Issue published July 15, 2003

O riginating in the thyroid, the prohormone thyroxine is converted to triiodothyronine, which is essential in brain development, growth, and metabolism. A study in this issue reveals a novel mechanism for controlling triiodothyronine production that provides the first example of enzyme activity being restored by deubiquitination.

T he molecular mechanisms underlying the putative role of osteopontin in the chronic inflammatory disease rheumatoid arthritis are unclear. A study in a murine model of arthritis now demonstrates that a specific antibody directed against the exposed osteopontin epitope SLAYGLR is capable of preventing inflammatory cell infiltration in arthritic joints.

A spirin has been shown to cause a reduction in the virulence of Staphylococcus aureus–associated endocarditis. A new study reveals that salicylic acid, the major metabolite of aspirin, acts at the level of transcription to downregulate the production of fibrinogen, fibronectin, and α-hemolysin — virulence factors necessary for bacterial replication in host tissues and, now, potential therapeutic targets.

A lthough a lysosomal, cathepsin B–dependent (Ctsb-dependent) pathway of apoptosis has been described, the contribution of this pathway to tissue damage remains unclear. Our aim was to ascertain if Ctsb inactivation attenuates liver injury, inflammation, and fibrogenesis after bile duct ligation (BDL). In 3-day BDL mice, hepatocyte apoptosis, mitochondrial cytochrome c release, and serum alanine aminotransferase (ALT) values were reduced in Ctsb^–/– versus Ctsb^+/+ animals. Likewise, R-3032 (a Ctsb inhibitor) also reduced these parameters in BDL WT mice. Both genetic and pharmacologic inhibition of Ctsb in the BDL mouse reduced (a) hepatic inflammation, as assessed by transcripts for CXC chemokines and neutrophil infiltration, and (b) fibrogenesis, as assessed by transcripts for stellate cell activation and sirius red staining for hepatic collagen deposition. These differences could not be ascribed to alterations in cholestasis. These findings support a prominent role for the lysosomal pathway of apoptosis in tissue injury and link apoptosis to inflammation and fibrogenesis. Ctsb inhibition may be therapeutic in liver diseases.

H ematopoietic stem cells rarely contribute to hepatic regeneration, however, the mechanisms governing their homing to the liver, which is a crucial first step, are poorly understood. The chemokine stromal cell–derived factor-1 (SDF-1), which attracts human and murine progenitors, is expressed by liver bile duct epithelium. Neutralization of the SDF-1 receptor CXCR4 abolished homing and engraftment of the murine liver by human CD34⁺ hematopoietic progenitors, while local injection of human SDF-1 increased their homing. Engrafted human cells were localized in clusters surrounding the bile ducts, in close proximity to SDF-1–expressing epithelial cells, and differentiated into albumin-producing cells. Irradiation or inflammation increased SDF-1 levels and hepatic injury induced MMP-9 activity, leading to both increased CXCR4 expression and SDF-1–mediated recruitment of hematopoietic progenitors to the liver. Unexpectedly, HGF, which is increased following liver injury, promoted protrusion formation, CXCR4 upregulation, and SDF-1–mediated directional migration by human CD34⁺ progenitors, and synergized with stem cell factor. Thus, stress-induced signals, such as increased expression of SDF-1, MMP-9, and HGF, recruit human CD34⁺ progenitors with hematopoietic and/or hepatic-like potential to the liver of NOD/SCID mice. Our results suggest the potential of hematopoietic CD34⁺/CXCR4⁺cells to respond to stress signals from nonhematopoietic injured organs as an important mechanism for tissue targeting and repair.

I FN-α activates the signal transducer and activator of transcription (STAT) family of proteins; however, it is unknown whether IFN-α exerts its antitumor actions primarily through a direct effect on malignant cells or by stimulating the immune system. To investigate the contribution of STAT1 signaling within the tumor, we generated a STAT1-deficient melanoma cell line, AGS-1. We reconstituted STAT1 into AGS-1 cells by retroviral gene transfer. The resulting cell line (AGS-1^STAT1) showed normal regulation of IFN-α–stimulated genes (e.g., H2k, ISG-54) as compared with AGS-1 cells infected with the empty vector (AGS-1^MSCV). However, mice challenged with the AGS-1, AGS-1^STAT1, and AGS-1^MSCV cell lines exhibited nearly identical survival in response to IFN-α treatment, indicating that restored STAT1 signaling within the tumor did not augment the antitumor activity of IFN-α. In contrast, STAT1^–/– mice could not utilize exogenous IFN-α to inhibit the growth of STAT1^+/+ melanoma cells in either an intraperitoneal tumor model or in the adjuvant setting. The survival of tumor-bearing STAT1^–/– mice was identical regardless of treatment (IFN-α or PBS). Additional cell depletion studies demonstrated that NK cells mediated the antitumor effects of IFN-α. Thus, STAT1-mediated gene regulation within immune effectors was necessary for mediating the antitumor effects of IFN-α in this experimental system.

I t has been shown that osteopontin (OPN) plays a pivotal role in the pathogenesis of rheumatoid arthritis (RA). However, the molecular mechanism of OPN action is yet to be elucidated. Splenic monocytes obtained from arthritic mice exhibited a significant capacity for cell migration toward thrombin-cleaved OPN but not toward full-length OPN. Migratory monocytes expressed α9 and α4 integrins. Since cleavage of OPN by thrombin exposes the cryptic epitope recognized by α9 and α4 integrins, we investigated the role of the cryptic epitope SLAYGLR in a murine RA model by using a specific antibody (M5) reacting to SLAYGLR sequence. The M5 antibody could abrogate monocyte migration toward the thrombin-cleaved form of OPN. Importantly, M5 antibody could inhibit the proliferation of synovium, bone erosion, and inflammatory cell infiltration in arthritic joints. Thus, we demonstrated that a cryptic epitope, the SLAYGLR sequence of murine OPN, is critically involved in the pathogenesis of a murine model of RA.

T he type 2 iodothyronine deiodinase (D2) is an integral membrane ER-resident selenoenzyme that activates the pro-hormone thyroxine (T4) and supplies most of the 3,5,3′-triiodothyronine (T3) that is essential for brain development. D2 is inactivated by selective conjugation to ubiquitin, a process accelerated by T4 catalysis and essential for the maintenance of T3 homeostasis. A yeast two-hybrid screen of a human-brain library with D2 as bait identified von Hippel–Lindau protein–interacting deubiquitinating enzyme-1 (VDU1). D2 interaction with VDU1 and VDU2, a closely related deubiquitinase, was confirmed in mammalian cells. Both VDU proteins colocalize with D2 in the ER, and their coexpression prolongs D2 half-life and activity by D2 deubiquitination. VDU1, but not VDU2, is markedly increased in brown adipocytes by norepinephrine or cold exposure, further amplifying the increase in D2 activity that results from catecholamine-stimulated de novo synthesis. Thus, deubiquitination regulates the supply of active thyroid hormone to brown adipocytes and other D2-expressing cells.

T he serine/threonine kinase Akt/PKB plays key roles in the regulation of cell growth, survival, and metabolism. It remains unclear, however, whether the functions of individual Akt/PKB isoforms are distinct. To investigate the function of Akt2/PKBβ, mice lacking this isoform were generated. Both male and female Akt2/PKBβ-null mice exhibit mild growth deficiency and an age-dependent loss of adipose tissue or lipoatrophy, with all observed adipose depots dramatically reduced by 22 weeks of age. Akt2/PKBβ-deficient mice are insulin resistant with elevated plasma triglycerides. In addition, Akt2/PKBβ-deficient mice exhibit fed and fasting hyperglycemia, hyperinsulinemia, glucose intolerance, and impaired muscle glucose uptake. In males, insulin resistance progresses to a severe form of diabetes accompanied by pancreatic β cell failure. In contrast, female Akt2/PKBβ-deficient mice remain mildly hyperglycemic and hyperinsulinemic until at least one year of age. Thus, Akt2/PKBβ-deficient mice exhibit growth deficiency similar to that reported previously for mice lacking Akt1/PKBα, indicating that both Akt2/PKBβ and Akt1/PKBα participate in the regulation of growth. The marked hyperglycemia and loss of pancreatic β cells and adipose tissue in Akt2/PKBβ-deficient mice suggest that Akt2/PKBβ plays critical roles in glucose metabolism and the development or maintenance of proper adipose tissue and islet mass for which other Akt/PKB isoforms are unable to fully compensate.

< i>Neph1-deficient mice develop nephrotic syndrome at birth, indicating the importance of this protein in the development of a normal glomerular filtration barrier. While the precise subcellular localization of Neph1 remains unknown, its relationship with other components of the glomerular filtration barrier is of great interest in this field. In this paper, we localize the expression of Neph1 to the glomerular slit diaphragm by immunogold electron microscopy in rodents and describe its direct interaction with two other components of the slit diaphragm, nephrin and ZO-1. Both native and recombinant Neph1 associate with each other as dimers and multimers and interact with nephrin via their extracellular segments. Disruption of the Neph1-nephrin interaction in vivo by injecting combinations of individual subnephritogenic doses of anti-Neph1 and anti-nephrin results in complement- and leukocyte-independent proteinuria with preserved foot processes. This disruption modestly reduces Neph1 and nephrin protein expression in podocytes and dramatically reduces ZO-1 protein expression via the interaction of ZO-1 PDZ domains with the cytoplasmic tail of Neph1, independent of changes in mRNA expression of all three genes. The interaction between nephrin and Neph1 is specific and not shared by either protein with P-cadherin, another integral slit diaphragm protein. The interaction between nephrin and Neph1 therefore appears to be an important determinant of glomerular permeability.

A spirin has been previously shown to reduce the in vivo virulence of Staphylococcus aureus in experimental endocarditis, through antiplatelet and antimicrobial mechanisms. In the present study, salicylic acid, the major in vivo metabolite of aspirin, mitigated two important virulence phenotypes in both clinical and laboratory S. aureus strains: α-hemolysin secretion and fibronectin binding in vitro. In addition, salicylic acid reduced the expression of the α-hemolysin gene promoter, hla, and the fibronectin gene promoter, fnbA. Transcriptional analysis, fluorometry, and flow cytometry revealed evidence of salicylic acid–mediated activation of the stress-response gene sigB. Expression of the sigB-repressible global regulon sarA and the global regulon agr were also mitigated by salicylic acid, corresponding to the reduced expression of the hla and fnbA genes in vitro. Studies in experimental endocarditis confirmed the key roles of both sarA and sigB in mediating the antistaphylococcal effects of salicylic acid in vivo. Therefore, aspirin has the potential to be an adjuvant therapeutic agent against endovascular infections that result from S. aureus, by downmodulating key staphylococcal global regulons and structural genes in vivo, thus abrogating relevant virulence phenotypes.

I nducible costimulatory molecule (ICOS) plays a pivotal role in T cell activation and Th1/Th2 differentiation. ICOS blockade has disparate effects on immune responses depending on the timing of blockade. Its role in transplantation immunity, however, remains incompletely defined. We used a vascularized mouse cardiac allograft model to explore the role of ICOS signaling at different time points after transplantation, targeting immune initiation (early blockade) or the immune effector phase (delayed blockade). In major histocompatibility–mismatched recipients, ICOS blockade prolonged allograft survival using both protocols but did so more effectively in the delayed-treatment group. By contrast, in minor histocompatibility–mismatched recipients, early blockade accelerated rejection and delayed blockade prolonged graft survival. Alloreactive CD4⁺ T cell expansion and alloantibody production were suppressed in both treatment groups, whereas only delayed blockade resulted in suppression of effector CD8⁺ T cell generation. After delayed ICOS blockade, there was a diminished frequency of allospecific IL-10–producing cells and an increased frequency of both IFN-γ– and IL-4–producing cells. The beneficial effects of ICOS blockade in regulating allograft rejection were seen in the absence of CD28 costimulation but required CD8⁺ cells, cytotoxic T lymphocyte antigen-4, and an intact signal transducer and activator of transcription–6 pathway. These data define the complex functions of the ICOS-B7h pathway in regulating alloimmune responses in vivo.

S trategies to stimulate endogenous surfactant production require a detailed understanding of the regulation of lipogenesis in alveolar type II cells. We developed culture conditions in which keratinocyte growth factor (KGF) stimulates fatty acid and phospholipid synthesis. KGF stimulated acetate incorporation into phosphatidylcholine, disaturated phosphatidylcholin, and phosphatidylglycerol more than 5% rat serum alone. To determine the mRNA levels of lipogenic enzymes and transport proteins, we analyzed gene expression by oligonucleotide microarrays. KGF increased the mRNA levels for fatty acid synthase, stearoyl-CoA desaturase-1 (SCD-1), and epidermal fatty acid–binding protein more than rat serum alone. In addition, KGF increased the mRNA levels of the transcription factors CCAAT/enhancer-binding protein α (C/EBPα) and C/EBPδ as well as SREBP-1c (ADD-1), but not PPARγ. These changes in C/EBPα and C/EBPδ were confirmed by in situ hybridization. SCD-1 was also found to be highly expressed in alveolar type II cells in vivo. Furthermore, KGF increased protein levels of fatty acid synthase, C/EBPα, C/EBPδ, SREBP-1, epidermal fatty acid–binding protein, and SCD. Finally, the liver X receptor agonist T0901317 increased acetate incorporation and SREBP-1 but not SREBP-2 protein levels. In summary, KGF stimulates lipogenesis in type II cells by a coordinated expression of lipogenic enzymes and transport proteins regulated by C/EBP isoforms and SREBP-1c.

N O prevents atherogenesis and inflammation in vessel walls by inhibition of cell proliferation and cytokine-induced endothelial expression of adhesion molecules and proinflammatory cytokines. Reduced NO production due to inhibition of either eNOS or iNOS may therefore reinforce atherosclerosis. Patients with end-stage renal failure show markedly increased mortality due to atherosclerosis. In the present study we tested the hypothesis that uremic toxins are responsible for reduced iNOS expression. LPS-induced iNOS expression in mononuclear leukocytes was studied using real-time PCR. The iNOS expression was blocked by addition of plasma from patients with end-stage renal failure, whereas plasma from healthy controls had no effect. Hemofiltrate obtained from patients with end-stage renal failure was fractionated by chromatographic methods. The chromatographic procedures revealed a homogenous fraction that inhibits iNOS expression. Using gas chromatography/mass spectrometry, this inhibitor was identified as phenylacetic acid. Authentic phenylacetic acid inhibited iNOS expression in a dose-dependent manner. In healthy control subjects, plasma concentrations were below the detection level, whereas patients with end-stage renal failure had a phenylacetic acid concentration of 3.49 ± 0.33 mmol/l (n = 41). It is concluded that accumulation of phenylacetic acid in patients with end-stage renal failure inhibits iNOS expression. That mechanism may contribute to increased atherosclerosis and cardiovascular morbidity in patients with end-stage renal failure.

S erum anti–T cell receptor (TCR) Ab’s are involved in immune regulation directed against pathogenic T cells in experimental models of autoimmune diseases. Our identification of a dominant T cell population expressing the Vβ5.1 TCR gene (TCRBV5-1), which is responsible for the production of pathogenic anti-acetylcholine receptor (AChR) autoantibodies in HLA-DR3 patients with early-onset myasthenia gravis (EOMG), prompted us to explore the occurrence, reactivity, and regulatory role of anti-TCR Ab’s in EOMG patients and disease controls with clearly defined other autoantibodies. In the absence of prior vaccination against the TCR, EOMG patients had elevated anti-Vβ5.1 Ab’s of the IgG class. This increase was restricted largely to EOMG cases with HLA-DR3 and with less severe disease, and it predicted clinical improvement in follow-up studies. EOMG patient sera containing anti-TCR Ab’s bound specifically the native TCR on intact Vβ5.1-expressing cells and specifically inhibited the proliferation and IFN-γ production of purified Vβ5.1-expressing cells to alloantigens in mixed lymphocyte reaction and the proliferation of a Vβ5.1-expressing T cell clone to an AChR peptide, indicating a regulatory function for these Ab’s. This evidence of spontaneously active anti-Vβ5.1 Ab’s in EOMG patients suggests dynamic protective immune regulation directed against the excess of pathogenic Vβ5.1-expressing T cells. Though not sufficient to prevent a chronic, exacerbated autoimmune process, it might be boosted using a TCR peptide as vaccine.

T he worldwide increase in the prevalence of multi-antibiotic–resistant bacteria has threatened the physician’s ability to provide appropriate therapy for infections. The relationship between antimicrobial drug concentration and infecting pathogen population reduction is of primary interest. Using data derived from mice infected with the bacterium Pseudomonas aeruginosa and treated with a fluoroquinolone antibiotic, a mathematical model was developed that described relationships between antimicrobial drug exposures and changes in drug-susceptible and -resistant bacterial subpopulations at an infection site. Dosing regimens and consequent drug exposures that amplify or suppress the emergence of resistant bacterial subpopulations were identified and prospectively validated. Resistant clones selected in vivo by suboptimal regimens were characterized. No mutations were identified in the quinolone resistance–determining regions of gyrA/B or parC/E. However, all resistant clones demonstrated efflux pump overexpression. At base line, MexAB-OprM, MexCD-OprJ, and MexEF-OprN were represented in the drug-resistant population. After 28 hours of therapy, MexCD-OprJ became the predominant pump expressed in the resistant clones. The likelihood of achieving resistance-suppression exposure in humans with a clinically prescribed antibiotic dose was determined. The methods developed in this study provide insight regarding how mathematical models can be used to identify rational dosing regimens that suppress the amplification of the resistant mutant population.

T he generation of Ig-secreting cells (ISCs) from memory B cells requires interactions between antigen-specific (Ag-specific) B cells, T cells, and dendritic cells. This process must be strictly regulated to ensure sufficient humoral immunity while avoiding production of pathogenic autoantibodies. BAFF, a member of the TNF family, is a key regulator of B cell homeostasis. BAFF exerts its effect by binding to three receptors — transmembrane activator of and CAML interactor (TACI), B cell maturation antigen (BCMA), and BAFF receptor (BAFF-R). To elucidate the contribution of BAFF to the differentiation of B cells into ISCs, we tracked the fate of human memory B cells stimulated with BAFF or CD40L. BAFF and CD40L significantly increased the overall number of surviving B cells. This was achieved via distinct mechanisms. CD40L induced proliferation of nondifferentiated blasts, while BAFF prevented apoptosis of ISCs without enhancing proliferation. The altered responsiveness of activated memory B cells to CD40L and BAFF correlated with changes in surface phenotype such that expression of CD40 and BAFF-R were reduced on ISCs while BCMA was induced. These results suggest BAFF may enhance humoral immunity in vivo by promoting survival of ISCs via a BCMA-dependent mechanism. These findings have wide-ranging implications for the treatment of human immunodeficiencies as well as autoimmune diseases.