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

Discrete localization of phospholipase Cβ3 and diacylglycerol kinase ι along the renal proximal tubules of normal rat kidney and gentamicin-induced changes in their expression

Many signaling enzymes have multiple isozymes that are localized discretely at varying molecular levels in different compartments of cells where they play specific roles. In this study, among the various isozymes of phospholipase C (PLC) and diacylglycerol kinase (DGK), which work sequentially in the phosphoinositide cycle, both PLCβ3 and DGKι were found in renal brush-border microvilli, but found to replace each other along the proximal tubules: PLCβ3 in the proximal straight tubules (PST) of the outer stripe of the outer medulla (OSOM) and the medullary ray (MR), and DGKι in the proximal convoluted tubules (PCT) in the cortex and partially in the PST of the MR. Following daily injection of gentamicin for 1 week, the expression of PLCβ3 and DGKι was transiently enhanced, as demonstrated by western blot, and the increases were found to most likely occur in their original sites, that is, in the brush borders of the PST for PLCβ3 and in the PCT for DGKι. These findings showing differences in expression along the tubules suggest that the exertion of reabsorption and secretion through various ion channels and transporters in the microvillus membranes and the maintenance of microvillus turnover are regulated by a PLC-mediated signal with the balance shifted toward relative augmentation of the DAG function in the PST, and by a DGK-mediated signal with the balance shifted to relative augmentation of the phosphatidic acid function in the PCT. Our results also suggest the possibility that these isozymes are potential diagnostic signs for the early detection of acute kidney injury caused by gentamicin.

Involvement of the peripheral cholinergic muscarinic system in the compensatory ovarian hypertrophy in the rat

In the present experiments, unilateral ovariectomy (ULO) induced compensatory hypertrophy (COH) of the remaining rat ovary (60%-85% increase in ovarian weight, total proteins, and total RNA and DNA). An increased thymidine uptake preceded the organ enlargement. COH was inhibited by i.p.-administered muscarinic antagonist propantheline (dose-dependently) or botulinum toxin delivered locally to the ovary. The effects were reversed by bethanecol i.p. (a muscarinic agonist). In sham ULO animals, [3H]-scopolamine binding to ovarian membranes indicated the existence of muscarinic receptors (Kd 2.5 nM, Bmax 12 fmol/mg proteins, Hill 1.0). The ovarian 1,2-diacylglycerol (DAG) was 120-150 pmol/mg tissue and did not react to carbachol in vitro (50 microM). At 15 minutes after ULO, the [3H]-scopolamine binding was unchanged (Kd 2.6 nM, Bmax 12.6 fmol/mg tissue, Hill 1.0), but the ovarian DAG was increased (280-350 pmol/mg tissue) and increased further in response to carbachol (460-550 pmol/mg tissue). After ULO, ovarian DAG remained continuously responsive to carbachol. The ULO-induced DAG increase and enhanced susceptibility to carbachol were inhibited by the botulinum toxin or atropine pretreatments. Abdominal vagotomy done immediately before ULO also inhibited the ULO-induced DAG increase and DAG responsiveness to carbachol. However, when the vagotomy was performed 10 mins after ULO, the ovarian DAG remained responsive to carbachol in vitro. The data suggest that the peripheral cholinergic system, including the ovarian muscarinic receptors, stimulates COH. This is apparently associated with the ULO-induced coupling of the ovarian muscarinic receptors to phosphoinositide (PI) breakdown. Vagus plays a role in the occurrence of the changed muscarinic receptor-PI breakdown relationship in the remaining ovary.

Hepatocyte growth factor activates phosphoinositide 3-kinase C2 beta in renal brush-border plasma membranes

Upon stimulation of renal cortical slices with hepatocyte growth factor (HGF), inositol lipid metabolism was studied in basal-lateral plasma membranes (BLM) and brush-border plasma membranes (BBM). Whereas in BLM rapid increases in 1,2-diacylglycerol, PtdIns(3,4,5)P(3) and PtdIns(3,4)P(2) were observed, suggesting that in BLM HGF activates both phospholipase C (PLC) and phosphoinositide 3-kinase (PI3K), in BBM only HGF-induced transient accumulation of PtdIns3P was seen, which was temporarily delayed from signalling events in BLM and could be blocked by the PtdIns-specific-PLC inhibitor ET-18-OCH(3) and the calpain inhibitor calpeptin, suggesting that 3-kinase activation in BBM lies downstream of PLC activation in BLM and is a calpain-mediated event. Moreover, the increase in immunoprecipitable PI3K-C2 beta activity, which is sensitive to wortmannin (10 nM) and shows strong preference for PtdIns over PtdIns4P as a substrate, was observed only in BBM upon stimulation of renal cortical slices with HGF and could be mimicked by the Ca(2+) ionophore A23187 and blocked by the cell-penetrant Ca(2+) chelator BAPTA-AM [1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid tetrakis(acetoxymethyl ester)]. On Western blots PI3K-C2 beta revealed a single immunoreactive band of 180 kDa in BLM and BBM, while after stimulation with HGF a gel shift of 18 kDa was noticed only in BBM, suggesting that the observed enzyme activation is achieved by proteolysis. When BBM were subjected to short-term (15 min) exposure to mu-calpain, a similar gel shift together with an increase in PI3K-C2 beta activity was observed, when compared with the BBM harvested after HGF stimulation. The above-mentioned gel shift and increase in PI3K-C2 beta activity could be prevented by the calpain inhibitor calpeptin. The data presented in this report show that in renal cells there is a spatial separation of the inositol lipid signalling system between BLM and BBM, and that HGF causes activation of PLC and PI3K primarily in BLM, which leads to calpain-mediated activation of PI3K-C2 beta in BBM with a concomitant increase in PtdIns3P.

The effect of big endothelin-1 in the proximal tubule of the rat kidney

1. An obligatory step in the biosynthesis of endothelin-1 (ET-1) is the conversion of its inactive precursor, big ET-1, into the mature form by the action of specific, phosphoramidon-sensitive, endothelin converting enzyme(s) (ECE). Disparate effects of big ET-1 and ET-1 on renal tubule function suggest that big ET-1 might directly influence renal tubule function. Therefore, the role of the enzymatic conversion of big ET-1 into ET-1 in eliciting the functional response (generation of 1,2-diacylglycerol) to big ET-1 was studied in the rat proximal tubules. 2. In renal cortical slices incubated with big ET-1, pretreatment with phosphoramidon (an ECE inhibitor) reduced tissue immunoreactive ET-1 to a level similar to that of cortical tissue not exposed to big ET-1. This confirms the presence and effectiveness of ECE inhibition by phosphoramidon. 3. In freshly isolated proximal tubule cells, big ET-1 stimulated the generation of 1,2-diacylglycerol (DAG) in a time- and dose-dependent manner. Neither phosphoramidon nor chymostatin, a chymase inhibitor, influenced the generation of DAG evoked by big ET-1. 4. Big ET-1-dependent synthesis of DAG was found in the brush-border membrane. It was unaffected by BQ123, an ETA receptor antagonist, but was blocked by bosentan, an ETA.B-nonselective endothelin receptor antagonist. 5. These results suggest that the proximal tubule is a site for the direct effect of big ET-1 in the rat kidney. The effect of big ET-1 is confined to the brush-border membrane of the proximal tubule, which may be the site of big ET-1 sensitive receptors.

Arachidonic Acid Mediates Interferon-γ–Induced Sphingomyelin Hydrolysis and Monocytic Marker Expression in HL-60 Cell Line

The biochemical signaling mechanisms involved in transducing the effects of interferon-γ (IFN-γ) on human leukemia-derived HL-60 cell differentiation are not completely understood. Recent studies established the existence of a sphyngomyelin (SM) cycle that operates in response to the action of IFN-γ on HL-60 cells, but the mechanisms by which IFN-γ induces the SM hydrolysis remain unexplored. In this study, biochemical events mediating IFN-γ effects on SM turnover and their specificity and role in HL-60 differentiation were investigated. The activation of the SM cycle by IFN-γ occurred rapidly, with a decrease of approximately 20% in the SM level observed after 60 minutes with a concomitant increase in ceramide level. Treatment of HL-60 cells with IFN-γ did not influence the 1,2-diacylglycerol concentration, intracellular Ca2+ concentration, or phospholipase D activity. IFN-γ stimulated a rapid release of arachidonic acid (AA) from HL-60 cells; the effect was abolished by the pretreatment of cells with pertussis toxin, suggesting a role for a pertussis-toxin–sensitive G protein in IFN-γ–mediated activation of phospholipase A2 (PLA2 ). At 4 to 120 hours after the stimulation of the cells with IFN-γ, a significant increase in the particulate and soluble PLA2 activity was observed, corresponding to an increase in the level of immunoreactive cPLA2 in both cytosol and membrane fractions. The treatment of cells with tyrosine kinase inhibitor herbimycin A completely abolished the effect of IFN-γ on PLA2 activity in membrane and cytosolic fractions, but had no effect on IFN-γ–mediated early AA release suggesting dual mechanism of PLA2 activation. Melittin, potent activator of PLA2 , and AA mimicked the effect of IFN-γ on SM hydrolysis. Pretreatment of HL-60 cells with the PLA2 inhibitor, bromophenacyl bromide (BPB), or pertussis toxin abolished the effect of IFN-γ on SM hydrolysis; exogenous addition of AA overcame the effects of BPB and pertussis toxin. Long-term exposure (5 days) of HL-60 cells to IFN-γ caused an increase in nitroblue tetrazolium (NBT)-reducing and nonspecific esterase (NSE) activity and induced expression of FcγRI (CD64) without significant effects on cell number, adherence, or fagocytic activity. The treatment of cells with AA or melittin induced NBT, NSE, and CD64 expression to the level similar to that observed with IFN-γ, and no further increase was observed with the combination of IFN-γ and AA or IFN-γ and melittin. Treatment of HL-60 cells with indomethacin, an inhibitor of cyclo-oxygenase, and nordihydroguaiaretic acid (NDGA), an inhibitor of lipoxygenase, had no effects on IFN-γ–mediated induction of CD64 expression. These studies indicate a key role for the phospholipase A2/AA pathway, as an early biochemical signal elicited by the occupation of IFN-γ–receptor, in mediating IFN-γ induction of the SM cycle and phenotypic changes associated with differentiation of HL-60 along monocytic lineage.

Arachidonic Acid Mediates Interferon-γ–Induced Sphingomyelin Hydrolysis and Monocytic Marker Expression in HL-60 Cell Line

The biochemical signaling mechanisms involved in transducing the effects of interferon-γ (IFN-γ) on human leukemia-derived HL-60 cell differentiation are not completely understood. Recent studies established the existence of a sphyngomyelin (SM) cycle that operates in response to the action of IFN-γ on HL-60 cells, but the mechanisms by which IFN-γ induces the SM hydrolysis remain unexplored. In this study, biochemical events mediating IFN-γ effects on SM turnover and their specificity and role in HL-60 differentiation were investigated. The activation of the SM cycle by IFN-γ occurred rapidly, with a decrease of approximately 20% in the SM level observed after 60 minutes with a concomitant increase in ceramide level. Treatment of HL-60 cells with IFN-γ did not influence the 1,2-diacylglycerol concentration, intracellular Ca2+ concentration, or phospholipase D activity. IFN-γ stimulated a rapid release of arachidonic acid (AA) from HL-60 cells; the effect was abolished by the pretreatment of cells with pertussis toxin, suggesting a role for a pertussis-toxin–sensitive G protein in IFN-γ–mediated activation of phospholipase A2 (PLA2 ). At 4 to 120 hours after the stimulation of the cells with IFN-γ, a significant increase in the particulate and soluble PLA2 activity was observed, corresponding to an increase in the level of immunoreactive cPLA2 in both cytosol and membrane fractions. The treatment of cells with tyrosine kinase inhibitor herbimycin A completely abolished the effect of IFN-γ on PLA2 activity in membrane and cytosolic fractions, but had no effect on IFN-γ–mediated early AA release suggesting dual mechanism of PLA2 activation. Melittin, potent activator of PLA2 , and AA mimicked the effect of IFN-γ on SM hydrolysis. Pretreatment of HL-60 cells with the PLA2 inhibitor, bromophenacyl bromide (BPB), or pertussis toxin abolished the effect of IFN-γ on SM hydrolysis; exogenous addition of AA overcame the effects of BPB and pertussis toxin. Long-term exposure (5 days) of HL-60 cells to IFN-γ caused an increase in nitroblue tetrazolium (NBT)-reducing and nonspecific esterase (NSE) activity and induced expression of FcγRI (CD64) without significant effects on cell number, adherence, or fagocytic activity. The treatment of cells with AA or melittin induced NBT, NSE, and CD64 expression to the level similar to that observed with IFN-γ, and no further increase was observed with the combination of IFN-γ and AA or IFN-γ and melittin. Treatment of HL-60 cells with indomethacin, an inhibitor of cyclo-oxygenase, and nordihydroguaiaretic acid (NDGA), an inhibitor of lipoxygenase, had no effects on IFN-γ–mediated induction of CD64 expression. These studies indicate a key role for the phospholipase A2/AA pathway, as an early biochemical signal elicited by the occupation of IFN-γ–receptor, in mediating IFN-γ induction of the SM cycle and phenotypic changes associated with differentiation of HL-60 along monocytic lineage.

Different endothelin receptor subtypes are involved in phospholipid signalling in the proximal tubule of rat kidney

Phospholipid signalling mediated by endothelin (ET) receptor subtypes was studied in the rat proximal tubule. In freshly isolated proximal tubule cells, ET-1, ET-2 and sarafotoxin S6c (S6c) evoked an increase in 1,2-diacylglycerol (DAG), inositol 1,4,5-trisphosphate (InsP3) and phosphocholine (PCho), suggesting stimulation of both phosphatidyl-inositol 4,5-bisphosphate- and phosphatidyl-choline-specific phospholipase C (PLC), while ET-3 increased only DAG and PCho, presumably via phosphatidyl-choline-dependent PLC. Renal cortical slices were also stimulated by the above-mentioned agonists, followed by isolation of either brush border (BBM) or basolateral (BLM) membranes for which mass measurements of inositol lipids and DAG were performed. In BBM, DAG increased in response to ET-1, ET-2 and ET-3, and was followed by protein kinase C (PKC) translocation to the BBM, while in BLM, DAG formation and translocation of PKC were observed only in response to ET-3, suggesting spatial segregation of signalling systems between two membane domains of proximal tubule cells. Tyrphostine, pertussis toxin (PTX) or cholera toxin (CTX) did not influence ET-mediated signalling in either of the membranes, suggesting involvement of PTX- and CTX-insensitive G-protein-mediated stimulation of PLCbeta by ET receptors. ET-dependent stimulation of PLC in BBM and BLM was used as a tool to examine the presence of different ET receptor subtypes in these two cell membrane domains. BQ123, an inhibitor of ETA receptors, did not prevent ET-1-mediated signalling in BBM, but an ETA,B antagonist, bosentan, inhibited ET-3-mediated signalling in BBM. In addition, an ETB agonist, S6c, stimulated PLC in BBM. Neither BQ123 nor bosentan inhibited ET-3 signalling in BLM. Therefore, these data strongly suggest the presence of ETB receptors coupled to phosphatidyl-inositol 4,5-bisphosphate- and phosphatidyl-choline-dependent PLC in BBM and ETC receptors linked to phosphatidyl-choline-dependent PLC in BLM.

Protein kinase C isoforms in rat kidney proximal tubule: acute effect of angiotensin II

The present study examined the effect of phorbol esters, Ca2+, and angiotensin II (ANG II) on protein kinase C (PKC) isoforms in the rat proximal tubule. The immunoblot analysis of PKC isoforms of particulate and cytosolic fractions of proximal tubules revealed immunoreactive proteins when antibodies against PKC-alpha, -delta, -epsilon, and -zeta, but not -beta and -gamma were used. Phorbol dibutyrate (PDBU) induced the translocation of PKC-alpha, -delta, and -epsilon, whereas an inactive phorbol ester had no effect. PDBU and ionomycin increased particulate PKC specific activity from 0.67 +/- 0.09 to 1.56 +/- 0.18 and 0.96 +/- 0.04 pmol.microgram protein-1.2 min-1, respectively. ANG II (10(-7) M) induced a time-dependent increase in particulate PKC-alpha immunoreactivity observed after 2 min and maintained for 12 min. Particulate PKC-epsilon immunoreactivity increased after 4 min. Meanwhile, PKC-delta and -zeta were not modified by ANG II. Accordingly, ANG II elicited a rise in the specific activity of the particulate PKC, which increased to 0.89 +/- 0.09 pmol.micrograms protein-1.2 min-1 after 2 min. This was inhibited by a preincubation in the presence of 10(-5) M losartan, specific inhibitor of angiotensin subtype 1 receptors. These data indicate that PKC-alpha and -epsilon are potential candidates to regulate the activity of Na+/H+ and Na(+)-HCO3- transporters because they are translocated with a time course fitting with that of the reported effect of ANG II on those transporters.