Issue published October 15, 2001

A nimals with mutations in the leptin receptor (ObR) exhibit an obese phenotype that is indistinguishable from that of leptin deficient ob/ob mice. ObR is expressed in many tissues, including brain, and the relative importance of leptin’s effects on central versus peripheral sites has not been resolved. To address this, we generated mice with neuron-specific (ObR^SynIKO) and hepatocyte-specific (ObR^AlbKO) disruption of ObR. Among the ObR^SynIKO mice, the extent of obesity was negatively correlated with the level of ObR in hypothalamus and those animals with the lowest levels of ObR exhibited an obese phenotype. The obese mice with low levels of hypothalamic ObR also show elevated plasma levels of leptin, glucose, insulin, and corticosterone. The hypothalamic levels of agouti-related protein and neuropeptide Y RNA are increased in these mice. These data indicate that leptin has direct effects on neurons and that a significant proportion, or perhaps the majority, of its weight-reducing effects are the result of its actions on brain. To explore possible direct effects of leptin on a peripheral tissue, we also characterized ObR^AlbKO mice. These mice weigh the same as controls and have no alterations in body composition. Moreover, while db/db mice and ObR^SynIKO mice have enlarged fatty livers, ObR^AlbKO mice do not. In summary, these data suggest that the brain is a direct target for the weight-reducing and neuroendocrine effects of leptin and that the liver abnormalities of db/db mice are secondary to defective leptin signaling in the brain.

C ushing syndrome is caused by an excess of adrenocorticotropic hormone (ACTH) production by neuroendocrine tumors, which subsequently results in chronic glucocorticoid excess. We found that retinoic acid inhibits the transcriptional activity of AP-1 and the orphan receptors Nur77 and Nurr1 in ACTH-secreting tumor cells. Retinoic acid treatment resulted in reduced pro-opiomelanocortin transcription and ACTH production. ACTH inhibition was also observed in human pituitary ACTH-secreting tumor cells and a small-cell lung cancer cell line, but not in normal cells. This correlated with the expression of the orphan receptor COUP-TFI, which was found in normal corticotrophs but not in pituitary Cushing tumors. COUP-TFI expression in ACTH-secreting tumor cells blocked retinoic acid action. Retinoic acid also inhibited cell proliferation and, after prolonged treatment, increased caspase-3 activity and induced cell death in ACTH-secreting cells. In adrenal cortex cells, retinoic acid inhibited corticosterone production and cell proliferation. The antiproliferative action and the inhibition of ACTH and corticosterone produced by retinoic acid were confirmed in vivo in experimental ACTH-secreting tumors in nude mice. Thus, we conclude that the effects of retinoic acid combine in vivo to reverse the endocrine alterations and symptoms observed in experimental Cushing syndrome.

T he role of processing in antigen (Ag) presentation and T cell activation in experimental allergic encephalomyelitis (EAE) was evaluated in wild-type mice, mice that selectively express either Ii p31 or p41, and mice completely deficient in Ii or H-2M. We demonstrate that processing of myelin oligodendrocyte glycoprotein (MOG) is required for presentation of the dominant encephalitogenic MOG epitope, p35-55. Ii p31- and p41-expressing mice developed EAE with similar incidence to wild-type mice, although p41 mice had a more severe course. Ag-presenting cells (APCs) from Ii- or H-2M–deficient mice could present p35-55, but not MOG, demonstrating that these APCs could not process native MOG. Ii- and H-2M–deficient mice were not susceptible to EAE by immunization with p35-55 or MOG or by adoptive transfer of encephalitogenic T cells. However, CD4⁺ T cells from p35-55–immunized H-2M–deficient mice proliferated, secreted IFN-γ, and transferred EAE to wild-type, but not H-2M–deficient, mice. Thus, EAE resistance in H-2M–deficient mice is not due to an inability of APCs to present p35-55, or an intrinsic defect in the encephalitogenic T cell repertoire, but reflects a defect in APC function. Our results indicate that processing is required for initial Ag presentation and CNS T cell activation and suggest that autopathogenic peptides of CNS autoantigen may not be readily available for presentation without processing.

H yperplasia of pulmonary artery smooth muscle cells (PA-SMCs) is a hallmark pathological feature of primary pulmonary hypertension (PPH). Here we found that PA-SMCs from patients with PPH grow faster than PA-SMCs from controls when stimulated by serotonin or serum and that these effects are due to increased expression of the serotonin transporter (5-HTT), which mediates internalization of indoleamine. In the presence of 5-HTT inhibitors, the growth stimulatory effects of serum and serotonin were markedly reduced and the difference between growth of PA-SMCs from patients and controls was no longer observed. As compared with controls, the expression of 5-HTT was increased in cultured PA-SMCs as well as in platelets and lungs from patients with PPH where it predominated in the media of thickened pulmonary arteries and in onion-bulb lesions. The L-allelic variant of the 5HTT gene promoter, which is associated with 5-HTT overexpression and increased PA-SMC growth, was present in homozygous form in 65% of patients but in only 27% of controls. We conclude that 5-HTT activity plays a key role in the pathogenesis of PA-SMC proliferation in PPH and that a 5HTT polymorphism confers susceptibility to PPH.

B ullous pemphigoid (BP) is an inflammatory subepidermal blistering disease associated with an IgG autoimmune response to the hemidesmosomal protein BP180. Passive transfer of antibodies to the murine BP180 (mBP180) ectodomain triggers a blistering skin disease in mice that depends on complement activation and neutrophil infiltration and closely mimics human BP. In the present study, we show that mast cells (MCs) play a crucial role in experimental BP. Wild-type mice injected intradermally with pathogenic anti-mBP180 IgG exhibited extensive MC degranulation in skin, which preceded neutrophil infiltration and subsequent subepidermal blistering. In contrast, mice genetically deficient in MCs or MC-sufficient mice pretreated with an inhibitor of MC degranulation failed to develop BP. Further, MC-deficient mice reconstituted in skin with MCs became susceptible to experimental BP. Despite the activation of complement to yield C3a and C5a, in the absence of MCs, accumulation of neutrophils at the injection site was blunted. The lack of response due to MC deficiency was overcome by intradermal administration of a neutrophil chemoattractant, IL-8, or by reconstitution of the injection sites with neutrophils. These findings provide the first direct evidence to our knowledge that MCs play an essential role in neutrophil recruitment during subepidermal blister formation in experimental BP.

S timulation of the hypothalamic-pituitary-adrenal (HPA) axis by proinflammatory cytokines results in increased release of glucocorticoid that restrains further development of the inflammatory process. IL-6 has been suggested to stimulate the HPA axis during immune activation independent of the input of hypothalamic corticotropin-releasing hormone (CRH). We used the corticotropin-releasing hormone–deficient (Crh^+/+) mouse to elucidate the effect of CRH deficiency on IL-6 expression and IL-6–induced HPA axis activation during turpentine-induced inflammation. We demonstrate that during inflammation CRH is required for a normal adrenocorticotropin hormone (ACTH) increase but not for adrenal corticosterone rise. The paradoxical increase of plasma IL-6 associated with CRH deficiency suggests that IL-6 release during inflammation is CRH-dependent. We also demonstrate that adrenal IL-6 expression is CRH-dependent, as its basal and inflammation-induced expression is blocked by CRH deficiency. Our findings suggest that during inflammation, IL-6 most likely compensates for the effects of CRH deficiency on food intake. Finally, we confirm that the HPA axis response is defective in Crh^+/+/IL-6^+/+ mice. These findings, along with the regulation of IL-6 by CRH, support the importance of the interaction between the immune system and the HPA axis in the pathophysiology of inflammatory diseases.

M etformin is a widely used drug for treatment of type 2 diabetes with no defined cellular mechanism of action. Its glucose-lowering effect results from decreased hepatic glucose production and increased glucose utilization. Metformin’s beneficial effects on circulating lipids have been linked to reduced fatty liver. AMP-activated protein kinase (AMPK) is a major cellular regulator of lipid and glucose metabolism. Here we report that metformin activates AMPK in hepatocytes; as a result, acetyl-CoA carboxylase (ACC) activity is reduced, fatty acid oxidation is induced, and expression of lipogenic enzymes is suppressed. Activation of AMPK by metformin or an adenosine analogue suppresses expression of SREBP-1, a key lipogenic transcription factor. In metformin-treated rats, hepatic expression of SREBP-1 (and other lipogenic) mRNAs and protein is reduced; activity of the AMPK target, ACC, is also reduced. Using a novel AMPK inhibitor, we find that AMPK activation is required for metformin’s inhibitory effect on glucose production by hepatocytes. In isolated rat skeletal muscles, metformin stimulates glucose uptake coincident with AMPK activation. Activation of AMPK provides a unified explanation for the pleiotropic beneficial effects of this drug; these results also suggest that alternative means of modulating AMPK should be useful for the treatment of metabolic disorders.

T he transplantation of neuronal cells and tissues represents a promising approach for the treatment of incurable neurodegenerative diseases. Indeed, it has been reported recently that retinal transplantation can rescue photoreceptor cells and delay age-related changes in various retinal layers in rodents. However, retinal grafts deteriorate progressively after placement in recipients’ eyes. Here we investigated whether a host’s immune response elicited toward the graft contributes to its deterioration. Using an ELISA spot assay, we measured T cell responses to retinal tissues placed in the vitreous cavity of syngeneic and allogeneic mice. We found that allogeneic retinas induced potent alloimmune responses mediated by T cells secreting type 1 cytokines (IFN-γ and IL-2). No response was found in mice engrafted with syngeneic retinas. In addition, all syngeneic retinal grafts displayed no signs of tissue damage (at 55 days), while the majority of allogeneic retinas deteriorated as early as 12 days after placement. Next, we showed that anti-donor responses occurred within two phenotypically and functionally distinct T cell subsets: CD4⁺ T cells secreting IL-2 and CD8⁺ T cells producing IFN-γ. Importantly, CD4⁺ T cells were necessary and sufficient to cause graft deterioration, while CD8⁺ T cells did not contribute to this process.

M ultiple lines of evidence suggest that CD4⁺ lymphocytes initiate autoimmune responses against myelin antigens in multiple sclerosis (MS). The increased frequency of activated myelin-specific cells in MS patients indicates that the activation of autoreactive cells represents a central event in the pathogenesis of the disease. We identified a CD4⁺ subpopulation that is characterized phenotypically by the persistent absence of surface CD28 expression and functionally by CD28-independent activation and Th1 cytokine secretion. Owing to their costimulation-independent activation and their expression of a full agonist signaling activation pattern, CD4⁺CD28^– cells have the potential to initiate autoimmune responses in the central nervous system, a compartment devoid of professional antigen presenting cells. Long-term memory CD4⁺CD28^– cells produce high amounts of IFN-γ and maximally upregulate IFN-γ and IL-12Rβ2 chain expression in the absence of costimulation. They exhibit prominent growth characteristics and increased survival after activation, likely related to their persistent lack of CTLA-4 surface expression. The CD4⁺CD28^– population is expanded in a subgroup of MS patients. Myelin basic protein-specific cells detected in this cell subset may play an important role in the inflammatory response within the central nervous system.

W e examined the kenetics of p15 methylation and expression during myeloid development. We treated human cord blood CD34⁺ cells with either GM-CSF alone or in combination with stem cell factor and followed methylation at this locus using bisulfite genomic sequencing. CD34⁺ cells were found to be either fully methylated or completely unmethylated at 27 CpG dinucleotide sites in exon 1 and at 18 CpG sites in the promoter region of the p15 gene. A time-course study showed that the percentage of the allelic methylation of p15 CpG island increased to approximately 50% to 60% until 7 days after cytokine stimulation, then decreased to less than 10% after 21 days. The methylation was also observed in bone marrow CD34⁺ cells exposed to GM-CSF. p15 expression varied inversely with methylation. Expression was negligible or at low levels until 14 days, after which it increased substantially. The frequency of myeloid colony-forming cells in the progeny decreased and myeloid-specific markers increased in the later stages. Based on our observations on cells grown with GM-CSF and 5-aza-2′-deoxycytidine, DNA methylation of the p15 promoter region CpG island appears to be associated with proliferation rather than differentiation of normal human myeloid progenitors.

A lthough insulin regulates metabolism in both brown and white adipocytes, the role of these tissues in energy storage and utilization is quite different. Recombination technology using the Cre-loxP approach allows inactivation of the insulin receptor in a tissue-specific manner. Mice lacking insulin receptors in brown adipocytes show an age-dependent loss of interscapular brown fat but increased expression of uncoupling protein-1 and -2. In parallel, these mice develop an insulin-secretion defect resulting in a progressive glucose intolerance, without insulin resistance. This model provides direct evidence for not only a role for the insulin receptors in brown fat adipogenesis, the data also suggest a novel role of brown adipose tissue in the regulation of insulin secretion and glucose homeostasis.

H ypoparathyroidism is characterized by hypocalcemia, hyperphosphatemia, and absent or markedly reduced circulating concentrations of parathyroid hormone. The transcription factor GCMB is predominantly, if not exclusively, expressed in parathyroid cells and is critical for development of the parathyroid glands in mice. Thus, in the present study we examined the GCMB gene, mapped to 6p23–24, as a candidate for isolated hypoparathyroidism. We defined the boundaries of the five exons of the human GCMB gene and then identified a large intragenic mutation in the GCMB genes of the proband of an extensive kindred with isolated hypoparathyroidism. Her parents and several other unaffected relatives were heterozygous for the mutation. Despite an absence of any history of consanguinity, microsatellite analysis showed shared genotypes that flanked the GCMB gene over a span of 5 cM, suggesting that both of the proband’s GCMB alleles had been derived from a single common ancestor. Analysis of additional, unrelated cases did not disclose the same mutation. We conclude that homozygous loss of function of the GCMB gene impairs normal parathyroid gland embryology and is responsible for isolated hypoparathyroidism in a subset of patients with this disease.

I nfection with Helicobacter pylori causes chronic gastritis, which is characterized by a dense mucosal infiltration by inflammatory cells such as monocytes/macrophages. H. pylori–induced inflammation is a risk factor for the development of gastric adenocarcinoma, but the mechanisms involved in H. pylori–associated carcinogenesis are poorly understood. A cecropin-like H. pylori peptide, Hp(2-20), was found to be a monocyte chemoattractant and activated the monocyte NADPH-oxidase to produce oxygen radicals. The receptors mediating monocyte activation were identified as FPRL1 and the monocyte-specific orphan receptor FPRL2. Hp(2-20)–activated monocytes inhibited lymphocytes with antitumor properties, such as CD56⁺ natural killer (NK) cells and CD3ε⁺ T cells. The changes observed in NK cells and T cells — a reduced antitumor cytotoxicity, downregulation of CD3ζ expression, and apoptosis — were mediated by Hp(2-20)–induced oxygen radicals. Histamine, a gastric mucosal constituent, rescued NK cells and T cells from inhibition and apoptosis by suppressing Hp(2-20)–induced oxygen radical formation. We conclude that H. pylori expression of this monocyte-activating peptide contributes to its ability to attract and activate monocytes and reduces the function and viability of antineoplastic lymphocytes. These novel mechanisms may be subject to local, histaminergic regulation in the gastric mucosa.

P roduction of prostaglandin E₂ (PGE₂) is enhanced during inflammation, and this lipid mediator can dramatically modulate immune responses. There are four receptors for PGE₂ (EP1–EP4) with unique patterns of expression and different coupling to intracellular signaling pathways. To identify the EP receptors that regulate cellular immune responses, we used mouse lines in which the genes encoding each of the four EP receptors were disrupted by gene targeting. Using the mixed lymphocyte response (MLR) as a model cellular immune response, we confirmed that PGE₂ has potent antiproliferative effects on wild-type responder cells. The absence of either the EP1 or EP3 receptors did not alter the inhibitory response to PGE₂ in the MLR. In contrast, when responder cells lacked the EP2 receptor, PGE₂ had little effect on proliferation. Modest resistance to PGE₂ was also observed in EP4^–/– responder cells. Reconstitution experiments suggest that EP2 receptors primarily inhibit the MLR through direct actions on T cells. Furthermore, PGE₂ modulates macrophage function by activating the EP4 receptor and thereby inhibiting cytokine release. Thus, PGE₂ regulates cellular immune responses through distinct EP receptors on different immune cell populations: EP2 receptors directly inhibit T cell proliferation while EP2 and EP4 receptors regulate antigen presenting cells functions.