HDL CAUSES MESANGIAL CELL MITOGENESIS THROUGH A TYROSINE KINASE-DEPENDENT RECEPTOR MECHANISM

Hypercholesterolemia and mesangial cell proliferation have been propos ed to play a role in the progression of glomerulosclerosis in diabetic nephropathy and other renal diseases. Although LDL is mitogenic for a nd cytotoxic to mesangial cells, the effect of HDL on these cells is u nknown. HDL stimulates fibroblast mitogenesis and is the principal cho lesterol-bearing lipoprotein in the rat, the experimental model for st udying the effect of hyperlipidemia on renal disease. Insulin is mitog enic in several cell systems, and its levels are increased in serum in non-insulin-dependent diabetes mellitus. This study investigates whet -her HDL acts as a growth factor in mesangial cells and whether it fun ctions in parallel with insulin. It was found that HDL at protein conc entrations between 10 and 500 mu g/ml, both alone and in the presence of 100 nM insulin, increased DNA synthesis in mesangial cells (129 to 165% of control for HDL alone; 140 to 235% for HDL plus insulin), wher eas HDL at 1000 mu g/ml and greater inhibited mesangial cell prolifera tion. Insulin alone at 100 nM stimulated [H-3]thymidine incorporation in the same cell system (145% of control); the mitogenic effect of ins ulin was additive to that of HDL. Purified apo A-I had a similar effec t, but at significantly lower concentrations. Specific binding of HDL to mesangial cells was demonstrated (B-max [binding constant] of 5.19 +/- 0.70 x 10(-7) mu mol of HDL bound/mg cell protein and K-b of 2.83 +/- 0.22 nM). Tetranitromethane alters apo A-I, preventing binding to its cognate receptor. Tetranitromethane-modified HDL did not bind to m esangial cells and had no effect on [H-3]thymidine incorporation. Addi tion of HDL to mesangial cells caused an immediate transient increase in free intracellular calcium in several representative mesangial cell s, similar to the response seen with platelet-derived growth factor. T he mitogenic effect of HDL was not altered after attenuation of cellul ar protein kinase C activity, but the stimulatory effect of HDL alone and in combination with insulin on DNA synthesis was completely elimin ated after inhibition of cellular tyrosine kinases by 24-h pretreatmen t with 0.25 mu M herbimycin A. Thus, HDL binds to a specific apo A-I-d ependent receptor, promotes DNA synthesis, and initiates second-messen ger events by a tyrosine kinase-dependent and protein kinase C-indepen dent mechanism.