Epidermal growth factor stimulates the incorporation of phosphate into phosphatidic acid and phosphoinositides but does not affect phosphoinositide breakdown by phospholipase C in renal cortical slices

Inositol lipid signalling occurs in brush-border membranes during initiation of compensatory renal growth in the rat

Using highly specific mass assays, concentrations of inositol lipids and 1,2-diacylglycerol (DAG) were determined in plasma membranes isolated from rat kidney cortex. Significantly higher concentrations of inositol lipids were determined in brush-border (BBM) than in basal-lateral (BLM) plasma membranes, although DAG concentrations were similar in both. After unilateral nephrectomy, a decrease in PtdIns(4,5)P2 and PtdIns4P, with a concomitant increase in DAG and translocation of protein kinase C (PKC), were observed in BBM but not in BLM isolated from the remaining kidney. On the other hand, stimulation of renal cortical slices with insulin-like growth factor II (IGF-II) or phenylephrine caused similar effects in BLM but not in BBM. Stimulation of phospholipase C activity with translocation of PKC only to BBM in one kidney was also induced by occlusion of blood flow through the contralateral kidney for 15 min. At 30 min after the occlusion was removed and reflow established, DAG concentration and the amount of PKC in BBM returned to control values. These results suggest that an early signal after unilateral nephrectomy is transmitted to cells through BBM and can be switched on and off by blood occlusion and reflow through the contralateral kidney, while hormonal signals caused by IGF-II and phenylephrine are transmitted to cells through BLM.

Mitogenic signal transduction in normal and transformed 32D hematopoietic cells

We studied mitogenic signal transduction in normal and oncogene-transformed 32D cells, a murine hematopoietic cell line that is normally dependent on interleukin-3 (IL3) for proliferation and survival. The formation of second messengers was measured in normal cells stimulated with IL3, and in cells transfected with foreign growth factor receptor genes and stimulated with appropriate growth factors. We also measured the steady-state level of second messengers in 32D cells transformed by erbB, abl, and src oncogenes which abrogate growth factor requirement. We found that IL3 stimulated the formation of diacylglycerol independently of inositol lipid turnover, but concomitantly with increased turnover of phosphatidylcholine. Epidermal growth factor (EGF), and platelet-derived growth factor (PDGF) stimulated the 'classical' turnover of inositol lipids with formation of diacylglycerol and calcium-mobilizing inositol phosphates. Colony stimulating factor-1 triggered inositol lipid turnover, although to a much lower extent than EGF and PDGF. Transformed cells showed elevated levels of diacylglycerol together with increased turnover of phosphoinositides and phosphatidylcholine. Taken together these results indicate that different growth factors and oncoproteins associate with multiple signalling pathways in 32D cells.

Mercury chloride as a possible phospholipase C activator: effect on angiotensin II-induced [Ca++]i transient in the rat early proximal tubule

In our previous report (Biochem. Biophys. Res. Commun. 165(3), 1221-1228, 1989), we have demonstrated the biphasic increase of intracellular free calcium concentration ([Ca++]i) induced by angiotensin II (ANG II) in isolated rat early proximal tubule (S1). The present study was undertaken to determine the effect of HgCl2 on ANG II-induced [Ca++]i increase using Fura-2. HgCl2 (10(-10) M2-10(-8) M) potentiated the [Ca++]i increase induced by ANG II (10(-11) M) in a dose-dependent manner. To determine the mechanism of stimulatory effect by HgCl2 on ANG II-induced [Ca++]i increase, nephron segments were pretreated with 10(-4) M propranolol, a phospholipase C inhibitor. The stimulatory effect by 10(-9) M HgCl2 in 10(-11) M ANG II-induced [Ca++]i increase was completely inhibited by propranolol. Moreover, 10(-4) M propranolol completely blocked the stimulatory effect of HgCl2 on ANG II-mediated IP3 production. This study suggests for the first time that HgCl2 stimulates the [Ca++]i increment induced by ANG II, possibly through an activation of phospholipase C.

Epidermal growth factor-stimulated phosphoinositide hydrolysis in cultured rat inner medullary collecting tubule cells. Regulation by G protein, calcium, and protein kinase C

Epidermal growth factor (EGF) exhibits specific saturable binding to cultured rat inner medullary collecting tubule cells and stimulates inositol trisphosphate (IP3) production by these cells in a dose-dependent fashion. EGF-stimulated IP3 production is enhanced by GTP gamma s or AIF4- and is inhibited by GDP beta s or pertussis toxin. Alterations in extracellular Ca2+ have no effect on either basal or EGF-stimulated IP3 production. Similarly, treatment with EGTA which decreases cytosolic Ca2+ is without effect. In contrast, treatment with ionomycin which increases cytosolic Ca2+ has no effect on basal IP3 production but enhances the response to EGF. Activation of protein kinase C inhibits IP3 production in response to either EGF or AIF4-. These studies demonstrate the occurrence of EGF-stimulated phospholipase C activity in the rat inner medullary collecting duct. Stimulation by EGF is transduced by a pertussis toxin-sensitive G protein, unaffected by alterations in extracellular Ca2+, insensitive to a decrement in cytosolic Ca2+, enhanced by an increase in cytosolic Ca2+, and inhibited by protein kinase C.

Insulin-like growth factor I stimulates phospholipid synthesis in renal cortical slices without production of inositol phosphate

The effects of insulin-like growth factor I (IGF-I) on the metabolism of phospholipids in renal cortical slices were examined using either sodium [32P]orthophosphate or myo-[3H]inositol. IGF-I was found to increase the incorporation of phosphate into phospholipids about 2-3-times above control values, leading to an increase in the concentration of total phospholipid phosphorus of 20% above control value after 1 h of incubation. The increased incorporation of phosphate into phospholipids could be prevented by 10 microM cycloheximide, while with 1 microM TPA (12-O-tetradecanoylphorbol 13-acetate) it could not. Insulin was also found to increase the incorporation of phosphate into phospholipids, but only if its concentration was at least 100-times higher than that of IGF-I. When phospholipids were prelabelled, IGF-I neither decreased the level of 32P in phospholipids nor stimulated the formation of inositol phosphates. The results show that IGF-I stimulates phospholipid synthesis without production of inositol phosphates in renal cortical slices.