Background: Recent studies in experimental animals show that insulin-like growth factor 1 (IGF1) plays a trophic role during development and tissue injury and that microglia are important sources of IGF1. However, little information is available regarding the expression, regulation, and function of IGF1 and related proteins in human brain cells. In the current study, we examined the expression of IGF1 and IGF2 in human microglia in vivo and in vitro.
Methods: Expression of IGF1 and IGF2 was examined by immunohistochemistry in post-mortem human brain sections derived from HIV+ and HIV- brains. In primary cultures of human fetal microglia, IGF1 and IGF2 mRNA and protein expression was examined by Q-PCR, ELISA, and Western blot analysis. Additionally, the role of IGF1 and IGF2 in neuroprotection was examined in primary human neuronal glial cultures.
Results: Immunohistochemistry of human brain tissues showed that nonparenchymal cells (vessels and meninges), as well as parenchymal microglia and macrophages were positive for IGF1, in both HIV encephalitis and control brains, while IGF2 was undetectable. Cultured microglia expressed IGF1 mRNA and produced pg/ml levels of IGF1 protein; this was significantly suppressed by proinflammatory mediators, such as lipopolysaccharide (LPS), poly(I:C), and IFNγ. The Th2 cytokines IL-4 and IL-13 had no significant effect, but the cAMP analog (dibutyryl cAMP) significantly increased IGF1 production. In contrast, microglial IGF2 mRNA and protein (determined by Western blot) were upregulated by LPS. IGF1 receptor (IGF1R) immunoreactivity was predominantly expressed by neurons, and both IGF1 and IGF2 significantly protected neurons from cytokine (IL-1/IFNγ) induced death.
Conclusions: Our study in human brain tissues and cells indicates that microglia are important sources of neurotrophic growth factors IGF1 and IGF2, and that microglial activation phenotypes can influence the growth factor expression. Importantly, our results suggest that chronic neuroinflammation and upregulation of proinflammatory cytokines could lead to neurodegeneration by suppressing the production of microglia-derived neuronal growth factors, such as IGF1.