Regulation of nerve growth factor secretion and mRNA expression by bacterial lipopolysaccharide in primary cultures of rat astrocytes.

The present work was undertaken to study the effect of bacterial lipopolysaccharide (LPS), a potent activator of the host inflammatory response, on the synthesis of nerve growth factor (NGF) by newborn rat brain astrocytes. Treatment of primary rat astroglial cells cultured in chemically defined medium with LPS resulted in a dose-dependent accumulation of NGF mRNA, and an increased release of NGF protein in the cell medium. NGF mRNA levels were maximal after 24 hr of stimulation (8-fold increase), whereas extracellular NGF peaked after 72 hours of treatment (17-fold increase). This dramatic increase of extracellular NGF was abrogated if cells were treated with actinomycin D or cycloheximide, a fact which implies that the accumulation of extracellular NGF by LPS-treated cells requires DNA transcription and RNA translation. Stimulation of NGF synthesis and secretion was: (i) unaffected by treatment with the protein kinase C inhibitor bisindolylmaleimide, and (ii) prevented by forskolin and 3-isobutyl-1-methylxanthine, two agents which increase cAMP levels. Inhibition of LPS effect was also obtained with apigenin, a proposed inhibitor of the mitogen-activated protein kinase pathway. Results thus show that LPS stimulates NGF synthesis by astroglial cells through a mechanism that is independent of protein kinase C (PKC), antagonized by cAMP-elevating agents, and probably mediated by the mitogen-activated protein kinase cascade. The data raise the possibility that LPS exerts stimulatory effects on NGF synthesis that are independent of those elicited by astrocyte-derived inflammatory lymphokines such as IL-1beta, TNF alpha or TGF beta1.