Down-regulation of the expression of rat inhibitor-of-apoptosis protein-1 and -3 during transforming growth factor-beta1-mediated apoptosis in rat brain microglia

Potential neuroprotective role of transforming growth factor β1 (TGFβ1) in the brain

TGFβ1 is a growth factor that is known to be expressed in most neurodegenerative diseases and after vascular accidents in the brain. TGFβ1 downregulates the activity of activated microglia and promotes astrogliosis. It also prevents cell death by a known mechanism dependant on astrocytes and the secretion of the plasminogen activator inhibitor 1 (PAI-1). This mechanism can provide light on what is the mechanism of action of TGFβ1 as a protective factor and it can provide the pharmacological principles in which this pathway could be used with therapeutic purposes. TGFβ1 is upregulated in most neurodegenerative diseases, however, its expression appears dramatically blocked in Huntington's disease, the fastest of those diseases in progress after the onset. This fact suggests that TGFβ1 slows down the neurodegenerative process, preventing tissue damage and neural apoptotic death. However, the exact details of TGFβ1 action are still unknown and the physiological roles on the diseases are still mysterious. Interestingly, all the data regarding the roles of TGFβ1 in health and disease have been also confirmed with the use of transgenic knockouts and TGFβ1 overexpressing mice. What possibly came as a surprise from the study of TGFβ1 overexpressing models is that combining its neuroprotective and antiproliferative effects, this cytokine generates a significant disruption in the hippocampal circuitry with its consequent learning and memory deficit.

The neuroprotective functions of transforming growth factor beta proteins

Transforming growth factor beta (TGF-β) proteins are multifunctional cytokines whose neural functions are increasingly recognized. The machinery of TGF-β signaling, including the serine kinase type transmembrane receptors, is present in the central nervous system. However, the 3 mammalian TGF-β subtypes have distinct distributions in the brain suggesting different neural functions. Evidence of their involvement in the development and plasticity of the nervous system as well as their functions in peripheral organs suggested that they also exhibit neuroprotective functions. Indeed, TGF-β expression is induced following a variety of types of brain tissue injury. The neuroprotective function of TGF-βs is most established following brain ischemia. Damage in experimental animal models of global and focal ischemia was shown to be attenuated by TGF-βs. In addition, support for their neuroprotective actions following trauma, sclerosis multiplex, neurodegenerative diseases, infections, and brain tumors is also accumulating. The review will also describe the potential mechanisms of neuroprotection exerted by TGF-βs including anti-inflammatory, -apoptotic, -excitotoxic actions as well as the promotion of scar formation, angiogenesis, and neuroregeneration. The participation of these mechanisms in the neuroprotective effects of TGF-βs during different brain lesions will also be discussed.

Fas activation increases neural progenitor cell survival

Although there is a sizable amount of research focusing on adult neural progenitor cells (NPCs) as a therapeutic approach for many neurodegenerative diseases, including multiple sclerosis, little is known about the pathways that govern NPC survival and apoptosis. Fas, a member of the death receptor superfamily, plays a well-characterized role in the immune system, but its function in neural stem cells remains uncertain. Our study focuses on the effects of Fas on NPC survival in vitro. Activation of Fas by recombinant Fas ligand (FasL) did not induce apoptosis in murine NPCs in culture. In fact, both an increase in the amount of viable cells and a decrease in apoptotic and dying cells were observed with FasL treatment. Our data indicate that FasL-mediated adult NPC neuroprotection is characterized by a reduction in apoptosis, but not increased proliferation. Further investigation of this effect revealed that the antiapoptotic effects of FasL are mediated by the up-regulation of Birc3, an inhibitor of apoptosis protein (IAP). Conversely, the observed effect is not the result of altered caspase activation or FLIP (Fas-associated death domain-like interleukin-1beta-converting enzyme inhibitory protein) up-regulation, which is known to inhibit caspase-8-mediated cell death in T cells. Our data indicate that murine adult NPCs are resistant to FasL-induced cell death. Activation of Fas increased cell survival by decreasing apoptosis through Birc3 up-regulation. These results describe a novel pathway involved in NPC survival.

Chunghyuldan attenuates brain microglial inflammatory response

Microglial cells are the prime effectors in immune and inflammatory responses of the central nervous system (CNS). During pathological conditions, the activation of these cells helps restore CNS homeostasis. However, chronic microglial activation endangers neuronal survival through the release of various proinflammatory molecules and neurotoxins. Thus, negative regulators of microglial activation have been considered as potential therapeutic candidates to target stroke and neurodegenerative diseases. Chunghyuldan, a combinatorial drug consisting of Scutellariae Radix, Coptidis Rhizoma, Phellodendri Cortex, Gardeniae Fructus, and Rhei Rhizoma, has an inhibitory effect on stroke recurrence in patients with small-vessel disease. It has also been reported to confer antihypertensive, antihyperlipidemic, and antiinflammatory effects. The aim of this study was to examine whether Chunghyuldan suppresses microglial activation. Chunghyuldan was effective at inhibiting LPS-induced nitric oxide (NO) release from rat brain microglia. Real-time reverse transcriptase PCR analysis revealed that pretreatment of rat brain microglia with Chunghyuldan attenuated the LPS-induced expression of mRNAs encoding inducible NO synthase, tumor necrosis factor (TNF)-α, interleukin-1β, and cyclooxygenase-2. In rat brain microglia, Chunghyuldan reduced the LPS-stimulated production of TNF-α and prostaglandin E2. In addition, Chunghyuldan significantly decreased LPS-induced phosphorylation of the ERK1/2 and p38 signaling proteins. These results suggest that Chunghyuldan provide neuroprotection by reducing the release of various proinflammatory molecules from activated microglia.

Shikonins attenuate microglial inflammatory responses by inhibition of ERK, Akt, and NF-kappaB: neuroprotective implications

Microglial cells are the prime effectors in immune and inflammatory responses of the central nervous system (CNS). During pathological conditions, the activation of these cells helps restore CNS homeostasis. However, chronic microglial activation endangers neuronal survival through the release of various proinflammatory molecules and neurotoxins. Thus, negative regulators of microglial activation have been considered as potential therapeutic candidates to target neurodegeneration, such as that in Alzheimer's and Parkinson's diseases. Shikonin, a naphthoquinone pigment from the root of Lithospermum erythrorhizon, has long been used as an ointment for wound healing in traditional oriental medicine. Shikonin has been reported to have antibacterial, antitumor, and anti-inflammatory effects. The aim of this study was to examine whether shikonin represses microglial activation. In a study of shikonin and five of its derivatives, isobutyrylshikonin (IBS) and isovalerylshikonin (IVS) were the most effective at inhibiting LPS-induced nitric oxide (NO) release from microglial cells. Reverse transcriptase real-time PCR analysis revealed that pretreatment of rat brain microglia with IBS and IVS attenuated the LPS-induced expression of mRNAs encoding inducible NO synthase, tumor necrosis factor (TNF)-alpha, interleukin-1beta, and cyclooxygenase-2. In rat brain microglia, IBS and IVS reduced the LPS-stimulated production of TNF-alpha and prostaglandin E2. In addition, IBS and IVS significantly decreased LPS-induced IkappaB-alpha phosphorylation and NF-kappaB DNA binding activity, as well as the phosphorylation of the ERK1/2 and Akt signaling proteins. In organotypic hippocampal slice cultures, propidium iodide staining revealed prominent cell death in the hippocampal layer after 72h of LPS treatment. Both IBS and IVS clearly blocked the effect of LPS on hippocampal cell death and inhibited LPS-induced NO production in culture medium. These results suggest that IBS and IVS provide neuroprotection by reducing the release of various proinflammatory molecules from activated microglia.

Lipopolysaccharide induces hypoxia-inducible factor-1 alpha mRNA expression and activation via NADPH oxidase and Sp1-dependent pathway in BV2 murine microglial cells

Hypoxia-inducible factor-1 (HIF-1), the key transcription factor of hypoxia-inducible genes, is known to be involved in inflammation and immune response, but little is known about the regulation of HIF-1 during microglial activation. Thus, we examined effect of lipopolysaccharide (LPS) on HIF-1 activation and its signaling mechanism in BV2 microglial cells. LPS induced HIF-1alpha mRNA and protein expression as well as HIF-1 transcriptional activation. Moreover, HIF-1alpha knockdown by small interfering RNA (siRNA) decreased LPS-induced expression of hypoxia responsive genes, VEGF, iNOS, and COX-2. We then showed that LPS-induced HIF-1alpha mRNA expression was blocked by an antioxidant, NADPH oxidase inhibitors, and siRNA of gp91phox, a subunit of NADPH oxidase. In addition, we showed that specific pharmacological inhibitors of PI 3-kinase and protein kinase C decreased LPS-induced HIF-1alpha mRNA expression. Finally, we showed that inhibition of transcription factor Sp1 by mithramycin A or Sp1 siRNA decreased LPS-induced HIF-1alpha mRNA and protein expression. Consistently, LPS increased Sp1 DNA binding and its transcriptional activity. Taken together, these results suggest that LPS induces HIF-1alpha mRNA expression and activation via NADPH oxidase and Sp1 in BV2 microglia.