SMAD4 is Involved in the Development of Endotoxin Tolerance in Microglia

Abstract

Initial exposure of macrophages to LPS induces hyporesponsiveness to a second challenge with LPS, a phenomenon termed LPS tolerance. Smad4 plays important roles in the induction of LPS tolerance. However, the function of Smad4 in microglia remains unknown. Here we show that expression of Smad4 was highly up-regulated in LPS-tolerized mouse cerebral cortex. Smad4 was mostly colocalized with microglia, rarely with neurons. Using a microglia cell line, BV2, we find that LPS activates endogenous Smad4, inducing its migration into the nucleus and increasing its expression. Smad4 significantly suppressed TLR-triggered production of proinflammatory cytokines (IL-6), increased anti-inflammatory cytokine in LPS-tolerized microglia. Moreover, IL-6 concentrations in culture supernatants after second LPS challenge are higher in SMAD4 small interfering RNA (siRNA) BV2 cells than control siRNA BV2 cells, indicating failure to induce tolerance in absence of Smad4 signaling. In our study, we conclude that both in vivo and in vitro, Smad4 signaling is required for maximal induction of endotoxin tolerance.

A unique hybrid characteristic having both pro- and anti-inflammatory phenotype transformed by repetitive low-dose lipopolysaccharide in C8-B4 microglia

“…It is generally known that a single LPS treatment induces pro-inflammatory molecules, while repetitive LPS treatment suppresses the induction of pro-inflammatory molecules, a phenomenon known as "LPS tolerance". Indeed, it has been reported that pro-inflammatory molecules such as IL-1β, IL-6, and TNF-α are suppressed in microglia by repetitive high-dose LPS treatment [17][18][19][20][21][22] . Although IL-1β, IL-6, and TNF-α were suppressed in REPELL-microglia similarly to data of past reports with a high-dose LPS model, REPELL-microglia did not exhibit global downregulation of pro-inflammatory molecules, but instead showed high expression of Nos2, Ccl1, IL-12B, and CD86 (Fig.…”

Microglial SMAD4 regulated by microRNA-146a promotes migration of microglia which support tumor progression in a glioma environment

“…While the roles of SMAD2/3 have been widely studied in microglia [ 55 , 56 ], the role of SMAD4 in microglial activation, specifically in context of the TGFβ signaling has remained unclear. There is evidence showing that SMAD4 is upregulated in LPS-activated microglia and acts as a negative feedback inhibitor of NFκB, a pro-inflammatory signaling response in the activated microglia [ 57 ]. In the present study, glioma-associated microglia expressed SMAD4 in human glioblastoma tumors in vivo and microglia exposed to GCM showed increased expression of SMAD4 in vitro .…”

“…Moreover, this study demonstrates that glioma-associated microglia promote tumorigenesis through SMAD4 expression, since glioma cells treated with conditioned medium derived from shSMAD4 microglial cells showed decreased viability. This decreased viability of glioma cells is also possible via SMAD4-induced negative regulation of NF-κB pathway in microglia, since recent studies have shown that SMAD4 knockdown in microglia induced pro-inflammatory cytokine, IL-6 in an NF-κB dependent manner [ 57 ]. Overall, these results indicate that the possible interaction between SMAD4 and NF-κB in glioma-associated microglia may determine the tumor progression.…”

Prolonged lipopolysaccharide exposure induces transient immunosuppression in BV2 microglia

“…Microglial cells are the central nervous system's (CNS) histiocytes that sense various stimuli in their territory and correspondingly shuffle between surveillance and activated state. These innate immune cells act as local sentinels that function right from early embryogenesis to autumn years of life (Liu et al, 2016). Besides, the physiological function of brain repair and scavenging apoptotic cells; they are also implicated in pathological conditions (Hanisch & Kettenmann, 2007).…”