition of fibrosis and the inhibitory effect involves TGF‐β1 suppression and MMP‐1/TIMP‐1 regulation. Anat Rec, 299:1348–1357, 2016. © 2016 Wiley Periodicals, Inc.     

Novel single nucleotide polymorphisms in the 3′‐UTR of the TGFβRI and TGFβRIII genes

Transforming growth factor beta (TGFβ) family members are multifunctional cytokines that play a key role in cellular growth, proliferation and differentiation. Transmembrane signalling by TGFβ occurs via a complex of the serine/threonine kinases TGFβ type 1 (TGFβRI), type 2 (TGFβRII), and type 3 (TGFβRIII) receptors. Previous studies have implicated TGFβ receptors (TGFβR) in a variety of important hereditary clinical disorders. Mutations of the TGFβR genes have been observed in several human cancers. The aim of this study was to identify and confirm novel single nucleotide polymorphisms (SNPs) in TGFβRI and RIII and to determine the relative allele and genotype frequencies of these SNPs. SNPs were identified from the examination of sequence alignments held in databases and were confirmed by DNA sequencing. A polymerase chain reaction–restriction fragment length polymorphism (PCR‐RFLP) method was devised for genotyping TGFβ receptor polymorphisms. DNA samples from 91 controls were examined. The observed heterozygosities of TGFβRI and TGFβRIII gene polymorphisms in the control population were 43 and 33%, respectively, suggesting these SNPs could be useful markers in disease association studies.     

MicroRNAs in metabolism

MicroRNAs (miRNAs) have within the past decade emerged as key regulators of metabolic homoeostasis. Major tissues in intermediary metabolism important during development of the metabolic syndrome, such as β‐cells, liver, skeletal and heart muscle as well as adipose tissue, have all been shown to be affected by miRNAs. In the pancreatic β‐cell, a number of miRNAs are important in maintaining the balance between differentiation and proliferation (miR‐200 and miR‐29 families) and insulin exocytosis in the differentiated state is controlled by miR‐7, miR‐375 and miR‐335. MiR‐33a and MiR‐33b play crucial roles in cholesterol and lipid metabolism, whereas miR‐103 and miR‐107 regulates hepatic insulin sensitivity. In muscle tissue, a defined number of miRNAs (miR‐1, miR‐133, miR‐206) control myofibre type switch and induce myogenic differentiation programmes. Similarly, in adipose tissue, a defined number of miRNAs control white to brown adipocyte conversion or differentiation (miR‐365, miR‐133, miR‐455). The discovery of circulating miRNAs in exosomes emphasizes their importance as both endocrine signalling molecules and potentially disease markers. Their dysregulation in metabolic diseases, such as obesity, type 2 diabetes and atherosclerosis stresses their potential as therapeutic targets. This review emphasizes current ideas and controversies within miRNA research in metabolism.