HDL causes mesangial cell mitogenesis through a tyrosine kinase-dependent receptor mechanism

Hypercholesterolemia and mesangial cell proliferation have been proposed to play a role in the progression of glomerulosclerosis in diabetic nephropathy and other renal diseases. Although LDL is mitogenic for and cytotoxic to mesangial cells, the effect of HDL on these cells is unknown. HDL stimulates fibroblast mitogenesis and is the principal cholesterol-bearing lipoprotein in the rat, the experimental model for studying the effect of hyperlipidemia on renal disease. Insulin is mitogenic in several cell systems, and its levels are increased in serum in non-insulin-dependent diabetes mellitus. This study investigates whether HDL acts as a growth factor in mesangial cells and whether it functions in parallel with insulin. It was found that HDL at protein concentrations between 10 and 500 microg/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), whereas HDL at 1000 microg/ml and greater inhibited mesangial cell proliferation. Insulin alone at 100 nM stimulated 3H]thymidine incorporation in the same cell system (145% of control); the mitogenic effect of insulin was additive to that of HDL. Purified apo A-I had a similar effect, 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) micromol 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 mesangial cells and had no effect on 3H]thymidine incorporation. Addition of HDL to mesangial cells caused an immediate transient increase in free intracellular calcium in several representative mesangial cells, similar to the response seen with platelet-derived growth factor. The mitogenic effect of HDL was not altered after attenuation of cellular protein kinase C activity, but the stimulatory effect of HDL alone and in combination with insulin on DNA synthesis was completely eliminated after inhibition of cellular tyrosine kinases by 24-h pretreatment with 0.25 microM herbimycin A. Thus, HDL binds to a specific apo A-I-dependent receptor, promotes DNA synthesis, and initiates second-messenger events by a tyrosine kinase-dependent and protein kinase C-independent mechanism.