Effects of cholera toxin and isobutylmethylxanthine on growth of human fibroblasts

Effects of lipids on ENaC activity in cultured mouse cortical collecting duct cells

Direct effects on epithelial Na+ channels (ENaC) activity by lipids, e.g., arachidonic acid (AA), eicosatetraynoic acid (ETYA), linoleic acid (LA), stearic acid (SA), hydroxyeicosatetraenoic acid (HETE), 11,12-epoxyeicosatrienoic acid (EET), (PGF2), and (PGE2), in cultured mouse cortical collecting duct (M1) cells were clarified by using single-channel recordings in this study. In a cell-attached recording, a bath application of 10 microM AA significantly reduced the ENaC open probability (NPo), whereas 10 microM ETYA or 5 microM LA only induced a slight inhibition. The inside-out recording as a standard protocol was thereafter performed to examine effects of these lipids on ENaC activity. Within 10 min after the formation of the inside-out configuration, the NPo of ENaC in cultured mouse cortical collecting duct (M1) cells remained relatively constant. Application of ETYA or LA or SA exhibited a similar inhibition on the channel NPo when applied to the extracellular side, suggesting that fatty acids could exert a nonspecific inhibition on ENaC activity. 11,12-EET, a metabolite of AA via the cytochrome P450 epoxygenase pathway, significantly inhibited the ENaC NPo, whereas 20-HETE, a metabolite of AA via the hydroxylase pathway, only caused a small inhibition of the ENaC NPo, to a similar degree as that seen with ETYA and LA. However, both PGE2 and PGF2alpha significantly enhanced the ENaC NPo. These results suggest that fatty acids exert a nonspecific effect on ENaC activity due to the interaction between the channel proximity and the lipid. The opposite effects of 11,12-EET and prostaglandin (PG) implicate different mechanisms in regulation of ENaC activity by activation of epoxygenase and cyclooxygenase.

Role of prostaglandins in the pathogenesis of X-linked hypophosphatemia

X-linked hypophosphatemia is an X-linked dominant disorder resulting from a mutation in the PHEX gene. PHEX stands for phosphate-regulating gene with endopeptidase activity, which is located on the X chromosome. Patients with X-linked hypophosphatemia have hypophosphatemia due to renal phosphate wasting and low or inappropriately normal levels of 1,25-dihydroxyvitamin D. The renal phosphate wasting is not intrinsic to the kidney but likely due to an increase in serum levels of fibroblast growth factor-23 (FGF-23), and perhaps other phosphate-wasting peptides previously known as phosphatonins. Patients with X-linked hypophosphatemia have short stature, rickets, bone pain and dental abscesses. Current therapy is oral phosphate and vitamin D which effectively treats the rickets and bone pain but does not adequately improve short stature. In this review, we describe recent observations using Hyp mice; mice with the same mutation as patients with X-linked hypophosphatemia. We have recently found that Hyp mice have abnormal renal prostaglandin production, which may be an important factor in the pathogenesis of this disorder. Administration of FGF-23 in vivo results in phosphaturia and an increase in prostaglandin excretion, and FGF-23 increases proximal tubule prostaglandin production in vitro. In Hyp mice, indomethacin improves the phosphate transport defect in vitro and in vivo. Whether indomethacin has the same effect in patients with X-linked hypophosphatemia is unknown.

Correction of proximal tubule phosphate transport defect in Hyp mice in vivo and in vitro with indomethacin

X-linked hypophosphatemia is the most prevalent inherited form of rickets. In this disorder, rickets results from hyperphosphaturia and inappropriately normal levels of 1,25(OH)2-vitamin D. Current therapy with oral phosphate and vitamin D improves the rickets, but has significant morbidity and does not significantly affect the short stature and hypophosphatemia. In the present study, we demonstrate that Hyp mice, which have a mutation homologous to that in patients with X-linked hypophosphatemia, have a 2-fold greater urinary prostaglandin E2 (PGE2) excretion than C57/B6 mice. To determine whether PGs were involved in the pathogenesis of this disorder, Hyp and C57/B6 mice received i.p. injections with vehicle or indomethacin (1 mg/kg of body weight twice daily for 4 days) and were studied approximately 12 h after the last dose of indomethacin. In the Hyp mice, indomethacin treatment decreased the fractional excretion of phosphate from 13.0 +/- 3.2% to 2.2 +/- 1.1% (P < 0.05), and increased serum phosphate from 2.9 +/- 0.2 mg/dl to 4.1 +/- 0.2 mg/dl (P < 0.05). There was no effect of indomethacin in C57/B6 mice. Indomethacin did not affect serum creatinine or inulin clearance, demonstrating that the normalization of urinary phosphate excretion was not caused by changes in glomerular filtration rate. Indomethacin treatment increased renal brush border membrane vesicle NaPi-2 protein abundance in Hyp mice to levels comparable to that of C57/B6 mice, but had no effect in C57/B6 mice. In vitro isolated perfused proximal tubule studies demonstrate directly that 10-6 M bath indomethacin normalized the phosphate transport defect in Hyp mice but had no effect on C57/B6 mice. In conclusion, there is dysregulation of renal PG metabolism in Hyp mice, and indomethacin treatment normalizes the urinary excretion of phosphate by a direct tubular effect.

Modulation of nicotinic AChR channels by prostaglandin E2 in chick sympathetic ganglion neurons

The effects of prostaglandin E2 (PGE2), an important metabolite of arachidonic acid, were studied on the activity of nicotinic AChR channels in cultured chick sympathetic ganglion neurons. In whole cell recordings, PGE2 (25 nM) inhibited significantly the ACh-evoked macroscopic current. In cell-attached patch recordings, PGE2 significantly inhibited single AChR channel currents as a result of a decrease in the frequency of channel opening, with no change in open time and conductance. PGE2 did not alter the extent or rate of agonist-induced desensitization of the AChR channels. These effects are specific since the related compound PGD2 had no effect on AChR channel function. Because there is an abundant endogenous production of PGE2 within sympathetic ganglia in response to certain stimuli, the inhibition of AChR channel function by PGE2 could serve an important role to modulate synaptic transmission in the sympathetic nervous system.

Arachidonic acid inhibits hormone-stimulated cAMP accumulation in the medullary thick ascending limb of the rat kidney by a mechanism sensitive to pertussis toxin

The possible regulation of adenosine 3',5'-cyclic monophosphate (cAMP) accumulation by arachidonic acid (AA) was studied in segments, microdissected from the rat kidney, which are sensitive to arginine vasopressin (AVP). In the presence of 5 microM indomethacin, the addition of 5 microM AA did not impair AVP-dependent cAMP accumulation (measured during 4 min at 35 degrees C) in the cortical or outer medullary collecting tubule, but decreased this response in the thick ascending limb with an inhibition much more pronounced in the medullary portion (MTAL) than in the cortical portion. In MTAL, the response to 10 nM AVP was inhibited by 34.4 +/- 9.6% (SEM) and 65.8 +/- 5.4% with 1 microM and 5 microM AA, respectively, N = 5 experiments. AVP-, glucagon- and calcitonin-sensitive cAMP levels in MTAL were inhibited by 5 microM AA to a similar extent. AA-induced inhibition was unaffected by the presence of inhibitors of AA metabolism: (1) either 10 microM indomethacin or 50 microM ibuprofen added to all media; (2) a 10-min pre-incubation and a 4-min incubation of MTAL samples with 10 microM eicosa-5,8,11,14-tetrayonic acid, (3) a 1-h preincubation with either 30 microM SKF-525A, 20 microM ketoconazole, or 20 microM nordihydroguariaretic acid. In contrast to AA, 11 other saturated or unsaturated fatty acids had no inhibitory effect on the AVP-dependent cAMP level. In fura-2-loaded MTAL samples, AA induced a slow increase of the intracellular calcium concentration ([Ca2+]i) which reached 21.0 +/- 3.8 nM and 92.9 +/- 21.4 nM over basal values (n = 11) at 2 min and 4 min, respectively, after the beginning of the superfusion of 5 microM AA. AA-induced inhibition of AVP-dependent cAMP accumulation was due neither to the increase in [Ca2+]i elicited by AA, nor to an activation of protein kinase C because this inhibition: (1) was not blocked when MTAL samples were incubated either in zero Ca2+ medium, or in the presence of 1,2-bis(2-aminophenoxy)ethane-N, N, N', N'-tetraacetic acid (BAPTA) to chelate [Ca2+]i, and (2) it was not reproduced by a pre-treatment of MTAL segments with a phorbol ester. Pre-incubation of MTAL (6 h at 35 degrees C) with 500 ng/ml pertussis toxin (PTX) prevented AA-induced inhibition: in the presence of PTX inhibition was 24.7 +/- 6.6% vs 10 nM AVP, as compared to 81.6 +/- 4.0% in control groups, i.e in the absence of PTX, N = 6.(ABSTRACT TRUNCATED AT 400 WORDS)

Purification and expansion of human Schwann cells in vitro

The ability to culture cells from the human nervous system provides new insight into the pathophysiology of neurological diseases and could be crucial to the development of gene replacement therapies and neural transplantation. We report that the proliferation of human Schwann cells isolated from paediatric and adult nerves is sustained in vitro by recombinant glial growth factor. Agents that increase intracellular cyclic cAMP were also mitogenic towards Schwann cells but suppress growth of contaminating fibroblasts. As the lifespan of highly enriched cultures can be extended for up to twelve population doublings, large numbers of cells can be generated from nerve biopsies.

Effect of prostaglandin E2 on agonist-stimulated cAMP accumulation in the distal convoluted tubule isolated from the rabbit kidney

The effects of calcitonin, vasoactive intestinal peptide (VIP), parathyroid hormone (PTH) and isoprenaline on intracellular cAMP accumulation were determined in the distal tubule (DCT) microdissected from collagenase-treated rabbit kidney. In DCTb (the initial "bright" portion) calcitonin (10 ng/ml) elicited a highly reproducible response 203.7 +/- 19.1 fmol cAMP mm-1 4 min-1 (SE,N = 13) whereas VIP-induced cAMP accumulation was less and more variable from one experiment to another (1 microM, 97.2 +/- 17.8 fmol mm-1 4 min-1, SE, N = 12). When used in combination, these two agonists were non-additive, indicating stimulation of a single pool of cAMP in DCTb. In DCTg, ("granular") which consists of at least two cell types, PTH (100 nM) elicited a marked, reproducible accumulation of cAMP (154.3 +/- 27.0 fmol mm-1 4 min-1; SE, N = 5). Isoprenaline (1 microM) and VIP (1 microM) induced much smaller increases in cAMP levels 20.9 +/- 2.7 and 29.4 +/- 4.1 fmol mm-1 4 min-1 (SE, N = 5) respectively, and, when used in combination, were non-additive, demonstrating that VIP and isoprenaline are active on the same cell type. In DCTb, prostaglandin E2 (PGE2) inhibited both calcitonin- and VIP-stimulated cAMP accumulation (calcitonin 57.8 +/- 2.7% inhibition, SE, N = 16; VIP, 80.6 +/- 2.1% inhibition, SE, N = 5). The EC50 values for calcitonin were 1.21 +/- 0.33 ng/ml and 1.83 +/- 0.25 ng/ml (SD, N = 3) in the absence and presence of PGE2 (300 nM) respectively with an IC50 for PGE2 of 26.3 +/- 6.3 nM (SE, N = 4).(ABSTRACT TRUNCATED AT 250 WORDS)

Synchronisation of cancer cell lines of human origin using methotrexate

U937 human histiocytic lymphoma cell line and SW626 ovarian carcinoma line of human origin were synchronised using very low, nontoxic concentrations (0.04-0.08 microM for 16-24 h) of methotrexate (MTX) under standard culture conditions. Satisfactory synchrony was achieved to study S phase events. Various kinetic behaviours and biological properties of the synchronised cells are considered for characterisation of the system. MTX-synchronisation was compared with that induced by aphidicolin (APC) alone and by serum deprivation and APC. In some cancer cell lines MTX appears to be the best choice for obtaining highly synchronised cell populations without cytotoxicity or physiological perturbations.

Antidiuretic hormone reduces the high PGE2 synthesis in papillary collecting duct of DI rats

PGE2 synthesis was measured along the nephron of Brattleboro (DI) rats, lacking ADH, and control LE rats, using an enzyme immunoassay. Experiments were performed in vitro, in the absence of exogenous arachidonic acid, using microdissected tubular segments. The effect of a chronic treatment of dDAVP was tested on three ADH sensitive tubular segments, medullary thick ascending limb (MTAL), medullary collecting tubule (OMCD) and papillary collecting duct (IMCD). No difference in PGE2 synthesis was present between LE and DI in glomerulus and tubular segments up to OMCD. In both strains, values were low in the proximal tubule and the loop of Henle, and gradually increased along the collecting tubule. In IMCD, PGE2 synthesis was much higher in DI (12.8 +/- 2.0 pg per 30 min per mm tubular length) than in LE (3.8 +/- 0.5, LE vs. DI p less than 0.001). In MTAL and OMCD, dDAVP treatment did not affect PGE2 synthesis. In IMCD, dDAVP reduced PGE2 synthesis to values (5.3 +/- 0.8 pg per 30 min per mm tubular length), which were not significantly different from those of LE. Neither oxytocin, which has been shown to be elevated in DI rats, nor furosemide, that reduced papillary osmolarity to values comparable to those of DI rats, were able to increase PGE2 synthesis in IMCD of LE rats. The mechanism of the increase in PGE2 synthesis in IMCD of DI rats, and of the inhibitory effect of dDAVP is yet unknown; it may participate to compensate for the lack of ADH in the Brattleboro rat.

Cyclic AMP inhibits mitogen-induced DNA synthesis in hamster fibroblasts, regardless of the signalling pathway involved

Mitogen-induced initiation of DNA synthesis in quiescent Chinese hamster lung fibroblasts (CCL39) is strongly inhibited by 8-Br cAMP and cAMP-evaluating agents (prostaglandin E1, cholera toxin, isobutylmethylxanthine). This inhibition is reversible and occurs very early in G0/G1. As exponential growth is much less affected by increased cAMP, we propose that cAMP inhibits an early signal essential for the exit from G0. CCL39 cells can be stimulated by alpha-thrombin, which activates phosphoinositide (PI) breakdown, as well as by mitogens (FGF or FGF + serotonin) which do not involve the PI pathway. Here we show that the action of both classes of mitogens is likewise inhibited by cAMP. Therefore, although PI breakdown is inhibited by cAMP in CCL39 cells, this effect cannot entirely account for th antimitogenic activity of cAMP. Other early steps of the mitogenic response must be also affected.

The biosynthesis and actions of prostaglandins in the renal collecting tubule and thick ascending limb

PGE2 formed by renal collecting tubules is an important factor in regulating NaCl and water reabsorption in the collecting tubule and medullary thick ascending limb. PGE2 appears to act, depending on its ambient concentration, via several different receptors present in these renal epithelia to modulate cAMP turnover in both positive and negative directions. These putative PGE receptors form a family of receptors, all coupled to G proteins, and this family of PGE receptors is homologous to the adrenergic receptor family.

Mitogenic and antimitogenic effects of cholera toxin-mediated cyclic AMP levels in 3T3 cells

The effect of time-controlled exposures to cholera toxin (CT) on intracellular levels of cyclic AMP (cAMP) and on the proliferative response of serum-stimulated 3T3 cells was investigated. Continuous exposure to CT caused up to 8-fold raises in cAMP content and inhibited DNA replication by delaying G1-S transition and by reducing the fraction of cells committed to DNA replication. In contrast, short exposures to CT during G0-G1 transition increased the fraction of cells responding to serum stimulation and potentiated the serum-induced morphological changes in the cell monolayer. A short exposure during late G1 phase, however, inhibited the onset of DNA synthesis but had little effect on ongoing DNA replication. The results indicate that cAMP has diverse and opposite effects on two defined restriction points in cell cycle control. Cyclic AMP was positively involved in the acquisition of the state of competence by quiescent cells (G0-G1 transition) but antagonistic on the onset of DNA replication (G1-S transition) in committed cells. The observations reconcile a number of controversial conclusions regarding the role of cAMP in cell cycle control.