Lipopolysaccharide stimulates differential expression of myristoylated protein kinase C substrates in murine microglia

Microglia rapidly respond to lipopolysaccharide (LPS) by transformation from resting to active states and secretion of several neuro- and immuno- regulators including tumour necrosis factor alpha (TNF-α), interleukin 1β (IL-1β), and interleukin 6 (IL-6). With longer LPS treatment, microglia are converted to reactive or phagocytic states with characteristics similar to macrophages in inflammation and injury processes. We have investigated LPS- mediated changes in two myristoylated substrates of protein kinase C (PKC): MARCKS (myristoylated alanine-rich C kinase substrate) and MRP (MARCKS- related protein). Within 6 hours of addition, LPS induced a twofold increase in [³H]myristoylated and immunoreactive MARCKS protein and a sevenfold increase in MRP. The differential effect of LPS on expression of MRP vs. MARCKS was even more dramatic at the level of transcription: S1 nuclease protection assays revealed a 40-fold increase in MRP mRNA levels (maximum at 4-6 hours), whereas a threefold increase was observed for MARCKS. TNFα and colony-stimulating factor 1 (CSF-1), two cytokines which are induced by LPS, did not reproduce the observed effect of LPS on MARCKS and MRP gene transcription. CSF-1 also induced differential transcription of MRP, but of lower magnitude (threefold) and more sustained than by LPS. Accordingly, these two substrates for PKC are differentially up-regulated by LPS, apparently independent of TNFα or CSF-1.