Microglial LRP1 modulates JNK activation: A signaling cascade that also regulates apolipoprotein E levels

Abstract

Apolipoprotein E (apoE) has been implicated in modulating the central nervous system (CNS) inflammatory response. However, the molecular mechanisms involved in the apoE-dependent immunomodulation are poorly understood. We hypothesize that apoE alters the CNS inflammatory response by signaling via low-density lipoprotein (LDL) receptor family members in glia. To address this hypothesis, we used a small bioactive peptide formed from the receptor-binding domain of apoE, apoE peptide (EP), to study LDL receptor family signaling in microglia. To model glial activation, we treated primary mouse microglia and the microglial cell line BV2 with lipopolysaccharide (LPS) and studied two inflammatory responses: an increase in nitric oxide production (NO) and a decrease in apoE production. We found that treatment of microglia with EP attenuated LPS-induced NO accumulation and apoE reduction via LDL receptor family members. We found that EP decreased JNK activation and thereby suppressed the LPS-induced JNK activation, which proved essential to increase NO and decrease apoE production. Next, we investigated whether activation of low-density lipoprotein receptor-related protein 1 (LRP1), a member of the LDL receptor family, mediated the anti-inflammatory effects of EP. To address this hypothesis, we studied LPS-induced inflammation and EP-induced signaling in primary microglia that were LRP1-deficient. In these studies, EP did not attenuate LPS-induced NO accumulation and JNK activation in LRP1-deficient microglia. Thus, we concluded that activation of LRP1 is essential for EP-induced anti-inflammatory properties in microglia. Lastly, we investigated the impact of JNK inhibition on apoE production in the brain. For these studies, we used two JNK inhibitors, JNK inhibitor I (L)-form (L-JNK1) and SP600125. We studied changes to apoE levels in primary glia treated with JNK inhibitors and mice injected with JNK inhibitor. Using APOE promoter luciferase reporter constructs, we found an increase in APOE promoter activity when JNK was inhibited. Our studies showed that APOE transcription and apoE protein levels are increased with JNK inhibition. We suggest that JNK inhibitors may prove useful by increasing apoE and its protective anti-inflammatory properties.