Search

The signaling pathways activated by the macrophage colony-stimulating factor (M-CSF) to promote survival of monocyte and macrophage lineage cells are not well established. In an effort to elucidate these pathways, we have used two cell types responsive to M-CSF: NIH 3T3 fibroblasts genetically engineered to express human M-CSF receptors (3T3-FMS cells) and human monocytes. M-CSF treatment induced M-CSF receptor tyrosine phosphorylation and recruitment of the p85 subunit of phosphatidylinositol 3-kinase (PI3K) to these receptors. These M-CSF receptor events correlated with activation of the serine/threonine kinase Akt. To clarify that PI3K products activate Akt in response to M-CSF, NIH 3T3 fibroblasts expressing mutant human M-CSF receptors (3T3-FMS(Y809F)) that fail to activate Ras in response to M-CSF also exhibit increased Akt kinase activity in response to M-CSF challenge. Furthermore, Akt appears to be the primary regulator of survival in 3T3-FMS cells, as transfection of genes encoding dominant-negative Akt isoforms into these fibroblasts blocked M-CSF-induced survival. In normal human monocytes, M-CSF increased the levels of tyrosine-phosphorylated proteins and induced Akt activation in a PI3K-dependent manner. The PI3K inhibitor LY294002 blocked M-CSF-mediated monocyte survival, an effect that was partially restored by caspase-9 inhibitors. These data suggest that M-CSF may induce cell survival through Akt-induced suppression of caspase-9 activation.

Localization of inflammation with two recently described radiotracers, 111In-labelled polyclonal IgG and 99Tcmalbumin colloid labelled leukocytes (Tc-WBC), was studied. Accumulation of activity was compared with lllIn-labelled leukocytes (In-WBC) using 131I human serum albumin as a control. Ratios of activity in a chemically induced abscess in the thigh of rabbits compared with normal muscle tissue were measured. The results showed that all agents localize in inflammation but Tc-WBC consistently localizes to a greater degree than the other agents. At 2 h the inflammed-to-normal ratios for Tc-WBC were 4, IgG 2.1, albumin 1.9 and In-WBC 1.7. The pattern of the ratios remained similar over the 18 h period of the study. The short time in which leukocytes can be labelled and the quality of the images obtained suggest that Tc-WBC imaging is the method of choice for this model.