BACTERIAL LIPOPOLYSACCHARIDE HAS STRUCTURAL SIMILARITY TO CERAMIDE AND STIMULATES CERAMIDE-ACTIVATED PROTEIN-KINASE IN MYELOID CELLS

Bacterial lipopolysaccharide (LPS), tumor necrosis factor (TNF)-alpha and interleukin-1 beta (IL-1 beta) stimulate similar cellular response s. TNF-alpha and IL-1 beta are known to initiate signaling through a p athway involving hydrolysis of sphingomyelin to ceramide (Kolesnick, R . N., and Golde, D. W. (1994) Cell 77, 325-328). In this system, ceram ide acts as a second messenger stimulating a ceramide-activated serine /threonine protein kinase. The present studies demonstrate that LPS, l ike TNF and IL-1, stimulates ceramide activated protein kinase activit y in human leukemia (HL-60) cells and in freshly isolated human neutro phils. Lipid A, the biologically active core of LPS, enhanced kinase a ctivity in a time- and concentration-dependent manner. As little as 10 nM lipid A was effective, and a maximal effect occurred with 500 nM l ipid A, increasing kinase activity 5-fold. Native LPS similarly induce d kinase activation. This effect of LPS was markedly enhanced by LPS b inding protein and required the LPS receptor CD14. In contrast to TNF and IL-1, LPS did not cause sphingomyelin hydrolysis and thus stimulat es ceramide-activated protein kinase without generating ceramide. Mole cular modeling showed strong structural similarity between ceramide an d a region of lipid A. Based on these observations, we propose that LP S stimulates cells by mimicking the second messenger function of ceram ide.