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 ce ll type similar to mesangial cells, we attempted to determine (1) whet her HB-EGF mRNA is present in the proliferative phase of experimental mesangial proliferative glomerulonephritis, and (2) some of the factor s which regulate its synthesis by mesangial cells. In this study we de monstrate that cultured rat mesangial cells (RMC) express HB-EGF mRNA and that transcript levels are markedly increased by serum with maxima l induction occurring within 2 h. Stimulation with individual cytokine s (EGF, TGF-alpha, PDGF, TGF-beta and TNF-alpha), by contrast, had onl y a minor effect. The increase in HB-EGF mRNA levels following additio n of serum was rapid, transient and independent of protein synthesis, features characteristic of immediate-early genes. In normal rat kidney s, there was no detectable glomerular expression of HB-EGF mRNA as det ermined by in situ hybridization, although occasional tubular cross se ctions were positive. Within 30 min after induction of the Thy-1.1 mod el, however, cells within the glomerulus and an increased number of tu bules were positive. The number of positive glomerular and tubular cel ls increased progressively at days 1 and 4 post-induction, but decline d by day 9 and had returned to background at day 15. Within the glomer ulus, HB-EGF was expressed by cells of Bowman's capsule and cells with in the glomerular tuft. Using (a) combined in situ hybridization and i mmunohistochemical staining of the same section and (b) staining of se quential sections by immunohistochemistry or in situ hybridization, it was found that intrinsic glomerular cells which did not stain with th e macrophage marker ED-1 expressed HB-EGF mRNA. Although some were pro bably 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 glomerul ar cells expressing HB-EGF mRNA and to define the functional role of H B-EGF in the Thy-1.1 model.