Heparin-binding epidermal growth factor-like growth factor, an immediate-early gene for mesangial cells, is up-regulated in the Thy-1.1 model.

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) is one of five well-described growth factors which bind to and activate the EGF receptor. Since cultured mesangial cells synthesize HB-EGF and this cytokine is a potent mitogen for smooth muscle cells (SMC), a cell type similar to mesangial cells, we attempted to determine (1) whether HB-EGF mRNA is present in the proliferative phase of experimental mesangial proliferative glomerulonephritis, and (2) some of the factors which regulate its synthesis by mesangial cells. In this study we demonstrate that cultured rat mesangial cells (RMC) express HB-EGF mRNA and that transcript levels are markedly increased by serum with maximal induction occurring within 2 h. Stimulation with individual cytokines (EGF, TGF-alpha, PDGF, TGF-beta and TNF-alpha), by contrast, had only a minor effect. The increase in HB-EGF mRNA levels following addition of serum was rapid, transient and independent of protein synthesis, features characteristic of immediate-early genes. In normal rat kidneys, there was no detectable glomerular expression of HB-EGF mRNA as determined by in situ hybridization, although occasional tubular cross sections were positive. Within 30 min after induction of the Thy-1.1 model, however, cells within the glomerulus and an increased number of tubules were positive. The number of positive glomerular and tubular cells increased progressively at days 1 and 4 post-induction, but declined by day 9 and had returned to background at day 15. Within the glomerulus, HB-EGF was expressed by cells of Bowman's capsule and cells within the glomerular tuft. Using (a) combined in situ hybridization and immunohistochemical staining of the same section and (b) staining of sequential sections by immunohistochemistry or in situ hybridization, it was found that intrinsic glomerular cells which did not stain with the macrophage marker ED-1 expressed HB-EGF mRNA. Although some were probably glomerular epithelial cells because of their peripheral location, it was uncertain whether mesangial cells were also positive. Future studies will be directed towards firmer identification of the glomerular cells expressing HB-EGF mRNA and to define the functional role of HB-EGF in the Thy-1.1 model.