Sulforaphane rescues amyloid-β peptide-mediated decrease in MerTK expression through its anti-inflammatory effect in human THP-1 macrophages.
J Neuroinflammation. 2018 Mar 12 ;15(1):75. Epub 2018 Mar 12. PMID: 29530050
Kyoung A Jhang
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-β (Aβ) in proinflammatory environment has not yet been ascertained.
METHODS: The objective of this study was to determine the underlying mechanism involved in Aβ-mediated decrease in MerTK expression through Aβ-mediated regulation of MerTK expression and its modulation by sulforaphane in human THP-1 macrophages challenged with Aβ1-42. We used protein preparation, Cainflux fluorescence imaging, nuclear fractionation, Western blotting techniques, and small interfering RNA (siRNA) knockdown to perform our study.
RESULTS: Aβ1-42 elicited a marked decrease in MerTK expression along with increased intracellular Calevel and induction of proinflammatory cytokines such as IL-1β and TNF-α. Ionomycin A and thapsigargin also increased intracellular Calevels and production of IL-1β and TNF-α, mimicking the effect of Aβ1-42. In contrast, the Aβ1-42-evoked responses were attenuated by depletion of Cawith ethylene glycol tetraacetic acid. Furthermore, recombinant IL-1β or TNF-α elicited a decrease in MerTK expression. However, immunodepletion of IL-1β or TNF-α with neutralizing antibodies significantly inhibited Aβ1-42-mediated downregulation of MerTK expression. Notably, sulforaphane treatment potently inhibited Aβ1-42-induced intracellular Calevel and rescued the decrease in MerTK expression by blocking nuclear factor-κB (NF-κB) nuclear translocation, thereby decreasing IL-1β and TNF-α production upon Aβ1-42 stimulation. Such adverse effects of sulforaphane were replicated by BAY 11-7082, a NF-κB inhibitor. Moreover, sulforaphane's anti-inflammatory effects on Aβ1-42-induced production of IL-1β and TNF-αwere significantly diminished by siRNA-mediated knockdown of MerTK, confirming a critical role of MerTK in suppressing Aβ1-42-induced innate immune response.
CONCLUSION: These findings implicate that targeting of MerTK with phytochemical sulforaphane as a mechanism for preventing Aβ1-42-induced neuroinflammation has potential to be applied in AD therapeutics.