Mathis Grossmann

G Protein-coupled Receptors I. DIVERSITY OF RECEPTOR-LIGAND INTERACTIONS

Nearly 2000 G protein-coupled receptors (GPCRs) have been reported since bovine opsin was cloned in 1983 (1) and the b-adrenergic receptor in 1986 (2). They are classified into over 100 subfamilies according to the sequence homology, ligand structure, and receptor function. A substantial degree of amino acid homology is found among members of a particular subfamily, but comparisons between subfamilies show significantly less or no similarity. Mutations have been observed that relate to a wide spectrum of hereditary and somatic disorders and diseases from cancer to infertility. These mutant receptors are incapable of binding ligand or generating normal signals, constitutively generate signals, or are not appropriately expressed on the cell surface. On the other hand, some mutations are beneficial. For example, a mutation in a chemokine (CCR5) receptor, which is a co-receptor for human immunodeficiency virus (HIV), prevents binding of HIV to target cells and consequently prevents HIV viral infection among the majority of homozygotes with this mutation (3). Although the majority of GPCRs mediate signal transduction via G proteins, emerging evidence indicates that some of these receptors are also capable of sending signals via alternative signal molecules, e.g. Jak2 kinase, phospholipase Cg, or protein kinase C. These alternative pathways are an indication of the overall diversity occurring in the GPCR superfamily. Furthermore, there are putative seven transmembrane molecules, which do not appear to be coupled to a G protein. The most striking difference has been observed in the sites and modes of ligand binding and signal generation, which not only manifests the diversity but also indicates the availability of numerous alternative approaches to clinical and industrial applications. In this review, we describe the general structure and ligand interactions of the receptors, and in the following review, Kobilka’s group (4) focuses on the conformational changes during receptor activation.

Genetic approaches in mice to understand Rel/NF-κB and IκB function : transgenics and knockouts

Rel/NF-κB transcription factors have been implicated in regulating a wide variety of genes important in cellular processes that include cell division, cell survival, differentiation and immunity. Here genetic models in which various Rel/NF-κB and IκB proteins have either been over-expressed or deleted in mice will be reviewed. Although expressed fairly ubiquitously, homozygous disruption of individual Rel/NF-κB genes generally affects the development of proper immune cell function. One exception is rela, which is essential for embryonic liver development. The disruption of genes encoding the individual subunits of the IκB kinase, namely IKKα and IKKβ, has demonstrated that IKKβ transmits the response to most common NF-κB inducing agents, whereas IKKα has an unexpected role in keratinocyte differentiation. Future studies will no doubt focus on the effect of multiple gene disruptions of members of this signaling pathway, on tissue-specific disruptions of these genes, and on the use of these mice as models for human diseases.

Low testosterone levels are common and associated with insulin resistance in men with diabetes.

CONTEXT Low testosterone levels are common in men with type 2 diabetes and may be associated with insulin resistance. OBJECTIVE We investigated prevalence of testosterone deficiency and the relationship between testosterone and insulin resistance in a large cohort of men with type 2 and type 1 diabetes. DESIGN The study was a cross-sectional survey of 580 men with type 2 diabetes and 69 men with type 1 diabetes. A subgroup of 262 men with type 2 diabetes was then reassessed after a median of 6 months. RESULTS Forty-three percent of men with type 2 diabetes had a reduced total testosterone, and 57% had a reduced calculated free testosterone. Only 7% of men with type 1 diabetes had low total testosterone. By contrast, 20.3% of men with type 1 diabetes had low calculated free testosterone, similar to that observed in type 2 diabetes (age-body mass index adjusted odds ratio = 1.4; 95% confidence interval = 0.7-2.9). Low testosterone levels were independently associated with insulin resistance in men with type 1 diabetes as well as type 2 diabetes. Serial measurements also revealed an inverse relationship between changes in testosterone levels and insulin resistance. CONCLUSIONS Testosterone deficiency is common in men with diabetes, regardless of the type. Testosterone levels are partly influenced by insulin resistance, which may represent an important avenue for intervention, whereas the utility of testosterone replacement remains to be established in prospective trials.

The anti‐apoptotic activities of Rel and RelA required during B‐cell maturation involve the regulation of Bcl‐2 expression

Rel and RelA, individually dispensable for lymphopoiesis, serve unique functions in activated B and T cells. Here their combined roles in lymphocyte development were examined in chimeric mice repopulated with c‐rel−/− rela−/− fetal liver hemopoietic stem cells. Mice engrafted with double‐mutant cells lacked mature IgMloIgDhi B cells, and numbers of peripheral CD4+ and CD8+ T cells were markedly reduced. The absence of mature B cells was associated with impaired survival that coincided with reduced expression of bcl‐2 and A1. bcl‐2 transgene expression not only prevented apoptosis and increased peripheral B‐cell numbers, but also induced further maturation to an IgMloIgDhi phenotype. In contrast, the survival of double‐mutant T cells was normal and the bcl‐2 transgene could not rectify the peripheral T‐cell deficit. These findings indicate that Rel and RelA serve essential, albeit redundant, functions during the later antigen‐independent stages of B‐ and T‐cell maturation, with these transcription factors promoting the survival of peripheral B cells in part by upregulating Bcl‐2.

The regulation and roles of Rel/NF-kappa B transcription factors during lymphocyte activation.

The activation of B and T cells by a wide range of stimuli can rapidly induce specific gene expression via a mechanism that promotes the nuclear translocation of different Rel/nuclear factor-kappa B (NF-kappa B) transcription factors which are normally resident in the cytoplasm. Recent findings highlight the crucial roles of specific Rel/NF-kappa B family members in the processes of cell division, apoptosis and differentiation that accompany lymphocyte activation.

Low Testosterone in Men with Type 2 Diabetes: Significance and Treatment

CONTEXT The relationship between testosterone and diabetes in men is an important issue, given that one third of U.S. men aged 65 yr or older have diabetes, with a similar percentage having low testosterone levels. EVIDENCE ACQUISITION The medical literature from 1970 to March 2011 was reviewed for key articles. EVIDENCE SYNTHESIS In population-based studies, low testosterone is commonly associated with type 2 diabetes and the metabolic syndrome, and it identifies men with an adverse metabolic profile. The difference in testosterone levels between men with diabetes compared to men without diabetes is moderate and comparable in magnitude to the effects of other chronic diseases, suggesting that low testosterone may be a marker of poor health. Although the inverse association of testosterone with diabetes is partially mediated by SHBG, low testosterone is linked to diabetes via a bidirectional relationship with visceral fat, muscle, and possibly bone. There is consistent evidence from randomized trials that testosterone therapy alters body composition in a metabolically favorable manner, but changes are modest and have not consistently translated into reductions in insulin resistance or improvements in glucose metabolism. CONCLUSIONS The key response to the aging, overweight man with type 2 diabetes and subnormal testosterone levels should be implementation of lifestyle measures such as weight loss and exercise, which, if successful, raise testosterone and provide multiple health benefits. Although approved therapy for diabetes should be used, testosterone therapy should not be given to such men until benefits and risks are clarified by adequately powered clinical trials.

Increase in visceral and subcutaneous abdominal fat in men with prostate cancer treated with androgen deprivation therapy.

Objective  Androgen deprivation therapy (ADT) for prostate cancer is associated with increases in fat mass and risk of type 2 diabetes; however, the relationship between sex steroid deficiency and abdominal fat distribution remains controversial.

A novel EPAS1/HIF2A germline mutation in a congenital polycythemia with paraganglioma

Congenital polycythemias have diverse etiologies, including mutations in the hypoxia sensing pathway. These include HIF2A at exon 12, VHL gene (Chuvash polycythemia), and PHD2 mutations, which in one family was also associated with recurrent pheochromocytoma/paraganglioma (PHEO/PGL). Over the past two decades, we have studied seven unrelated patients with sporadic congenital polycythemia who subsequently developed PHEO/PGL with, until now, no discernible molecular basis. We now report a polycythemic patient with a novel germline HIF2AF374Y (exon 9) mutation, inherited from his mother, who developed PHEO/PGL. We show that this is a gain-of-function mutation and demonstrate no loss-of-heterozygosity or additional somatic mutation of HIF2A in the tumor, indicating HIF2AF374Y may be predisposing rather than causative of PHEO/PGL. This report, in view of two other concomitantly reported PHEO/PGL patients with somatic mutations of HIF2A and polycythemia, underscores the PHEO/PGL-promoting potential of mutations of HIF2A that alone are not sufficient for PHEO/PGL development.

Thyrotoxicosis during sunitinib treatment for renal cell carcinoma

Context  Sunitinib malate is an oral tyrosine kinase inhibitor used in the treatment of renal cell carcinoma (RCC) and gastrointestinal stromal tumours. Hypothyroidism has been observed in patients treated with sunitinib, but the mechanism whereby sunitinib induces hypothyroidism is unknown.

Substitution of the Seat-belt Region of the Thyroid-stimulating Hormone (TSH) β-Subunit with the Corresponding Regions of Choriogonadotropin or Follitropin Confers Luteotropic but Not Follitropic Activity to Chimeric TSH

The region between the 10th and 12th cysteine (Cys88-Cys105 in human thyroid-stimulating hormone β-subunit (hTSHβ)) of the glycoprotein hormone β-subunits corresponds to the disulfide-linked seat-belt region. It wraps around the common α-subunit and has been implicated in regulating specificity between human choriogonadotropin (hCG) and human follicle-stimulating hormone (hFSH), but determinants of hTSH specificity are unknown. To characterize the role of this region for hTSH, we constructed hTSH chimeras in which the entire seat-belt region Cys88-Cys105 or individual intercysteine segments Cys88-Cys95 and Cys95-Cys105 were replaced with the corresponding sequences of hCG and hFSH or alanine cassettes. Alanine cassette mutagenesis of hTSH showed that the Cys95-Cys105 segment of the seat-belt was more important for TSH receptor binding and signal transduction than the Cys88-Cys95 determinant loop region. Replacing the entire seat-belt of hTSHβ with the hCG sequence conferred full hCG receptor binding and activation to the hTSH chimera, whereas TSH receptor binding and activation were abolished. Conversely, introduction of the hTSHβ seat-belt sequence into hCGβ generated an hCG chimera that bound to and activated the TSH receptor but not the CG/lutropin (LH) receptor. In contrast, an hTSH chimera bearing hFSH seat-belt residues did not possess any follitropic activity, and its thyrotropic activity was only slightly reduced. This may in part be due to the fact that the net charge of the seat-belt is similar in hTSH and hFSH but different from hCG. However, exchanging other regions of charge heterogeneity between hTSHβ and hFSHβ did not confer follitropic activity to hTSH. Thus, exchanging the seat-belt region between hTSH and hCG switches hormonal specificity in a mutually exclusive fashion. In contrast, the seat-belt appears not to discriminate between the TSH and the FSH receptors, indicating for the first time that domains outside the seat-belt region contribute to glycoprotein hormone specificity.