Hyperlipidemia, especially hypercholesterolemia, may contribute to glomerulosclerosis as it does to atherosclerosis. Low density lipoprotein (LDL) stimulates the production of extracellular matrix by mesangial cells in culture as well as the proliferation of mesangial cells. This study was carried out to examine the effects of LDL on the type IV collagen (CIV) production by cultured rat mesangial cells (CRMC). Subconfluent CRMC monolayers which were grown in RPMI with 20% lipid-free fetal calf serum for 48 h were challenged with LDL (0, 50,100,150 and 200 μg/ml) for another 48 h. LDL was prepared from normal human plasma. Mesangial cell proliferation was examined by [3H]-thymidine uptake. Production of CIV was evaluated as the expression of CIV on the cell surface by flow-cytometric analysis. The collagen synthesis was measured by the [3H]-proline uptake. Total RNA was extracted from CRMC at 6 and 24 h of incubation with 150 μg/ml LDL, and Northern blotting and hybridization was performed with cDNAs for αrCIV, for 72-kD collagenase and for tissue inhibitor of metalloproteinase (TIMP)-2. The amount of total mRNA was corrected with β-actin mRNA. Mesangial cell proliferation increased in all concentrations studied and had a peak value of 221% with 150 μg/ml of LDL. Expression of CIV increased by 30-60% in 100-200 μg/ml of LDL. Collagen synthesis also increased by 50-70% in 150-200 μg/ml of LDL. The mRNA ratio (procollagen α1(IV)/β-actin) increased to 133% at 24 h. The mRNA ratio (TIMP-2/β-actin) increased to 137% at 24 h. mRNA ratios at 6 h showed no change. LDL had little effect on collagenase mRNA expression. These results show that LDL stimulates, in addition to stimulated mesangial cell proliferation, collagen production through a combination of increased collagen synthesis and possibly decreased collagen degradation. Hyperlipidemia may contribute to the pathogenesis of glomerulosclerosis through the direct effect of LDL on mesang