Lysophosphatidic acid-induced calcium mobilization and proliferation in kidney proximal tubular cells

Towards quantitation of the effects of renal impairment and probenecid inhibition on kidney uptake and efflux transporters, using physiologically based pharmacokinetic modelling and simulations

Background and Objectives

The kidney is a major drug-eliminating organ. Renal impairment or concomitant use of transporter inhibitors may decrease active secretion and increase exposure to a drug that is a substrate of kidney secretory transporters. However, prediction of the effects of patient factors on kidney transporters remains challenging because of the multiplicity of transporters and the lack of understanding of their abundance and specificity. The objective of this study was to use physiologically based pharmacokinetic (PBPK) modelling to evaluate the effects of patient factors on kidney transporters.

Methods

Models for three renally cleared drugs (oseltamivir carboxylate, cidofovir and cefuroxime) were developed using a general PBPK platform, with the contributions of net basolateral uptake transport (T_up,b) and apical efflux transport (T_eff,a) being specifically defined.

Results and Conclusion

We demonstrated the practical use of PBPK models to: (1) define transporter-mediated renal secretion, using plasma and urine data; (2) inform a change in the system-dependent parameter (≥10-fold reduction in the functional ‘proximal tubule cells per gram kidney’) in severe renal impairment that is responsible for the decreased secretory transport activities of test drugs; (3) derive an in vivo, plasma unbound inhibition constant of T_up,b by probenecid (≤1 μM), based on observed drug interaction data; and (4) suggest a plausible mechanism of probenecid preferentially inhibiting T_up,b in order to alleviate cidofovir-induced nephrotoxicity.

Electronic supplementary material

The online version of this article (doi:10.1007/s40262-013-0117-y) contains supplementary material, which is available to authorized users.

Lysophosphatidic acid and renal fibrosis

The development of fibrosis involves a multitude of events and molecules. Until now the majority of these molecules were found to be proteins or peptides. But recent data show significant involvement of the phospholipid lysophosphatidic acid (LPA) in the development of pulmonary, liver and renal fibrosis. The latest data on the role of LPA and the G-protein-coupled LPA1 receptor in the development of renal fibrosis will be discussed. LPA1-receptor activation was found to be associated with increased vascular leakage and increased fibroblast recruitment in pulmonary fibrosis. Furthermore, in renal fibrosis LPA1-receptor activation stimulates macrophage recruitment and connective tissue growth factor expression. The observations make this receptor an interesting alternative and new therapeutic target in fibrotic diseases.

Regulatory Binding Partners and Complexes of NHE3

NHE3 is the brush-border (BB) Na+/H+exchanger of small intestine, colon, and renal proximal tubule which is involved in large amounts of neutral Na+absorption. NHE3 is a highly regulated transporter, being both stimulated and inhibited by signaling that mimics the postprandial state. It also undergoes downregulation in diarrheal diseases as well as changes in renal disorders. For this regulation, NHE3 exists in large, multiprotein complexes in which it associates with at least nine other proteins. This review deals with short-term regulation of NHE3 and the identity and function of its recognized interacting partners and the multiprotein complexes in which NHE3 functions.

Role of ATP in DNA synthesis of renal proximal tubule cells: involvement of calcium, MAPKs, and CDKs

Although ATP has been shown to act as a modulator in various kidney functions, its effect on renal proximal tubule cell (PTC) proliferation has not been elucidated. This study investigated the effect of ATP on cell proliferation and the effect of its related signal pathways on primary cultured PTCs. Treatment with >10(-5) M ATP for 1 h stimulated incorporation of thymidine and bromodeoxyuridine. ATP (10(-4) M)-induced stimulation of thymidine incorporation was blocked by suramin (a P2X and P2Y receptor antagonist), reactive blue 2 (a P2Y receptor antagonist), MRS-2159 (a P2X1 receptor antagonist), and MRS-2179 (a P2Y1 receptor antagonist). ATP increased intracellular Ca2+ concentration, which was blocked by suramin, methoxyverapamil, and EGTA. ATP-induced stimulation of cell proliferation was also blocked by EGTA (an extracellular Ca2+ chelator), methoxyverapamil (a Ca2+ antagonist), and nifedipine (an L-type Ca2+ channel blocker), suggesting a role for Ca2+ influx. ATP-induced phosphorylation of p38 and p44/42 MAPKs was blocked by nifedipine. ATP increased expression levels of cyclin-dependent kinase (CDK)-2, CDK-4, and cyclin E, which were blocked by suramin, reactive blue 2, MRS-2179, MRS-2159, and nifedipine. However, ATP decreased expression levels of p21WAF1/Cip1 and p27kip1. ATP-induced stimulation of thymidine incorporation and increase of CDK-2 and CDK-4 expression were blocked by SB-203580 (a p38 MAPK inhibitor) and PD-98059 (an MEK inhibitor), but not by SP-600125 (a JNK inhibitor). In conclusion, ATP stimulates proliferation by increasing intracellular Ca2+ concentration and activating p38, p44/42 MAPKs, and CDKs in PTCs.

Lysophosphatidic acid induces exocytic trafficking of Na(+)/H(+) exchanger 3 by E3KARP-dependent activation of phospholipase C

Lysophosphatidic acid (LPA) stimulates Na(+)/H(+) exchanger 3 (NHE3) activity in opossum kidney proximal tubule (OK) cells by increasing the apical membrane amount of NHE3. This occurs by stimulation of exocytic trafficking of NHE3 to the apical plasma membrane by an E3KARP-dependent mechanism. However, it is still unclear how E3KARP leads to the LPA-induced exocytosis of NHE3. In the current study, we demonstrate that stable expression of exogenous E3KARP increases LPA-induced phospholipase C (PLC) activation and subsequent elevation of intracellular Ca(2+) in opossum kidney proximal tubule (OK) cells. Pretreatment with U73122, a PLC inhibitor, prevented the LPA-induced NHE3 activation and the exocytic trafficking of NHE3. To understand how the elevation of intracellular Ca(2+) leads to the stimulation of NHE3, we pretreated OK cells with BAPTA-AM, an intracellular Ca(2+) chelator. BAPTA-AM completely blocked the LPA-induced increase of NHE3 activity and surface NHE3 amount by decreasing the LPA-induced exocytic trafficking of NHE3. Pretreatment with GF109203X, a PKC inhibitor, did not affect the percent of LPA-induced NHE3 activation and increase of surface NHE3 amount. From these results, we suggest that E3KARP plays a necessary role in LPA-induced PLC activation, and that PLC-dependent elevation of intracellular Ca(2+) but not PKC activation is necessary for the LPA-induced increase of NHE3 exocytosis.

Lack of stereospecificity in lysophosphatidic acid enantiomer-induced calcium mobilization in human erythroleukemia cells

Lysophosphatidic acid (LPA) is a lipid mediator that, among several other cellular responses, can stimulate cells to mobilize calcium (Ca2+). LPA is known to activate at least three different subtypes of G protein-coupled receptors. These receptors can then stimulate different kinds of G proteins. In the present study, LPA and LPA analogs were synthesized from (R)- and (S)-glycidol and used to characterize the ability to stimulate Ca2+ mobilization. The cytosolic Ca2+ concentration ([Ca2+]i) was measured in fura-2-acetoxymethylester-loaded human erythroleukemia (HEL) cells. Furthermore, a reverse transcriptase polymerase chain reaction was used to characterize LPA receptor subtypes expressed in HEL cells. The results show that HEL cells mainly express LPA1 and LPA2, although LPA3 might possibly be expressed as well. Moreover, LPA and its analogs concentration-dependently increased [Ca2+]i in HEL cells. The response involved both influx of extracellular Ca2+ and release of Ca2+ from intracellular stores. This is the first time the unnatural (S)-enantiomer of LPA, (S)-3-O-oleoyl-1-O-phosphoryl-glycerol, has been synthesized and studied according to its ability to activate cells. The results indicate that this group of receptors does not discriminate between (R)- and (S)-enantiomers of LPA and its analogs. When comparing ether analogs having different hydrocarbon chain lengths, the tetradecyl analog (14 carbons) was found to be the most effective in increasing [Ca2+]i. Pertussis toxin treatment of the HEL cells resulted in an even more efficient Ca2+ mobilization stimulated by LPA and its analogs. Furthermore, at repeated incubation with the same ligand no further increase in [Ca2+]i was obtained. When combining LPA with the ether analogs no suppression of the new Ca2+ signal occurred. All these findings may be of significance in the process of searching for specific agonists and antagonists of the LPA receptor subtypes.