Interleukin 1 (IL-1) exerts a number of biologic actions upon cultured glomerular mesangial cells (MC). These include stimulation of cellular proliferation and induction of prostaglandin and type IV collagenase secretion. It was determined that this activity, as with other polypeptide growth factors, was associated with the activation of specific MC plasma membrane protein kinases. Plasma membranes from cycling MC were incubated with purified IL-1 and (32P) ATP in the absence of calcium and cyclic nucleotides. Macrophage IL-1 stimulated the rapid phosphorylation of several plasma membrane proteins, the most significant of which were 52-55 kd, 46 kd, and 20 kd in size. Macrophage IL-1 induced specific membrane phosphorylation in concentrations as low as 1.5 x 10(-12) M, an effect obtained with equivalent concentrations of purified MC IL-1. The 46 kd phosphoprotein, which was the most prominent, was alkali-resistant and contained phosphotyrosine when examined by phosphoamino acid analysis. The 52-55 kd and 20 kd phosphoproteins were alkali-labile and contained phosphoserine. The 46 kd phosphoprotein was the major phosphoprotein recovered from Con A-Sepharose and IL-1 affinity columns. Induction of plasma membrane-associated protein kinase activity may represent one mechanism whereby IL-1 initiates mesangial cellular activation.

Glomerular mesangial cell (MC)--derived IL-1 may be an important factor in the development of the hypercellularity and sclerosis characteristic of many forms of glomerulonephritis. To define the regulation of IL-1 synthesis by human MC, Northern blot analyses were performed using specific probes for monocytic IL-1 alpha and beta mRNA. Proliferating MC expressed mRNA for both IL-1 alpha and beta, whereas nonproliferating MC contained no detectable IL-1 mRNA. Synchronized MC expressed IL-1 alpha and beta mRNA within 2 h of stimulation with serum. This serum effect could be reproduced with platelet-derived growth factor and epidermal growth factor. Immune precipitations of 35S-methionine-labeled cells indicate that the mesangial IL-1 is synthesized as a 33-kD precursor protein with a pI of 7.2. Extracellular mesangial IL-1 has a pI of 7.0 and molecular weight of 17 kD, consistent with its identification as IL-1 beta. Cellular proliferation in glomerular disease may be driven in part by peptide growth factor-mediated induction of mesangial IL-1 gene expression and protein synthesis.

A neutral proteinase, capable of degrading gelatin, has been found in both an active and a latent form in the medium from the culture of rat mesangial cells. The latent form had an Mr of 80,000-100,000 and could be activated with either 4-aminophenylmercuric acetate or prolonged incubation at neutral pH. The active form of the enzyme was extensively purified. The estimated Mr of the purified enzyme on gel filtration was approximately 200,000, indicating that the active enzyme formed aggregates. However, analysis by SDS/polyacrylamide-gel electrophoresis under reducing conditions showed two protein bands, with Mr 68,000 and 66,000. Both proteins were found to contain proteolytic activity when run on SDS/substrate gels. The enzyme was inhibited by EDTA and 1,10-phenanthroline, but not by inhibitors for cysteine, serine or aspartic proteinases. The enzyme did not digest fibronectin, bovine serum albumin, proteoglycan or interstitial collagen. The enzyme degraded pepsin-solubilized placental type V collagen at 31 degrees C, whereas similarly solubilized type IV collagen was only degraded at higher temperatures. In addition, the neutral proteinase degraded native soluble type IV collagen. It also had activity on insoluble type IV collagen of glomerular basement membrane. The above properties suggest that the mesangial neutral proteinase belongs to the gelatinase group of metalloproteinases and that it may play a role in the normal turnover of extracellular glomerular matrix.