Hyperosmotic mannitol activates basolateral NHE in proximal tubule from P-glycoprotein null mice

Mechanisms of P-Glycoprotein Regulation Under Exogenous and Endogenous Oxidative Stress In Vitro

We investigated the mechanisms of P-glycoprotein (P-gp) transporter regulation in Caco-2 cells under exogenous and endogenous oxidative stress (OS). Exogenous OS was modeled by exposure of the growth medium to hydrogen peroxide at concentrations of 0.1, 0.5, and 1 μM for 24 h or 10 μM for 72 h. Endogenous OS was modeled by incubating cells with DL-buthionine sulfoximine (BSO, gamma-glutamylcysteine synthetase inhibitor) at a concentration of 10, 50, and 100 μM for 24 h. The levels of intracellular reactive oxygen species (ROS) were assessed using MitoTracker Red CM-H2XRos fluorescent probes. Relative P-gp contents were analyzed using Western blot. Exogenous and endogenous OS was shown to increase relative to P-gp contents. An important role played by the Nrf2-Keap1 signaling pathway in increasing the P-gp contents under H2O2-induced exogenous OS was revealed using specific inhibitors. The transcription factor HIF1 is involved in the regulation of the P-gp levels under 24-hour exogenous OS, and the transcription factor CAR is involved in the regulation of transporter levels under 72-hour OS. All tested transcription factors and signaling pathways are involved in P-gp induction under endogenous OS. Most likely, this is associated with the bimodal effect of BSO on Pgp. On the one hand, BSO induces the development of OS; on the other, BSO, as a xenobiotic, is able to stimulate PXR and CAR, which, in turn, increase the P-gp contents.

Cell Volume Regulation in the Proximal Tubule of Rat Kidney : Proximal Tubule Cell Volume Regulation

We developed a dynamic model of a rat proximal convoluted tubule cell in order to investigate cell volume regulation mechanisms in this nephron segment. We examined whether regulatory volume decrease (RVD), which follows exposure to a hyposmotic peritubular solution, can be achieved solely via stimulation of basolateral K[Formula: see text] and [Formula: see text] channels and [Formula: see text]-[Formula: see text] cotransporters. We also determined whether regulatory volume increase (RVI), which follows exposure to a hyperosmotic peritubular solution under certain conditions, may be accomplished by activating basolateral [Formula: see text]/H[Formula: see text] exchangers. Model predictions were in good agreement with experimental observations in mouse proximal tubule cells assuming that a 10% increase in cell volume induces a fourfold increase in the expression of basolateral K[Formula: see text] and [Formula: see text] channels and [Formula: see text]-[Formula: see text] cotransporters. Our results also suggest that in response to a hyposmotic challenge and subsequent cell swelling, [Formula: see text]-[Formula: see text] cotransporters are more efficient than basolateral K[Formula: see text] and [Formula: see text] channels at lowering intracellular osmolality and reducing cell volume. Moreover, both RVD and RVI are predicted to stabilize net transcellular [Formula: see text] reabsorption, that is, to limit the net [Formula: see text] flux decrease during a hyposmotic challenge or the net [Formula: see text] flux increase during a hyperosmotic challenge.

Na+/H+ exchanger activity is increased in doxorubicin-resistant human colon cancer cells and its modulation modifies the sensitivity of the cells to doxorubicin

Multidrug resistant (MDR) tumor cells exhibit an altered pH gradient across different cell compartments, which favors a reduced intracellular accumulation of antineoplastic drugs and a decreased therapeutic effect. In our study, we have observed that the activity and expression of Na+/H+ exchanger (NHE), which is involved in the homeostasis of intracellular pH (pHi), are increased in doxorubicin-resistant (HT29-dx) human colon carcinoma cells in comparison with doxorubicin-sensitive HT29 cells. The pH(i) was significantly higher in HT29-dx cells, which accumulated less doxorubicin than HT29 cells. The NHE inhibitor 5-(N-ethyl-N-isopropyl)amiloride (EIPA) significantly reduced the pHi value and increased the intracellular accumulation of doxorubicin in both cell populations: in the presence of EIPA HT29-dx cells accumulated as much drug as control HT29 cells. On the other hand, monensin, a Na+/H+ ionophore mimicking NHE activation, and phorbol 12-myristate 13-acetate (PMA), which stimulates NHE, significantly increased the pHi and decreased the drug accumulation in HT29 cells to values similar to those observed in control HT29-dx cells. EIPA potentiated the cytotoxic effect of doxorubicin in HT29 cells, and made HT29-dx cells as sensitive to the cytotoxic effect of the drug as control HT29 cells. Instead, PMA and monensin made HT29 cells as insensitive to doxorubicin as HT29-dx cells. These results suggest that in MDR cells the higher cytosolic pH is likely to decrease drug accumulation, and that such resistance can be reverted by inhibiting the NHE activity. This result opens the possibility to revert MDR with the clinical use of NHE inhibitors.

Hyperosmotic urea activates basolateral NHE in proximal tubule from P-gp null and wild-type mice

Using the pH-sensitive fluorescent dye BCECF, we compared the effects of hyperosmotic urea on basolateral Na(+)/H(+) exchange (NHE) with those of hyperosmotic mannitol in isolated nonperfused proximal tubule S2 segments from mice lacking both the mdr1a and mdr1b genes (KO) and wild-type (WT) mice. All the experiments were performed in CO(2)/HCO-free HEPES solutions. Osmolality of the peritubular solution was raised from 300 to 500 mosmol/kgH(2)O by adding mannitol or urea. NHE activity was assessed by the Na(+)-dependent acid extrusion rate (J(H)) after an acid load with NH(4)Cl prepulse. In WT mice, hyperosmotic mannitol had no effect on J(H) at over the entire range of intracellular pH (pH(i)) studied (6.20-6.90), whereas in KO mice it increased J(H) at a pH(i) range of 6.20-6.45. In contrast, in both WT and KO mice, hyperosmotic urea increased J(H) at a pH(i) range of 6.20-6.90. In KO mice, J(H) in a hyperosmotic urea solution were similar to those in a hyperosmotic mannitol solution at a pH(i) range of 6.20-6.40 but were greater than in a hyperosmotic mannitol solution at a pH(i) range of 6.45-6.90. In WT mice, hyperosmotic urea caused an increase in V(max) without changing K(m) for peritubular Na(+). Staurosporine (the PKC inhibitor) inhibited hyperosmotic mannitol-induced NHE activation in KO mice, whereas it had no effect on hyperosmotic urea-induced NHE activation in WT or KO mice. Genistein (the tyrosine kinase inhibitor) inhibited hyperosmotic urea-induced NHE activation in WT and KO mice, whereas it caused no effect on hyperosmotic mannitol-induced NHE activation in KO mice. We conclude that hyperosmotic urea activates basolateral NHE via tyrosine kinase in tubules from both WT and KO mice, whereas hyperosmotic mannitol activates it via PKC only in tubules from KO mice.