THE RESISTANCE OF MACROPHAGE-LIKE TUMOR-CELL LINES TO GROWTH-INHIBITION BY LIPOPOLYSACCHARIDE AND PERTUSSIS TOXIN

The process of tumorigenesis is frequently associated with resistance to growth inhibition by physiological regulators of normal cells. Murine macrophage-like cell lines BAC1.2F5, RAW264, J774.1A and PU5/1.8 were resistant to growth inhibition by bacterial lipopolysaccharide (LPS) and pertussis toxin, agents that blocked growth of primary bone marrow-derived macrophages (BMDM) in the presence of macrophage colony-stimulating factor (CSF-1). The resistance of the CSF-1-dependent cell line BAC1.2F5 to growth inhibition by pertussis toxin argues against the possibility that pertussis toxin-sensitive G proteins are essential for the pathway of growth stimulation by CSF-1. Conversely, these data add further weight to the argument that LPS mediates some of its biological activities by mimicking the action of pertussis toxin and inhibiting G protein function. The resistance of cell lines to LPS and pertussis toxin was not correlated with any alteration in the expression of mRNA encoding any of three pertussis-toxin sensitive G protein alpha subunits. The pattern of G protein expression was consistent between primary cells and tumour cells, suggesting that this is a differentiation marker. In particular, G(i)alpha2 mRNA was expressed at remarkably high levels in all of the cells. The specificity of LPS resistance was investigated by studying down-regulation of CSF-I binding and induction of proto-oncogene c-fos and tumour necrosis factor (TNF) mRNA. BAC1.2F5 cells were LPS-resistant in each of these assays. In CSF-1 binding, RAW2 64 and J774.1A responded in the same way as bone marrow-derived macrophages but required higher doses of LPS, whereas c-fos and TNF mRNA were induced in these cells at concentrations that did not inhibit growth. In PU5/1.8 cells, CSF-I binding was already very low and was not further down-regulated, but c-fos and TNF mRNA was inducible by LPS. By contrast to primary macrophages. the cell lines did not respond to LPS with down-regulation of c-fms mRNA, which encodes the CSF-I receptor. Hence, the resistance of macrophage-like tumour cells to LPS and pertussis toxin was specific to the pathways controlling growth, and was correlated with altered regulation of the CSF-1 receptor.