CD28 deletion improves obesity-induced liver steatosis but increases adiposity in mice

BACKGROUND/OBJECTIVES

Lymphocytes have a critical role in visceral adipose tissue (AT) inflammation. The CD28 costimulatory molecule is required for lymphocyte activation and for the development of a functional regulatory T cells (Tregs) compartment; however, its role during obesity is unknown.

METHODS

During diet-induced obesity, we investigated the effects of selective interference with CD28 signaling using knockout mice (Cd28KO) and a CTLA4-Ig fusion protein inhibiting CD28-B7 interactions.

RESULTS

Cd28 deficiency decreased pathogenic T cells and Treg content within AT without changing the macrophages number. Cd28KO epididymal but not subcutaneous fat was characterized by enlarged adipocytes, reduced levels of inflammatory cytokines and increased Glut4, adiponectin and lipogenic enzyme mRNA levels. This was associated with reduced inflammation, fat accumulation and enhanced glucose metabolism in liver. Weight gain and fasting glucose tolerance were not affected. CTLA4-Ig injections reduced the number of T cells in epididymal AT (epiAT) but not the inflammatory cytokines levels and failed to improve liver fat accumulation.

CONCLUSIONS

Deletion of CD28 creates a new pro/anti-inflammatory balance in epiAT and liver and exerts a protective effect against hepatic steatosis.

DOK4 and DOK5: new Dok-related genes expressed in human T cells

Dok proteins are adapter proteins involved in signal transduction. Several intracellular proteins expressed in lymphocytes meet the criteria of membrane-associated adapter proteins such as members of the Dok family. To understand the role and the formation of multiprotein networks involving Dok proteins in T lymphocytes, we search for potential additional members of this family. Here, we describe the two new human dok-related genes DOK4 and DOK5 and present data showing the expression of DOK4 and DOK5 genes in T cells. These genes are the orthologues of mouse Dok4 and Dok5 genes. Based on analysis of phylogenetic trees and exon/intron structure of Dok family members, DOK4 and DOK5 define a subfamily within dok genes distinct from DOK1, DOK2 and DOK3. So, Dok-4 and Dok-5 molecules constitute a new group of adapter proteins in T cells, requiring further functional analysis.

Role of Tec kinase in nuclear factor of activated T cells signaling

The Tec protein kinase family includes Btk, Itk, Tec, Rlk and Bmx, which are critically involved in signals mediated by various cytokines and antigen receptors. Btk mutations cause severe immunodeficiencies, with defective B cell function. In T cells, Tec regulates cytokine production. However, the downstream targets of these Tec kinases are poorly defined. Here we report that overexpression of Tec in T cells can regulate gene transcription through the nuclear factor of activated T cells (NF-AT). Using different reporter gene constructs, we establish that Tec in transfected T cells dramatically induced NF-AT-dependent gene transcription, which was prevented by a dominant-negative mutant of NF-AT or by the immunosuppressive drug cyclosporin A. Tec appears to regulate NF-AT nuclear import. In addition, Tec influences cytoplasmic free calcium increase. Taken together, our results identify NF-AT as a major downstream target of Tec kinases that is critically involved in transcriptional gene regulation. These observations highlight signaling pathways regulated by Tec kinases and provide new pharmacological targets to regulate immune functions.

Analysis of immunoreceptor tyrosine-based activation motif (ITAM) binding to ZAP-70 by surface plasmon resonance

The signaling function of the T cell antigen receptor (TCR) is mediated via CD3 polypeptides, the cytoplasmic sequences of which bear conserved immunoreceptor tyrosine-based activation motifs (ITAM). ITAM are defined by two YxxL/I sequences separated by a six-eight amino acid long spacer. Upon antigen recognition, ITAM become phosphorylated on both tyrosine residues, creating a high affinity binding site for the tandem SH2 domains found in the protein tyrosine kinase ZAP-70. Using surface plasmon resonance, we further dissected the sequences required for the binding of ZAP-70 to each TCR-associated ITAM. First, we generated protein tyrosine phosphatase-resistant ITAM peptide analogs, in which difluorophosphonomethyl phenylalanyl (F2p) replaced both phosphotyrosines, and showed that those protein tyrosine phosphatase-resistant analogs bind ZAP-70 with high affinity, establishing a rational strategy for the design of novel pharmacological tools capable of interfering with TCR signaling function. Second, we substituted the five amino acids separating the two YxxL/I sequences of the CD3 zeta 1 ITAM with a non-peptidic linker made up of gamma-amino butyric acid units and demonstrated that the length of this intervening sequence rather than its chemical composition is essential for high affinity binding of phosphorylated ITAM to the ZAP-70 SH2 domains.

Signal transduction by CD28 costimulatory receptor on T cells. B7-1 and B7-2 regulation of tyrosine kinase adaptor molecules

This study compares the biochemical responses in T cells activated with the CD28 ligands B7-1 and B7-2. The patterns of tyrosine phosphorylation induced in T cells by these two CD28 ligands are identical, but clearly different from the tyrosine phosphorylation induced by the T cell receptor (TCR). The TCR regulates protein complexes mediated by the adapter Grb2 both in vivo and in vitro. In contrast, there is no apparent regulation of in vivo Grb2 complexes in response to B7-1 or B7-2. Rather, B7-1 and B7-2 both induce tyrosine phosphorylation of a different adaptor protein, p62. The regulation of p62 is a unique CD28 response that is not shared with the TCR. These data indicate that B7-1 and B7-2 induce identical tyrosine kinase signal transduction pathways. The data show also that the TCR and CD28 couple to different adapter proteins, which could explain the divergence of TCR and CD28 signal transduction pathways during T cell activation.

The role of p21ras in CD28 signal transduction: triggering of CD28 with antibodies, but not the ligand B7-1, activates p21ras

CD28 is a 44-kD homodimer expressed on the surface of the majority of human T cells that provides an important costimulus for T cell activation. The biochemical basis of the CD28 accessory signals is poorly understood. Triggering of the T cell antigen receptor (TCR) activates the p21ras proteins. Here we show that ligation of CD28 by a monoclonal antibody (mAb) also stimulates p21ras and induces Ras-dependent events such as stimulation of the microtubule-associated protein (MAP) kinase ERK2 and hyperphosphorylation of Raf-1. One physiological ligand for CD28 is the molecule B7-1. In contrast to the effect of CD28 mAb, the present studies show that interactions between CD28 and B7-1 do not stimulate p21ras signaling pathways. Two substrates for TCR-regulated protein tyrosine kinases (PTKs) have been implicated in p21ras activation in T cells: p95vav and a 36-kD protein that associates with a complex of Grb2 and the Ras exchange protein Sos. Triggering CD28 with both antibodies and B7-1 activates cellular PTKs, and we have exploited the differences between antibodies and B7-1 for p21ras activation in an attempt to identify critical PTK-controlled events for Ras activation in T cells. The data show that antibodies against TCR or CD28 induce tyrosine phosphorylation of both Vav and p36. B7-1 also induces Vav tyrosine phosphorylation but has no apparent effect on tyrosine phosphorylation of the Grb2-associated p36 protein. The intensity of the Vav tyrosine phosphorylation is greater in B7-1 than in TCR-stimulated cells. Moreover the kinetics of Vav tyrosine phosphorylation is prolonged in the B7-1-stimulated cells. These studies show that for CD28 signaling, the activation of p21ras correlates more closely with p36 tyrosine phosphorylation than with Vav tyrosine phosphorylation. However, the experiments demonstrate that Vav is a major substrate for B7-activated PTKs and hence could be important in CD28 signal transduction pathway.