Phosphotyrosine of macrophage by low-density lipoproteins from hemodialysis patients.

Background: Because of the possible importance of tyrosine phosphorylation in the signal transduction process, we investigated whether an interaction of low-density lipoprotein (LDL) from hemodialysis patients (HD-LDL) and human macrophages induces tyrosine-phosphorylated proteins in the macrophages.

Methods: Human monocyte-derived macrophages were incubated with HD-LDL (100 micrograms/ml) or native LDL (100 micrograms/ml) for 15 minutes at 37 degrees C. Whole cells were lyzed with Tris-HCl buffer containing vanadate and Triton X-100. After centrifugation, lyzed proteins were divided into Triton-soluble and -insoluble fractions. Both fractions (soluble and insoluble) were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and were electroblotted onto a polyvinylidene difluoride (PVDF) membrane. Immunoblotting was performed using an antibody against phosphotyrosine or c-Src.

Results: Several proteins in the range 40 to 100 kDa were found to be phosphorylated constitutively in the macrophages and not affected by the addition of HD-LDL. HD-LDL did not induce any tyrosine-phosphorylated proteins either in the soluble or insoluble fractions. Macrophages pretreated with tyrosine kinase inhibitor genestein drastically inhibited tyrosine phosphorylation of these proteins. The nonreceptor tyrosine kinase, c-Src p60, was also strongly tyrosine phosphorylated in the macrophages, and this was not enhanced by the stimulation of HD-LDL.

Conclusion: These data suggest that tyrosine autophosphorylated proteins may play a role in the early step of signal transduction in the macrophages.