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

ABSTRACT

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.