Sulforaphane rescues amyloid-ß peptide-mediated decrease in MerTK expression through its anti-inflammatory effect in human THP-1 macrophages.

BACKGROUND: Mer tyrosine kinase (MerTK) activity necessary for amyloid-stimulated phagocytosis strongly implicates that MerTK dysregulation might contribute to chronic inflammation implicated in Alzheimer's disease (AD) pathology. However, the precise mechanism involved in the regulation of MerTK expression by amyloid-ß (Aß) in proinflammatory environment has not yet been ascertained. METHODS: The objective of this study was to determine the underlying mechanism involved in Aß-mediated decrease in MerTK expression through Aß-mediated regulation of MerTK expression and its modulation by sulforaphane in human THP-1 macrophages challenged with Aß1-42. We used protein preparation, Ca2+ influx fluorescence imaging, nuclear fractionation, Western blotting techniques, and small interfering RNA (siRNA) knockdown to perform our study. RESULTS: Aß1-42 elicited a marked decrease in MerTK expression along with increased intracellular Ca2+ level and induction of proinflammatory cytokines such as IL-1ß and TNF-α. Ionomycin A and thapsigargin also increased intracellular Ca2+ levels and production of IL-1ß and TNF-α, mimicking the effect of Aß1-42. In contrast, the Aß1-42-evoked responses were attenuated by depletion of Ca2+ with ethylene glycol tetraacetic acid. Furthermore, recombinant IL-1ß or TNF-α elicited a decrease in MerTK expression. However, immunodepletion of IL-1ß or TNF-α with neutralizing antibodies significantly inhibited Aß1-42-mediated downregulation of MerTK expression. Notably, sulforaphane treatment potently inhibited Aß1-42-induced intracellular Ca2+ level and rescued the decrease in MerTK expression by blocking nuclear factor-κB (NF-κB) nuclear translocation, thereby decreasing IL-1ß and TNF-α production upon Aß1-42 stimulation. Such adverse effects of sulforaphane were replicated by BAY 11-7082, a NF-κB inhibitor. Moreover, sulforaphane's anti-inflammatory effects on Aß1-42-induced production of IL-1ß and TNF-α were significantly diminished by siRNA-mediated knockdown of MerTK, confirming a critical role of MerTK in suppressing Aß1-42-induced innate immune response. CONCLUSION: These findings implicate that targeting of MerTK with phytochemical sulforaphane as a mechanism for preventing Aß1-42-induced neuroinflammation has potential to be applied in AD therapeutics.

Resveratrol Ameliorates Tau Hyperphosphorylation at Ser396 Site and Oxidative Damage in Rat Hippocampal Slices Exposed to Vanadate: Implication of ERK1/2 and GSK-3ß Signaling Cascades.

The objective of this study was to investigate the effect of resveratrol (a natural polyphenolic phytostilbene) on tau hyperphosphorylation and oxidative damage induced by sodium orthovanadate (Na3VO4), the prevalent species of vanadium (vanadate), in rat hippocampal slices. Our results showed that resveratrol significantly inhibited Na3VO4-induced hyperphosphorylation of tau at the Ser396 (p-S396-tau) site, which is upregulated in the hippocampus of Alzheimer's disease (AD) brains and principally linked to AD-associated cognitive dysfunction. Subsequent mechanistic studies revealed that reduction of ERK1/2 activation was involved in the inhibitory effect of resveratrol by inhibiting the ERK1/2 pathway with SL327 mimicking the aforementioned effect of resveratrol. Moreover, resveratrol potently induced GSK-3ß Ser9 phosphorylation and reduced Na3VO4-induced p-S396-tau levels, which were markedly replicated by pharmacologic inhibition of GSK-3ß with LiCl. These results indicate that resveratrol could suppress Na3VO4-induced p-S396-tau levels via downregulating ERK1/2 and GSK-3ß signaling cascades in rat hippocampal slices. In addition, resveratrol diminished the increased extracellular reactive oxygen species generation and hippocampal toxicity upon long-term exposure to Na3VO4 or FeCl2. Our findings strongly support the notion that resveratrol may serve as a potential nutraceutical agent for AD.

S100A9 Exacerbates the Aß1-42-mediated Innate Immunity in Human THP-1 Monocytes.

The S100A9 protein is an important proinflammatory factor of innate immunity that has been proposed to participate in inflammation associated with the pathogenesis of Alzheimer's disease. Here, we provide insights into the potential roles of extracellular S100A9 in the interaction with the immune response in human THP-1 monocytic cells that have been challenged with amyloid ß1-42 (Aß1-42) monomers instead of oligomers. Extracellular S100A9 alone produced a stimulatory effect on tumor necrosis factor-α and interleukin-1ß, expression as well as released monocyte chemoattractant protein-1 into culture supernatants, which was accompanied by an increased level of matrix metalloproteinas-9 activity. Importantly, co-stimulation with S100A9 and Aß1-42 resulted in a marked enhancement of Aß1-42-mediated release of these proinflammatory mediators under the same experimental conditions, whereas heat inactivated S100A9 had little effect. Our findings clearly suggest that excess S100A9 protein may play an important role in the pathological processes of Alzheimer's disease by exacerbating the Aß1-42-induced innate immune response.

Sulforaphane exerts its anti-inflammatory effect against amyloid-ß peptide via STAT-1 dephosphorylation and activation of Nrf2/HO-1 cascade in human THP-1 macrophages.

Alzheimer's disease (AD) is the most common neurodegenerative disorder worldwide, accounting for most cases of dementia in elderly individuals, and effective therapies are still lacking. This study was designed to investigate the anti-inflammatory properties of sulforaphane against Aß1-42 monomers in human THP-1 microglia-like cells. The results showed that sulforaphane preferentially inhibited cathepsin B- and caspase-1-dependent NLRP3 inflammasome activation induced by mostly Aß1-42 monomers, an effect that potently reduced excessive secretion of the proinflammatory cytokine interleukin-1ß (IL-1ß). Subsequent mechanistic studies revealed that sulforaphane mitigated the activation of signal transducer and activator of transcription-1 induced by Aß1-42 monomers. Sulforaphane also increased nuclear factor erythroid 2-related factor 2 (Nrf2) nuclear translocation, which was followed by upregulation of heme-oxygenase 1 (HO-1). The anti-inflammatory effect of sulforaphane on Aß1-42-induced IL-1ß production was diminished by small interfering RNA-mediated knockdown of Nrf2 or HO-1. Moreover, sulforaphane significantly attenuated the levels of microRNA-146a, which is selectively upregulated in the temporal cortex and hippocampus of AD brains. The aforementioned effects of sulforaphane were replicated by the tyrosine kinase inhibitor, herbimycin A, and Nrf2 activator. These results indicate that signal transducer and activator of transcription-1 dephosphorylation, HO-1 and its upstream effector, Nrf2, play a pivotal role in triggering an anti-inflammatory signaling cascade of sulforaphane that results in decreases of IL-1ß release and microRNA-146a production in Aß1-42-stimulated human microglia-like cells. These findings suggest that the phytochemical sulforaphane has a potential application in AD therapeutics.

Norepinephrine provides short-term neuroprotection against Aß1-42 by reducing oxidative stress independent of Nrf2 activation.

Pathophysiological evidence correlating locus ceruleus neuron loss with increased Alzheimer's disease pathology suggests that norepinephrine (NE) is neuroprotective. Here, we evaluated the effects of NE on amyloid-ß (Aß)1-42-induced neurotoxicity and determined how NE exerts its actions in human SK-N-SH neurons. NE protected SK-N-SH cells against Aß1-42-induced neurotoxicity only after a 4-hour treatment. The ability of NE to reduce Aß1-42-induced neurotoxicity was independent of the adrenoceptor signaling pathway. Notably, NE downregulated Aß1-42-mediated increases in intracellular reactive oxygen species (ROS) production. However, NE did not affect Aß1-42-induced activation of the nuclear factor erythroid 2-related factor 2 (Nrf2) redox signaling pathway, known to be involved in oxidative stress. Among the antioxidants tested, N-acetyl cysteine and glutathione, which are not only ROS scavengers but also thiol-reducing agents, mimicked the protective effects of NE. Consistently, Kelch-like ECH-associating protein 1 inhibitors, which activated the Nrf2 pathway, failed to decrease Aß1-42-induced ROS generation and elicited no protection against Aß1-42. Taken together, these findings suggest that NE could exert neuroprotective function against Aß1-42 via redox cycling and reduction of intracellular oxidative stress regardless of downstream activation of the Nrf2 pathway.