Reduced expression of the Indy (‘I am Not Dead, Yet’) gene in lower organisms promotes longevity in a manner akin to caloric restriction. Deletion of the mammalian homolog of Indy (mIndy, Slc13a5) encoding for a plasma membrane associated citrate transporter expressed highly in the liver, protects mice from high-fat diet and aging-induced obesity and hepatic fat accumulation through a mechanism resembling caloric restriction. We aimed to study a possible role of mIndy in human hepatic fat metabolism. In obese, insulin resistant patients with NAFLD, hepatic mIndy expression was increased and mIndy expression was also independently associated with hepatic steatosis. In non-human primates, a two year high fat, high sucrose diet increased hepatic mIndy expression. Liver microarray analysis showed that high mIndy expression was associated with pathways involved in hepatic lipid metabolism and immunological processes. Interleukin-6 (IL-6) was identified as a regulator of mIndy by binding to its cognate receptor. Studies in human primary hepatocytes confirmed that IL-6 markedly induced mIndy transcription via the IL-6-receptor (IL-6R) and activation of the transcription factor Stat3 and a putative start site of the human mIndy promoter was determined. Activation of the IL-6-Stat3 pathway stimulated mIndy expression, enhanced cytoplasmic citrate influx and augmented hepatic lipogenesis in vivo. In contrast, deletion of mIndy completely prevented the stimulating effect of IL-6 on citrate uptake and reduced hepatic lipogenesis. These data show that mIndy is increased in liver of obese humans and non-human primates with NALFD. Moreover, our data identify mIndy as a target gene of IL-6 and determine novel functions of IL-6 via mINDY. Targeting human mINDY may have therapeutic potential in obese patients with NAFLD.

Reduced expression of the INDY (I'm not dead yet) tricarboxylate carrier increased the life span in different species by mechanisms akin to caloric restriction. Mammalian INDY homolog (mIndy, SLC13A5) gene expression seems to be regulated by hormonal and/or nutritional factors. The underlying mechanisms are still unknown. The current study revealed that mIndy expression and [14C]-citrate uptake was induced by physiological concentrations of glucagon via a cAMP-dependent and cAMP-responsive element–binding protein (CREB)–dependent mechanism in primary rat hepatocytes. The promoter sequence of mIndy located upstream of the most frequent transcription start site was determined by 5′-rapid amplification of cDNA ends. In silico analysis identified a CREB-binding site within this promoter fragment of mIndy. Functional relevance for the CREB-binding site was demonstrated with reporter gene constructs that were induced by CREB activation when under the control of a fragment of a wild-type promoter, whereas promoter activity was lost after site-directed mutagenesis of the CREB-binding site. Moreover, CREB binding to this promoter element was confirmed by chromatin immunoprecipitation in rat liver. In vivo studies revealed that mIndy was induced in livers of fasted as well as in high-fat-diet–streptozotocin diabetic rats, in which CREB is constitutively activated. mIndy induction was completely prevented when CREB was depleted in these rats by antisense oligonucleotides. Together, these data suggest that mIndy is a CREB-dependent glucagon target gene that is induced in fasting and in type 2 diabetes. Increased mIndy expression might contribute to the metabolic consequences of diabetes in the liver.

Hepatic insulin resistance is a major contributor to fasting hyperglycemia in patients with metabolic syndrome and type 2 diabetes. Circumstantial evidence suggests that cyclooxygenase products in addition to cytokines might contribute to insulin resistance. However, direct evidence for a role of prostaglandins in the development of hepatic insulin resistance is lacking. Therefore, the impact of prostaglandin E 2 (PGE 2 ) alone and in combination with interleukin-6 (IL-6) on insulin signaling was studied in primary hepatocyte cultures. Rat hepatocytes were incubated with IL-6 and/or PGE 2 and subsequently with insulin. Glycogen synthesis was monitored by radiochemical analysis; the activation state of proteins of the insulin receptor signal chain was analyzed by western blot with phosphospecific antibodies. In hepatocytes, insulin-stimulated glycogen synthesis and insulin-dependent phosphorylation of Akt-kinase were attenuated synergistically by prior incubation with IL-6 and/or PGE 2 while insulin receptor autophosphorylation was barely affected. IL-6 but not PGE 2 induced suppressors of cytokine signaling (SOCS3). PGE 2 but not IL-6 activated extracellular signal-regulated kinase 1/2 (ERK1/2) persistently. Inhibition of ERK1/2 activation by PD98059 abolished the PGE 2 -dependent but not the IL-6-dependent attenuation of insulin signaling. In HepG2 cells expressing a recombinant EP3-receptor, PGE 2 pre-incubation activated ERK1/2, caused a serine phosphorylation of insulin receptor substrate 1 (IRS1), and reduced the insulin-dependent Akt-phosphorylation. Conclusion: PGE 2 might contribute to hepatic insulin resistance via an EP3-receptor-dependent ERK1/2 activation resulting in a serine phosphorylation of insulin receptor substrate, thereby preventing an insulindependent activation of Akt and glycogen synthesis. Since different molecular mechanisms appear to be employed, PGE 2 may synergize with IL-6, which interrupted the insulin receptor signal chain, principally by an induction of SOCS, namely SOCS3. (HEPATOLOGY 2009;50: 781-790.)

SummaryBotulinum toxin is a bacterial toxin that inhibits neurotransmitter release from neurons and thereby causes a flaccid paralysis. It is used as drug to treat a number of serious ailments and, more frequently, for aesthetic medical interventions. Botulinum toxin for pharmacological applications is isolated from bacterial cultures. Due to partial denaturation of the protein, the specific activity of these preparations shows large variations. Because of its extreme potential toxicity, pharmacological preparations must be carefully tested for their activity. For the current gold standard, the mouse lethality assay, several hundred thousand mice are killed per year. Alternative methods have been developed that suffer from one or more of the following deficits: In vitro enzyme assays test only the activity of the catalytic subunit of the toxin. Enzymatic and cell based immunological assays are specific for just one of the different serotypes. The current study takes a completely different approach that overcomes these limitations: Neuronal cell lines were stably transfected with plasmids coding for luciferases of different species, which were N-terminally tagged with leader sequences that redirect the luciferase into neuro-secretory vesicles. From these vesicles, luciferases were released upon depolarization of the cells. The depolarization-dependent release was efficiently inhibited by botulinum toxin in a concentration range (1 to 100 pM) that is used in pharmacological preparations. The new assay might thus be an alternative to the mouse lethality assay and the immunological assays already in use.

Despite the implementation of cell-based replacement methods, the mouse lethality assay is still frequently used to determine the activity of botulinum toxin (BoNT) for medical use. One explanation is that due to the use of neoepitope-specific antibodies to detect the cleaved BoNT substrate, the currently devised assays can detect only one specific serotype of the toxin. Recently, we developed a cell-based functional assay, in which BoNT activity is determined by inhibiting the release of a reporter enzyme that is liberated concomitantly with the neurotransmitter from neurosecretory vesicles. In theory, this assay should be suitable to detect the activity of any BoNT serotype. Consistent with this assumption, the current study shows that the stimulus-dependent release of a luciferase from a differentiated human neuroblastoma-based reporter cell line (SIMA-hPOMC1-26-GLuc cells) was inhibited by BoNT-A and-C. Furthermore, this was also inhibited by BoNT-B and tetanus toxin to a lesser extent and at higher concentrations. In order to provide support for the suitability of this technique in practical applications, a dose–response curve obtained with a pharmaceutical preparation of BoNT-A closely mirrored the activity determined in the mouse lethality assay. In summary, the newly established cell-based assay may represent a versatile and specific alternative to the mouse lethality assay and other currently established cell-based assays.