Selective Pro-Inflammatory Activation of Astrocytes by High Mobility Group Box 1 Protein Signaling - New Insights to Neuroimmune Biology - page 48

Recently, emerging roles for HMGB1/RAGE have been proposed in neuropa-
thology and neuroinflammatory responses [22,23] . Normal brain, cultured neu-
rons, endothelial, and glial cells express HMGB1, and the protein level changes in
various brain regions in dependence of age [24,25] . A significant up-regulation of
brain HMGB1 has been shown to occur in response to cortical impact injury, sug-
gesting that this protein could play a role in the regeneration of the damaged tissue
[26] . Because microglia and astrocytes can export HMGB1 when challenged with
specific stimuli, and neurons also release this cytokine when exposed to glutamate
excitotoxic concentrations, the local amount of extracellular HMGB1 in the CNS
can rapidly increase [27-30] . Moreover, HMGB1 receptors undergo up-regulation in
inflammatory diseases of the CNS, thus contributing to enhancement of paracrine
and autocrine cytokine-like activities exerted by HMGB1 [31,32] . HMGB1 inhibi-
tors can prevent these pro-inflammatory responses, indicating that the cytokine plays
a nonredundant role as an inducer of neuroinflammation [33,34] . However, HMGB1
can also bind pro-inflammatory cytokines, such as IL-1, potentiating their activity
[35] . Hence, in the presence of low concentrations of IL-1, the inflammatory pro-
cess can be highly amplified by a concomitant release of the HMGB1 costimulus.
Glial cells of the CNS are implicated in a variety of neurological disorders and
are actively involved in local responses, including inflammation and tissue repair
processes [36,37] . Recently, microglia have been identified as a target of HMGB1.
Specifically, the cytokine released from dying neurons inhibits microglial amy-
loid beta (A) peptide 42 clearance and enhances the neurotoxicity of A42 [38] .
Because astrocytes are equipped with all the surface receptors for HMGB1 and this
cytokine activates the release of excitatory amino acids from subcellular particles of
astrocyte origin, here we have analyzed the possible functional role of HMGB1 on
these glial cells [39-41] . Astrocytes are the most abundant macroglial resident cells
of the CNS and play important roles in the homeostasis of the normal brain—but
can also assume different active states when exposed to specific insults or chemi-
cal stimuli [42] . In keeping with these observations, depending on the inflammatory
pattern expressed, these cells are able to produce a plethora of mediators of glial and
neuronal cell responses [43] . To determine the specific reactive phenotype induced
by HMGB1, we have explored the expression of astrocyte gene markers involved
in cell stress and death, immunity, signaling, adhesion, metabolism, and proteoly-
sis and the functional implications of this cell activation. Our results indicate that
HMGB1-stimulated astrocytes release several protein factors involved in the progres-
sion of inflammatory processes, including chemoattractants, toward immune effector
leukocytes.
3.2 Proteomic Analysis of Astrocytes Stimulated
with HMGB1
Astrocyte cultures with microglial contamination below 1% were prepared from brains
of 1- to 2-day-old rats [44] . The potential contribution of residual contaminating