Toll-like receptors control p38 and JNK MAPK signaling pathways in rat astrocytes differently, when cultured in normal or high glucose concentrations

Abstract

Astrocytes play a vital role in regulating central nervous system inflammation, energy metabolism and brain homeostasis. Unlike macrophages and microglia, which are cells of myeloid ancestry, astrocytes are of ectodermal origin. However, regulatory specificities of signaling pathways connecting inflammatory and metabolic processes are still largely unknown. We analyzed firstly cellular responses to toll-like receptor (TLR) agonists and secondly, modulation of the mRNA of the three isoforms of the transcription factors PPARs (peroxisome proliferator-activated receptors) in primary rat astrocytes exposed to normal glucose (5.5 mM) and high glucose (25 mM). Cell culturing of rat brain astrocytes for 2 days in high glucose did not alter cellular morphology, but i) enhanced the release of TNFα that was induced by TLR4 agonist LPS or TLR3 agonist PIC and the synthesis of prostaglandin E₂ (PGE₂), ii) changed the signaling pathways of TLR4/MAPK (increase in p38 MAPK, and decrease in JNK activities at early stages of TLR activation) and iii) modulated mRNA expression of PPARs. High glucose cultivation reduced PPARα and PPARβ mRNA levels, without altering PPARγ mRNA level and changed the sensitivity of expressions to agonists of TLR1/2 (PGN), TLR4 (LPS), TLR3 (PIC), and TLR5 (FGN). Differences between low and high glucose-adapted cells were obtained for agonists of TLR1/2 (PPARα, PPARβ), TLR4 (PPAR β), TLR3 (PPARα). In the TLR4/p38/PPARβ signaling pathway, there was a stimulatory connection in normal glucose but an inhibitory connection in high glucose. TLR4/JNK/activated PPARβ, TLR4/JNK/inhibited PPARγ both in cells adapted to normal or high glucose, but PPARα expression was not affected. As PPARs in astrocytes are involved in inflammatory processes in the form of the recently published PPAR triad, the changes in expression revealed here are most likely resulting in implications of high glucose in inflammatory processes. Our data underline the complexity of multiple regulatory interactions between inflammatory responses and energy metabolism in astrocytes.

Keywords: Astrocytes; High glucose; Inflammation; Mitogen-activated protein kinases (MAPK); Peroxisome proliferator-activated receptors (PPAR); Toll-like receptors (TLR).