Chemokines constitute a family of chemoattractant cytokines and are subdivided into four families on the basis of the number and spacing of the conserved cysteine residues in the N-terminus of the protein. Chemokines play a major role in selectively recruiting monocytes, neutrophils, and lymphocytes, as well as in inducing chemotaxis through the activation of G-protein-coupled receptors. Monocyte chemoattractant protein-1 (MCP-1/CCL2) is one of the key chemokines that regulate migration and infiltration of monocytes/macrophages. Both CCL2 and its receptor CCR2 have been demonstrated to be induced and involved in various diseases. Migration of monocytes from the blood stream across the vascular endothelium is required for routine immunological surveillance of tissues, as well as in response to inflammation. This review will discuss these biological processes and the structure and function of CCL2.

Monocyte chemoattractant protein-1 (MCP-1): an overview.

The TGF-β family comprises many structurally related differentiation factors that act through a heteromeric receptor complex at the cell surface and an intracellular signal transducing Smad complex. The receptor complex consists of two type II and two type I transmembrane serine/threonine kinases. Upon phosphorylation by the receptors, Smad complexes translocate into the nucleus, where they cooperate with sequence-specific transcription factors to regulate gene expression. The vertebrate genome encodes many ligands, fewer type II and type I receptors, and only a few Smads. In contrast to the perceived simplicity of the signal transduction mechanism with few Smads, the cellular responses to TGF-β ligands are complex and context dependent. This raises the question of how the specificity of the ligand-induced signaling is achieved. We review the molecular basis for the specificity and versatility of signaling by the many ligands through this conceptually simple signal transduction mechanism.

SPECIFICITY AND VERSATILITY IN TGF-β SIGNALING THROUGH SMADS

Accumulating evidence indicates that obesity is closely associated with an increased risk of metabolic diseases such as insulin resistance, type 2 diabetes, dyslipidemia and nonalcoholic fatty liver disease. Obesity results from an imbalance between food intake and energy expenditure, which leads to an excessive accumulation of adipose tissue. Adipose tissue is now recognized not only as a main site of storage of excess energy derived from food intake but also as an endocrine organ. The expansion of adipose tissue produces a number of bioactive substances, known as adipocytokines or adipokines, which trigger chronic low-grade inflammation and interact with a range of processes in many different organs. Although the precise mechanisms are still unclear, dysregulated production or secretion of these adipokines caused by excess adipose tissue and adipose tissue dysfunction can contribute to the development of obesity-related metabolic diseases. In this review, we focus on the role of several adipokines associated with obesity and the potential impact on obesity-related metabolic diseases. Multiple lines evidence provides valuable insights into the roles of adipokines in the development of obesity and its metabolic complications. Further research is still required to fully understand the mechanisms underlying the metabolic actions of a few newly identified adipokines.

Obesity and Its Metabolic Complications: The Role of Adipokines and the Relationship between Obesity, Inflammation, Insulin Resistance, Dyslipidemia and Nonalcoholic Fatty Liver Disease

C-reactive protein (CRP) is an acute inflammatory protein that increases up to 1000-fold at sites of infection or inflammation. CRP is produced as a homo-pentameric protein, termed native CRP (nCRP), which can irreversibly dissociate at sites of inflammation and infection into five separate monomers, termed monomeric CRP (mCRP). CRP is synthesised primarily in liver hepatocytes but also by smooth muscle cells, macrophages, endothelial cells, lymphocytes and adipocytes. Evidence suggests estrogen in the form of hormone replacement therapy influences CRP levels in the elderly. Having been traditionally utilised as a marker of infection and cardiovascular events, there is now growing evidence that CRP plays important roles in inflammatory processes and host responses to infection including the complement pathway, apoptosis, phagocytosis, nitric oxide (NO) release and the production of cytokines, particularly interleukin-6 and tumour necrosis factor-alpha. Unlike more recent publications, the findings of early work on CRP can seem somewhat unclear and at times conflicting since it was often not specified which particular CRP isoform was measured or utilised in experiments and whether responses attributed to nCRP were in fact possibly due to dissociation into mCRP or lipopolysaccharide contamination. In addition, since antibodies for mCRP are not commercially available, few laboratories are able to conduct studies investigating the mCRP isoform. Despite these issues and the fact most CRP research to date has focussed on vascular disorders, there is mounting evidence that CRP isoforms have distinct biological properties, with nCRP often exhibiting more anti-inflammatory activities compared to mCRP. The nCRP isoform activates the classical complement pathway, induces phagocytosis and promotes apoptosis. On the other hand, mCRP promotes the chemotaxis and recruitment of circulating leukocytes to areas of inflammation and can delay apoptosis. The nCRP and mCRP isoforms work in opposing directions to inhibit and induce NO production respectively. In terms of pro-inflammatory cytokine production, mCRP increases interleukin-8 and monocyte chemoattractant protein-1 production whereas nCRP has no detectable effect on their levels. Further studies are needed to expand on these emerging findings and fully characterise the differential roles each CRP isoform at sites of local inflammation and infection.

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Role of C-Reactive Protein at Sites of Inflammation and Infection

Infiltration of macrophages in solid tumours is associated with poor prognosis and correlates with chemotherapy resistance in most cancers. In mouse models of cancer, macrophages promote cancer initiation and malignant progression by stimulating angiogenesis, increasing tumour cell migration, invasion and intravasation and suppressing antitumour immunity. At metastatic sites, macrophages promote tumour cell extravasation, survival and subsequent growth. Each of these pro-tumoural activities is promoted by a subpopulation of macrophages that express canonical markers but have unique transcriptional profiles, which makes tumour-associated macrophages (TAMs) good targets for anticancer therapy in humans through either their ablation or their re-differentiation away from pro-tumoural towards antitumoural states. In this Review, we evaluate the state of the art of TAM-targeting strategies, focusing on the limitations and potential side effects of the different therapies such as toxicity, rebound effects and compensatory mechanisms. We provide an extensive overview of the different types of therapy used in the clinic and their limitations in light of known macrophage biology and propose new strategies for targeting TAMs.

Targeting macrophages: therapeutic approaches in cancer.

Vpr, the 96 amino acid long protein represents one of the auxiliary proteins of human immunodeficiency virus type-1 (HIV-1), which exhibits the ability to increase the rate of replication of the virus in T cells. Structurally, this protein is composed of several regions such as the acidic domain with alpha helix at the amino terminus, leucine-isoleucine-rich domain (LR) near the carboxyl terminus and an arginine-rich domain at the C-terminus. Here, we evaluated the ability of wild-type and a spectrum of Vpr mutants with altered amino acid residues within the three major domains of Vpr to regulate of transcription of the HIV-1 LTR. Our results revealed that alterations of amino acids within the LR domain at position 73 from arginine to serine, renders Vpr defective in stimulating transcription of the viral promoter in human T-lymphocytic and astrocytic cells. Mutations within the N- and C-terminal domains had little or no effect on the transcriptional activity of Vpr. Of interest, ectopic expression of this mutant protein exerts a negative effect on the ability of wild-type Vpr, as well as the viral transactivator, Tat, in augmenting viral gene transcription. Production of the mutant Vpr interferes with the replication of the wild-type and delta Vpr virus in the cells. Accordingly, a Vpr mutant virus containing the transition of arginine to serine at position 73 exhibited an inhibitory effect on the replication of wild-type virus. Our results provide a new avenue for the utilization of the variant of the HIV-1 regulatory protein, Vpr, in suppressing replication of the viral genome in infected cells.

Suppression of HIV-1 transcription and replication by a Vpr mutant.

Expression of the viral protein R, Vpr, of HIV-1 affects many biological events in host cells including cell cycle progression, and modulates HIV-1 gene transcription. Earlier studies implicating the cellular protein p21(WAF1) (p21) in regulation of HIV-1 transcription, led us to investigate the functional and physical interaction of Vpr and p21. Our results show that Vpr modestly activated HIV-LTR in cells lacking p21 gene. Here, we describe the mechanisms by which p21 and Vpr leading to stimulation of HIV-1 transcription. Data from the protein-protein interaction experiments revealed the ability of Vpr, p21 and p300 to form a complex. Further, we show that, Vpr interacts with the N- and the C-terminal domains of p21. Furthermore, in cells expressing Vpr, p21 localizes to both the cytoplasm and the nucleus. Interestingly, expression of Vpr alleviates p21-mediated inhibition of cell departure from G1 phase. Expression of a mutant Vpr, with arginine 73 altered to serine, did not affect the ability of p21 to cause cells arrest or its sub-cellular localization. These observations reveal a new cellular partner for Vpr, and provide a new therapeutic avenue for controlling HIV-1 expression.

The role of Vpr in the regulation of HIV-1 gene expression.

Polyomavirus BK (BKV) is a serious problem for immunocompromised patients, where latent virus can enter into the lytic cycle causing cytolytic destruction of host cells. BKV infects >80 % of the population worldwide during childhood and then remains in a latent state in the kidney. In the context of immunosuppression in kidney transplant patients, reactivation of the viral early promoter (BKVE) results in production of T antigen, enabling virus replication and transition from latency to the lytic phase, causing polyomavirus-associated nephropathy. Reactivation of BKV can also cause complications such as nephritis, atypical retinitis and haemorrhagic cystitis in AIDS patients. Here, the effects of human immunodeficiency virus type 1 (HIV-1) proteins Tat and Vpr on BKV transcription were investigated and it was demonstrated that Tat dramatically stimulated BKVE. Site-directed mutagenesis analysis of potential Tat-responsive transcriptional motifs complemented by an electrophoretic mobility shift assay (EMSA) showed that Tat activated BKVE by inducing binding of the NF-κB p65 subunit to a κB motif near the 3′ end of BKVE. In addition, a sequence within the 5′ UTR of BKVE transcripts (BKVE-TAR) was identified that is identical to the HIV-1 transactivation response (TAR) element. The BKVE-TAR sequence bound TAT in RNA EMSA assays and deletion of the BKVE-TAR sequence eliminated Tat transactivation of BKVE transcription. Thus, Tat positively affected BKVE transcription by a dual mechanism and this may be important in diseases involving BKV reactivation in AIDS patients.

Activation of early gene transcription in polyomavirus BK by human immunodeficiency virus type 1 Tat.

Cross talk between growth factors and viral and cellular factors alters neuronal signaling pathways: Implication for HIV-associated dementia

The transforming growth factor-β1 (TGF-β1) is a cytokine involved in many biological events inlcuding immunosuppression, angiogenesis, cell growth, and apoptosis Expression of TGF-β1 at the transcriptional level is controlled by a series of ubiquitous and specialized factors whose activities can be modulated by a variety of signaling events Here we demonstrate that activity of the TGF-β1 promoter is increased by C/EBPβ, a DNA-binding transcription factor whose activity can be influenced by several immunomodulators, in astrocytes and microglial cells Interestingly, expression of Smad3 and Smad4, the downstream regulators of the TGF-β1-signaling pathway, impairs the activity of C/EBPβ on the TGF-β1 promoter Further, we demonstrate that MH2, a common domain among Smads that has protein-binding activities, interacts with C/EBPβ and decreases its association with a region of the TGF-β1 promoter that is responsive to C/EBPβ activation Interestingly, the p65 subunit of nuclear factor-κB (NF-κB), which also interacts with C/EBPβ, cooperates with MH2 and decreased DNA-binding and transcriptional activities of C/EBPβ on the TGF-β1 promoter These observations indicate that an autoregulatory mechanism, involving the MH2 domain of Smads, modulates activation of the TGF-β1 promoter by C/EBPβ Further, our results show that the interplay between NF-κB and C/EBPβ has an impact on the ability of C/EBPβ to stimulate TGF-β1 transcription, hence, suggesting that the cross-communication of signaling pathways that modulate NF-κB and C/EBPβ may dictate the level of TGF-β1 promoter activity

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Bassel E. Sawaya

Papers

Suppression of HIV-1 transcription and replication by a Vpr mutant.

The role of Vpr in the regulation of HIV-1 gene expression.

Activation of early gene transcription in polyomavirus BK by human immunodeficiency virus type 1 Tat.

Cross talk between growth factors and viral and cellular factors alters neuronal signaling pathways: Implication for HIV-associated dementia

Evidence for Activation of the TGF-β1 Promoter by C/EBPβ and Its Modulation by Smads