Dynamics and localization of activin A expression in rat gastric ulcers

Androgen-Influenced Polarization of Activin A-Producing Macrophages Accompanies Post-pyelonephritic Renal Scarring

Ascending bacterial pyelonephritis, a form of urinary tract infection (UTI) that can result in hospitalization, sepsis, and other complications, occurs in ~250,000 US patients annually; uropathogenic Escherichia coli (UPEC) cause a large majority of these infections. Although UTIs are primarily a disease of women, acute pyelonephritis in males is associated with increased mortality and morbidity, including renal scarring, and end-stage renal disease. Preclinical models of UTI have only recently allowed investigation of sex and sex-hormone effects on pathogenesis. We previously demonstrated that renal scarring after experimental UPEC pyelonephritis is augmented by androgen exposure; testosterone exposure increases both the severity of pyelonephritis and the degree of renal scarring in both male and female mice. Activin A is an important driver of scarring in non-infectious renal injury, as well as a mediator of macrophage polarization. In this work, we investigated how androgen exposure influences immune cell recruitment to the UPEC-infected kidney and how cell-specific activin A production affects post-pyelonephritic scar formation. Compared with vehicle-treated females, androgenized mice exhibited reduced bacterial clearance from the kidney, despite robust myeloid cell recruitment that continued to increase as infection progressed. Infected kidneys from androgenized mice harbored more alternatively activated (M2) macrophages than vehicle-treated mice, reflecting an earlier shift from a pro-inflammatory (M1) phenotype. Androgen exposure also led to a sharp increase in activin A-producing myeloid cells in the infected kidney, as well as decreased levels of follistatin (which normally antagonizes activin action). As a result, infection in androgenized mice featured prolonged polarization of macrophages toward a pro-fibrotic M2a phenotype, accompanied by an increase in M2a-associated cytokines. These data indicate that androgen enhancement of UTI severity and resulting scar formation is related to augmented local activin A production and corresponding promotion of M2a macrophage polarization.

Activin A in Mammalian Physiology

Activins are dimeric glycoproteins belonging to the transforming growth factor beta superfamily and resulting from the assembly of two beta subunits, which may also be combined with alpha subunits to form inhibins. Activins were discovered in 1986 following the isolation of inhibins from porcine follicular fluid, and were characterized as ovarian hormones that stimulate follicle stimulating hormone (FSH) release by the pituitary gland. In particular, activin A was shown to be the isoform of greater physiological importance in humans. The current understanding of activin A surpasses the reproductive system and allows its classification as a hormone, a growth factor, and a cytokine. In more than 30 yr of intense research, activin A was localized in female and male reproductive organs but also in other organs and systems as diverse as the brain, liver, lung, bone, and gut. Moreover, its roles include embryonic differentiation, trophoblast invasion of the uterine wall in early pregnancy, and fetal/neonate brain protection in hypoxic conditions. It is now recognized that activin A overexpression may be either cytostatic or mitogenic, depending on the cell type, with important implications for tumor biology. Activin A also regulates bone formation and regeneration, enhances joint inflammation in rheumatoid arthritis, and triggers pathogenic mechanisms in the respiratory system. In this 30-yr review, we analyze the evidence for physiological roles of activin A and the potential use of activin agonists and antagonists as therapeutic agents.

Activin-A causes Hepatic stellate cell activation via the induction of TNFα and TGFβ in Kupffer cells

BACKGROUND & AIMS

TGFβ superfamily member Activin-A is a multifunctional hormone/cytokine expressed in multiple tissues and cells, where it regulates cellular differentiation, proliferation, inflammation and tissue architecture. High activin-A levels have been reported in alcoholic cirrhosis and non-alcoholic steatohepatitis (NASH). Our aim was to identify the cell types involved in the fibrotic processes induced by activin-A in liver and verify the liver diseases that this molecule can be found increased.

METHODS

We studied the effect of activin-A on mouse primary Kupffer cells (KCs) and Hepatic Stellate cells (HSCs) and the levels of activin-A and its inhibitor follistatin in the serum of patients from a large panel of liver diseases.

RESULTS

Activin-A is expressed by mouse hepatocytes, HSCs and Liver Sinusoid Endothelial cells but not KCs. Each cell type expresses different activin receptor combinations. HSCs are unresponsive to activin-A due to downregulation/desensitization of type-II activin receptors, while KCs respond by increasing the expression/production of TNFα και TGFβ1. In the presence of KCs or conditioned medium from activin-A treated KCs, HSCs switch to a profibrogenic phenotype, including increased collagen and αSMA expression and migratory capacity. Incubation of activin-A treated KC conditioned medium with antibodies against TNFα and TGFβ1 partially blocks its capacity to activate HSCs. Only patients with alcoholic liver diseases and NASH cirrhosis have significantly higher activin-A levels and activin-A/follistatin ratio.

CONCLUSIONS

Activin-A may induce fibrosis in NASH and alcoholic cirrhosis via activation of KCs to express pro-inflammatory molecules that promote HSC-dependent fibrogenesis and could be a target for future anti-fibrotic therapies.

The regulation and functions of activin and follistatin in inflammation and immunity

The activins are members of the transforming growth factor β superfamily with broad and complex effects on cell growth and differentiation. Activin A has long been known to be a critical regulator of inflammation and immunity, and similar roles are now emerging for activin B, with which it shares 65% sequence homology. These molecules and their binding protein, follistatin, are widely expressed, and their production is increased in many acute and chronic inflammatory conditions. Synthesis and release of the activins are stimulated by inflammatory cytokines, Toll-like receptor ligands, and oxidative stress. The activins interact with heterodimeric serine/threonine kinase receptor complexes to activate SMAD transcription factors and the MAP kinase signaling pathways, which mediate inflammation, stress, and immunity. Follistatin binds to the activins with high affinity, thereby obstructing the activin receptor binding site, and targets them to cell surface proteoglycans and lysosomal degradation. Studies on transgenic mice and those with gene knockouts, together with blocking studies using exogenous follistatin, have established that activin A plays critical roles in the onset of cachexia, acute and chronic inflammatory responses such as septicemia, colitis and asthma, and fibrosis. However, activin A also directs the development of monocyte/macrophages, myeloid dendritic cells, and T cell subsets to promote type 2 and regulatory immune responses. The ability of both endogenous and exogenous follistatin to block the proinflammatory and profibrotic actions of activin A has led to interest in this binding protein as a potential therapeutic for limiting the severity of disease and to improve subsequent damage associated with inflammation and fibrosis. However, the ability of activin A to sculpt the subsequent immune response as well means that the full range of effects that might arise from blocking activin bioactivity will need to be considered in any therapeutic applications.

Induction of follistatin precedes gastric transformation in gastrin deficient mice

We previously showed that antral gastric tumors develop in gastrin-deficient (Gas(-/-)) mice. Therefore Gas(-/-) mice were studied sequentially over 12 months to identify molecular mechanisms underlying gastric transformation. Fundic atrophy developed by 9 months in Gas(-/-) mice. Antral mucosal hyperplasia developed coincident with the focal loss of TFF1 and Muc5AC. Microarray analysis of 12 month Gas(-/-) tumors revealed an increase in follistatin, an activin/BMP antagonist. We found that elevated follistatin expression occurred in the proliferative neck zone of hyperplastic antrums, in antral tumors of Gas(-/-) mice, and also in human gastric cancers. Follistatin induced cyclin D1 and the trefoil factors TFF1 and TFF2 in a gastric cancer cell line. We concluded that antral hyperplasia in Gas(-/-) mice involves amplification of mucous cell lineages due to follistatin, suggesting its role in the development of antral gastric tumors.

Activin acts with nerve growth factor to regulate calcitonin gene-related peptide mRNA in sensory neurons

Calcitonin gene-related peptide (CGRP) increases in sensory neurons after inflammation and plays an important role in abnormal pain responses, but how this neuropeptide is regulated is not well understood. Both activin A and nerve growth factor (NGF) increase in skin after inflammation and induce CGRP in neurons in vivo and in vitro. This study was designed to understand how neurons integrate these two signals to regulate the neuropeptide important for inflammatory pain. In adult dorsal root ganglion neurons, NGF but not activin alone produced a dose-dependent increase in CGRP mRNA. When added together with NGF, activin synergistically increased CGRP mRNA, indicating that sensory neurons combine these signals. Studies were then designed to learn if that combination occurred at a common receptor or shared intracellular signals. Studies with activin IB receptor or tyrosine receptor kinase A inhibitors suggested that each ligand required its cognate receptor to stimulate the neuropeptide. Further, activin did not augment NGF-initiated intracellular mitogen-activated protein kinase signals but instead stimulated Smad phosphorylation, suggesting these ligands initiated parallel signals in the cytoplasm. Activin synergy required several NGF intracellular signals to be present. Because activin did not further stimulate, but did require NGF intracellular signals, it appears that activin and NGF converge not in receptor or cytoplasmic signals, but in transcriptional mechanisms to regulate CGRP in rat sensory neurons after inflammation.

Activin is a neuronal survival factor that is rapidly increased after transient cerebral ischemia and hypoxia in mice

One approach for developing targeted stroke therapies is to identify the neuronal protective and destructive signaling pathways and gene expression that follow ischemic insult. In some neural injury models, the transforming growth factor-beta family member activin can provide neuroprotective effects in vivo and promote neuronal survival. This study tests if activin supports cortical neurons after ischemic challenge in vitro and if signals after cerebral ischemia involve activin in vivo. In a defined cell culture model that uses hydrogen peroxide (H(2)O(2))-free radical stress, activin addition maintained neuronal survival. H(2)O(2) treatment increased activin mRNA twofold in surviving cortical neurons, and inhibition of activin with neutralizing antibodies caused neuronal death. These data identify activin gene changes as a rapid response to oxidative stress, and indicate that endogenous activin acts as a protective factor for cortical neurons in vitro. Similarly, after transient focal cerebral ischemia in adult mice, activin mRNA increased at 1 and 4 h ipsilateral to the infarct but returned to control values at 24 h after reperfusion. Intracellular activated smad signals were detected in neurons adjacent to the infarct. Activin was also increased after 2 h of 11% hypoxia. Activin mRNA increased at 1 h but not 4 or 24 h after hypoxia, similar to the time course of erythropoietin and vascular endothelial growth factor induction. These findings identify activin as an early-regulated gene response to transient ischemia and hypoxia, and its function in cortical neuron survival during oxidative challenge provides a basis to test activin as a potential therapeutic in stroke injury.

An overview of acetic acid ulcer models--the history and state of the art of peptic ulcer research

Four types of experimental chronic ulcer models, named acetic acid ulcer models, have been developed to examine the healing process of peptic ulcers, screen anti-ulcer drugs, and better evaluate the adverse effects of various anti-inflammatory drugs on the gastrointestinal mucosa. The model easily and reliably produces round, deep ulcers in the stomach and duodenum, allowing acetic acid ulcer production in mice, rats, Mongolian gerbils, guinea pigs, cats, dogs, miniature pigs, and monkeys. These ulcer models highly resemble human ulcers in terms of both pathological features and healing process. The models have been established over the past 35 years and are now used throughout the world by basic and clinical scientists. One of the characteristic features of acetic acid ulcers in rats is the spontaneous relapse of healed ulcers >100 d after ulceration, an endoscopically confirmed phenomenon. Indomethacin significantly delays the healing of acetic acid ulcers, probably by reducing endogenous prostaglandins and inhibiting angiogenesis in ulcerated tissue. Helicobacter pylori significantly delays healing of acetic acid ulcers and causes relapse of healed ulcers at a high incidence in Mongolian gerbils. Anti-secretory drugs (e.g. omeprazole), prostaglandin analogs, mucosal defense agents (e.g. sucralfate), and various growth factors all significantly enhance healing of acetic acid ulcers. Gene therapy with epidermal growth factor and vascular endothelial growth factor applied to the base of acetic acid ulcers in rats is effective in enhancing ulcer healing. Since an inhibitor of nitric oxide syntase prevents ulcer healing, nitric oxide might be involved in the mechanism underlying ulcer healing. We conclude that acetic acid ulcer models are quite useful for various studies related to peptic ulcers.

Activin induces tactile allodynia and increases calcitonin gene-related peptide after peripheral inflammation

Calcitonin gene-related peptide (CGRP) is a sensory neuropeptide important in inflammatory pain that conveys pain information centrally and dilates blood vessels peripherally. Previous studies indicate that activin A increases CGRP-immunoreactive (IR) sensory neurons in vitro, and following wound, activin A protein increases in the skin and more neurons have detectable CGRP expression in the innervating dorsal root ganglion (DRG). These data suggest some adult sensory neurons respond to activin A or other target-derived factors with increased neuropeptide expression. This study was undertaken to test whether activin contributes to inflammatory pain and increased CGRP and to learn which neurons retained plasticity. After adjuvant-induced inflammation, activin mRNA, but not NGF or glial cell line-derived neurotrophic factor, increased in the skin. To examine which DRG neurons increased CGRP immunoreactivity, retrograde tracer-labeled cutaneous neurons were characterized after inflammation. The proportion and size of tracer-labeled DRG neurons with detectable CGRP increased after inflammation. One-third of CGRP-IR neurons that appear after inflammation also had isolectin B4 binding, suggesting that some mechanoreceptors became CGRP-IR. In contrast, the increased proportion of CGRP-IR neurons did not appear to come from RT97-IR neurons. To learn whether central projections were altered after inflammation, CGRP immunoreactivity in the protein kinase Cgamma-IR lamina IIi was quantified and found to increase. Injection of activin A protein alone caused robust tactile allodynia and increased CGRP in the DRG. Together, these data support the hypothesis that inflammation and skin changes involving activin A cause some sensory neurons to increase CGRP expression and pain responses.