Gyorgy Baffy

Uncoupling protein-2 negatively regulates insulin secretion and is a major link between obesity, beta cell dysfunction, and type 2 diabetes

Abstract β cells sense glucose through its metabolism and the resulting increase in ATP, which subsequently stimulates insulin secretion. Uncoupling protein-2 (UCP2) mediates mitochondrial proton leak, decreasing ATP production. In the present study, we assessed UCP2s role in regulating insulin secretion. UCP2-deficient mice had higher islet ATP levels and increased glucose-stimulated insulin secretion, establishing that UCP2 negatively regulates insulin secretion. Of pathophysiologic significance, UCP2 was markedly upregulated in islets of ob/ob mice, a model of obesity-induced diabetes. Importantly, ob/ob mice lacking UCP2 had restored first-phase insulin secretion, increased serum insulin levels, and greatly decreased levels of glycemia. These results establish UCP2 as a key component of β cell glucose sensing, and as a critical link between obesity, β cell dysfunction, and type 2 diabetes.

Hepatocellular carcinoma in non-alcoholic fatty liver disease: an emerging menace.

Hepatocellular carcinoma (HCC) is a common cancer worldwide that primarily develops in cirrhosis resulting from chronic infection by hepatitis B virus and hepatitis C virus, alcoholic injury, and to a lesser extent from genetically determined disorders such as hemochromatosis. HCC has recently been linked to non-alcoholic fatty liver disease (NAFLD), the hepatic manifestation of obesity and related metabolic disorders such as diabetes. This association is alarming due to the globally high prevalence of these conditions and may contribute to the rising incidence of HCC witnessed in many industrialized countries. There is also evidence that NAFLD acts synergistically with other risk factors of HCC such as chronic hepatitis C and alcoholic liver injury. Moreover, HCC may complicate non-cirrhotic NAFLD with mild or absent fibrosis, greatly expanding the population potentially at higher risk. Major systemic and liver-specific molecular mechanisms involved include insulin resistance and hyperinsulinemia, increased TNF signaling pathways, and alterations in cellular lipid metabolism. These provide new targets for prevention, early recognition, and effective treatment of HCC associated with NAFLD. Indeed, both metformin and PPAR gamma agonists have been associated with lower risk and improved prognosis of HCC. This review summarizes current evidence as it pertains to the epidemiology, pathogenesis, and prevention of NAFLD-associated HCC.

Kupffer cells in non-alcoholic fatty liver disease: the emerging view.

Non-alcoholic fatty liver disease (NAFLD) has become the most common liver disorder of our times. Simple steatosis, a seemingly innocent manifestation of NAFLD, may progress into steatohepatitis and cirrhosis, but this process is not well understood. Since NAFLD is associated with obesity and insulin resistance, mechanisms that link lipid metabolism to inflammation offer insights into the pathogenesis. An important parallel between obesity-related pathology of adipose tissue and liver pertains to the emerging role of macrophages and evidence is growing that Kupffer cells critically contribute to progression of NAFLD. Toll-like receptors, in particular TLR4, represent a major conduit for danger recognition linked to Kupffer cell activation and this process may be perturbed at multiple steps in NAFLD. Steatosis may interfere with sinusoid microcirculation and hepatocellular clearance of microbial and host-derived danger signals, enhancing responsiveness of Kupffer cells. Altered lipid homeostasis in NAFLD may unfavourably affect TLR4 receptor complex assembly and sorting, interfere with signalling flux redistribution, promote amplification loops, and impair negative regulation including alternative activation of Kupffer cells. These events are further promoted by altered adipokine secretion and reactive oxygen species production. Specific targeting of these interactions may provide more effective strategies in the treatment of NAFLD.

The Mitochondrial Uncoupling Protein-2 Promotes Chemoresistance in Cancer Cells

Cancer cells acquire drug resistance as a result of selection pressure dictated by unfavorable microenvironments. This survival process is facilitated through efficient control of oxidative stress originating from mitochondria that typically initiates programmed cell death. We show this critical adaptive response in cancer cells to be linked to uncoupling protein-2 (UCP2), a mitochondrial suppressor of reactive oxygen species (ROS). UCP2 is present in drug-resistant lines of various cancer cells and in human colon cancer. Overexpression of UCP2 in HCT116 human colon cancer cells inhibits ROS accumulation and apoptosis after exposure to chemotherapeutic agents. Tumor xenografts of UCP2-overexpressing HCT116 cells retain growth in nude mice receiving chemotherapy. Augmented cancer cell survival is accompanied by altered NH(2)-terminal phosphorylation of the pivotal tumor suppressor p53 and induction of the glycolytic phenotype (Warburg effect). These findings link UCP2 with molecular mechanisms of chemoresistance. Targeting UCP2 may be considered a novel treatment strategy for cancer.

Expression of Uncoupling Protein-2 in Human Colon Cancer

Purpose: Cancer cell survival depends on adaptive mechanisms that include modulation of oxidative stress. One such mechanism may be via up-regulation of uncoupling protein-2 (UCP2), a mitochondrial inner membrane anion carrier recently found to provide cytoprotection in nontumor cells by acting as a sensor and negative regulator of reactive oxygen species production. We hypothesized that UCP2 expression may be increased in colon cancer as part of tumor adaptation. Experimental Design: UCP2 expression was characterized by real-time polymerase chain reaction and Western blotting using paired human colon adenocarcinoma and peritumoral specimens. Oxidant production was characterized by tissue malondialdehyde levels. Tissue microarrays constructed of 107 colon adenocarcinomas as well as representative specimens of hyperplastic polyps and tubular adenomas were used for UCP2 immunohistochemistry. Results: UCP2 mRNA and protein levels were 3- to 4-fold higher in adenocarcinomas, and UCP2 mRNA levels showed significant correlation with increased tumor tissue malondialdehyde contents. Immunohistochemistry on tissue microarrays showed positive staining for UCP2 in most adenocarcinomas (86.0%); positive staining for UCP2 was seen less often in tubular adenomas (58.8%) and rarely seen in hyperplastic polyps (11.1%). Conclusions: UCP2 expression is increased in most human colon cancers, and the level of expression appears to correlate with the degree of neoplastic changes. These findings may foster the idea that UCP2 is part of a novel adaptive response by which oxidative stress is modulated in colon cancer.

Uncoupling protein‐2 deficiency promotes oxidant stress and delays liver regeneration in mice

The control of liver regeneration remains elusive. Because reactive oxygen species (ROS) are able to mediate cell growth arrest and activate proteins that inhibit the cell cycle, ROS production may have a negative impact on liver regeneration. We examined how liver regeneration is affected by uncoupling protein‐2 (UCP2), an inner mitochondrial membrane carrier that senses and negatively regulates superoxide production. Liver regeneration was monitored up to 5 days and was found to be significantly delayed in UCP2−/− mice after partial hepatectomy. Apoptosis rates in UCP2+/+ and UCP2− /− liver remnants were similar, while parameters of cell proliferation indicated a diminished response in UCP2− /− mice with corresponding changes in the expression of key cell cycle regulatory proteins and prolonged activation of stress‐responsive protein kinase p38. Levels of malondialdehyde, a marker of ROS generation and oxidant stress, were elevated in UCP2− /− livers at every examined time point. Liver remnants of UCP2+ /+ mice 48 hours post‐hepatectomy showed a fourfold increase in the expression of UCP2 protein primarily detected in hepatocytes. In conclusion, our results suggest that absent or insufficient UCP2 function in the regenerating liver results in increased ROS production and negatively modulates the control of cell cycle. (HEPATOLOGY 2004;39:386–392.)

Uncoupling protein-2 and cancer.

Cancer cells respond to unfavorable microenvironments such as nutrient limitation, hypoxia, oxidative stress, and host defense by comprehensive metabolic reprogramming. Mitochondria are linked to this complex adaptive response and emerging evidence indicates that uncoupling protein-2 (UCP2), a mitochondrial inner membrane anion carrier, may contribute to this process. Effects of UCP2 on mitochondrial bioenergetics, redox homeostasis, and oxidant production in cancer cells may modulate molecular pathways of macromolecular biosynthesis, antioxidant defense, apoptosis, cell growth and proliferation, enhancing robustness and promoting chemoresistance. Elucidation of these interactions may identify novel anti-cancer strategies.

Obesity-associated mechanisms of hepatocarcinogenesis

Obesity has been recognized as a key component of the metabolic syndrome, a cluster of risk factors associated with diabetes and cardiovascular morbidity. In addition, obesity has been linked to higher frequency of cancers in a variety of tissues including the liver. Liver cancer most often occurs as hepatocellular carcinoma (HCC) complicating cirrhosis due to chronic viral infection or toxic injury and remains the third leading cause of cancer death in the world. However, HCC is increasingly diagnosed among individuals with obesity and related disorders. As these metabolic conditions have become globally prevalent, they coexist with well-established risk factors of HCC and create a unique challenge for the liver as a chronically diseased organ. Obesity-associated HCC has recently been attributed to molecular mechanisms such as chronic inflammation due to adipose tissue remodeling and pro-inflammatory adipokine secretion, ectopic lipid accumulation and lipotoxicity, altered gut microbiota, and disrupted senescence in stellate cells, as well as insulin resistance leading to increased levels of insulin and insulin-like growth factors. These mechanisms synergize with those occurring in chronic liver disease resulting from other etiologies and accelerate the development of HCC before or after the onset of cirrhosis. Increasingly common interactions between oncogenic pathways linked to obesity and chronic liver disease may explain why HCC is one of the few malignancies with rising incidence in developed countries. Better understanding of this complex process will improve our strategies of cancer prevention, prediction, and surveillance.

UNCOUPLING PROTEIN-2 AND NON-ALCOHOLIC FATTY LIVER DISEASE

Non-alcoholic fatty liver disease (NAFLD) has become the most common form of hepatic disorders in the developed world. NAFLD is part of the metabolic syndrome with insulin resistance as a primary underlying derangement. The natural history of NAFLD may extend from simple steatosis over steatohepatitis into cirrhosis and hepatocellular carcinoma. Among numerous factors shaping these transitions, uncoupling protein-2 (UCP2) may theoretically contribute to every stage of this disease. UCP2 is a recently identified fatty acid-responsive mitochondrial inner membrane carrier protein showing wide tissue distribution with a substantially increased presence in fatty liver. The biological functions of UCP2 are not fully elucidated and the greater part of our current knowledge has been obtained from animal experiments. These data suggest a role for UCP2 in lipid metabolism, mitochondrial bioenergetics, oxidative stress, apoptosis, and even carcinogenesis. Available evidence is reviewed and new concepts are considered to appraise the potential role of UCP2 in the pathogenesis of NAFLD.

Mitochondrial recoupling: a novel therapeutic strategy for cancer?

Recent findings link metabolic transformation of cancer cells to aberrant functions of mitochondrial uncoupling proteins (UCPs). By inducing proton leak, UCPs interfere with mitochondrial synthesis of adenosine 5′-triphosphate, which is also a key determinant of glycolytic pathways. In addition, UCP suppress the generation of superoxide, a byproduct of mitochondrial electron transport and a major source of oxidative stress. The near ubiquitous UCP2 becomes highly abundant in some cancers and may advance metabolic reprogramming, further disrupt tumour suppression, and promote chemoresistance. Here we review current evidence to suggest that inhibition of mitochondrial uncoupling may eliminate these responses and reveal novel anti-cancer strategies.