Effects of two bacterial products, muramyl dipeptide and endotoxin, on bone resorption in organ culture.

We have compared two components of bacterial cell walls, muramyl dipeptide (MDP) and lipopolysaccharide (LPS), for their effects on bone resorption as measured by the release of previously incorporated 45Ca. MDP is the smallest active component of peptidoglycan, whereas LPS is the active component of endotoxin. Fetal rat long bones were cultured for 5 days in a chemically defined medium supplemented with bovine serum albumin (BSA) or serum. LPS increased 45Ca release at concentrations of 0.03-1.0 microgram/ml. LPS further purified by electrolytic dialysis (ED-LPS) was active at 0.01 microgram/ml. ED-LPS was ineffective at such low concentrations in the presence of serum. The response to MDP was more variable than that to LPS, but bone resorption was stimulated at concentrations of 10(-7)-10(-5) M. MDP was less effective or inactive in medium supplemented with serum. Stereoisomers of MDP that do not have adjuvant activity caused minimal stimulation of bone resorption, whereas 6-0-steroyl MDP stimulated resorption at 10(-8) M. The stimulation of bone resorption by LPS and MDP was not inhibited by indomethacin. Both LPS and MDP increased lysosomal enzyme release in proportion to their effects on 45Ca release. LPS also markedly increased collagenase activity in the medium, but MDP did not. These results indicate that chemically different products of bacterial cell walls can stimulate bone resorption in vitro. These products may be distinguished by differences in dose response curve, serum inhibition, and collagenase release.