Cultured rat glomerular epithelial cells (GEC) were examined for their ability to release extracellular matrix-degrading proteinases with [3H]gelatin as substrate. GEC-conditioned media, under serum-free conditions, contained modest amounts of gelatinase activity (1 to 10 U/mg of protein); the activity was maximal at neutral pH, was inhibited by zinc chelators, was not inhibited by tissue inhibitor of metalloproteinase-2, and could not be further activated by trypsin or organomercurials. Gelatin substrate sodium dodecyl sulfate-polyacrylamide gels of GEC-conditioned medium revealed several zones of lysis, with molecular sizes of 150 kd (major band), and 220, 86 to 93, and 52 to 54 kd (minor bands). Northern blot analysis demonstrated that the GEC metalloproteinase(s) were distinct from the 68- to 72-kd type IV collagenase/gelatinase present in mesangial cells or the 92-kd type IV collagenase present in neutrophils. The GEC gelatinolytic activity also degraded insoluble type IV collagen in glomerular basement membrane in a dose-dependent manner. The major metalloproteinase activity responsible for the type IV collagen degradation has a molecular size of 150 kd with a type IV collagen substrate gel. Thus, GEC produce several neutral metalloproteinases, which, by virtue of their substrate specificity, may play an important role in glomerular basement membrane remodeling and in glomerular diseases characterized by alterations in basement membrane permeability.

Lipid A, the active component of bacterial endotoxin, stimulates multiple cell types, including glomerular mesangial cells (MC), and yet the molecular mechanisms of cell activation remain unclear. Lipid A, in its monosaccharyl form, structurally resembles the biologically active lipid phosphatidic acid (PA). Given this, it was postulated that lipid A activates cells by acting as a structural and functional mimetic of PA. Lipid A was found to specifically stimulate an MC lyso-PA acyl transferase activity, leading to enhanced synthesis of sn-2-unsaturated forms of PA. Sn-2-unsaturated PA itself, in contrast to sn-2-saturated PA, also stimulated the lyso-PA acyl transferase activity, a positive feedback feature previously noted with lyso-lecithin acyl transferase. Structure-function correlations demonstrated that the phosphate moieties in both PA and lipid A were necessary to feedback stimulation of lyso-PA acyl transferase (AT), as dephosphorylated lipid A and 2-unsaturated 1,2-sn-diacylglycerol had no stimulatory effect on lyso-PA AT. The biologic relevance of the lipid A and PA-mediated increases in lyso-PA acyl transferase activity was shown, whereby limited exposure to these lipids rapidly induced identical MC morphologic and functional alterations characteristic of cellular activation. By mimicking the stimulatory action of PA, per se, on lyso-PA acyl transferase activity, lipid A may initiate a positive feedback cycle of acylation, yielding increased amounts of PA enriched in unsaturated fatty acids. This newly synthesized PA may subsequently act as the proximal mediator of cellular activation.