Effects of phytoestrogens on sarcoplasmic/endoplasmic reticulum calcium ATPase 2a and Ca2+ uptake into cardiac sarcoplasmic reticulum.

Phytoestrogens are naturally occurring estrogenic compounds found in plants and plant products. These compounds are also known to exert cellular effects independent of their interactions with estrogen receptors. We studied the effects of the phytoestrogens phloretin, phloridzin, genistein, and biochanin A on Ca(2+) uptake into the cardiac muscle sarcoplasmic reticulum (SR). Genistein and biochanin A did not affect SR Ca(2+) uptake. On the other hand, phloretin and phloridzin decreased the maximum velocity of SR Ca(2+) uptake but did not affect the Hill coefficient or the Ca(2+) sensitivity of uptake. Measurements of the ATPase activity of the cardiac SR Ca(2+) pump (SERCA2a) revealed direct inhibitory effects of phloretin and phloridzin on SERCA2a. Neither compound induced a detectable change in the permeability of the SR membrane to Ca(2+). These results indicate that phloretin and phloridzin inhibit cardiac SR Ca(2+) uptake by directly inhibiting SERCA2a.

Mitochondrial dysfunction is a primary event in renal cell oxalate toxicity.

BACKGROUND: In cultured renal epithelial cells, exposure to oxalate, a constituent of many kidney stones, elicits a cascade of responses that often leads to cell death. Oxalate toxicity is mediated via generation of reactive oxygen species (ROS) in a process that depends at least in part upon lipid signaling molecules that are generated through membrane events that culminate in phospholipase A2 (PLA2) activation. The present studies asked whether mitochondria, a major site of ROS production, were targets of oxalate toxicity, and if so, whether mitochondrial responses to oxalate were mediated by PLA2 activation. METHODS: Effects of oxalate and various lipids on mitochondrial membrane potential (DeltaPsim) were measured in Madin-Darby canine kidney (MDCK) cell monolayers using 5,5',6,6'-tetrachloro 1,1',3,3'-tetraethylbenzimidazolylcarbocyanine iodide (JC-1), a DeltaPsim-sensitive dye. Other studies assayed caspases, serine proteases activated during apoptosis, in response to oxalate or lipid signaling molecules. Additional studies asked whether oxalate or lipids produced by PLA2 activation promoted ROS formation in isolated renal mitochondria. RESULTS: Oxalate exposure decreased MDCK cell DeltaPsim within 30 minutes, a response attenuated by arachidonyl trifluoromethyl ketone (AACOCF3), an inhibitor of cytosolic PLA2 (cPLA2). Exposure to arachidonic acid or to lysophosphatidylcholine (lyso-PC), lipid products of PLA2 activation, or to ceramide, another lipid signal generated in MDCK cells following oxalate exposure, also depolarized MDCK cell DeltaPsim and increased the number of caspase-positive cells. Isolated renal mitochondria responded to oxalate, arachidonic acid, lyso-PC, and ceramide by increasing their accumulation of ROS, lipid peroxides, and oxidized thiol proteins. CONCLUSION: These studies suggest that lipid signaling molecules released after oxalate-induced PLA2 activation trigger marked, rapid changes in mitochondrial function that may mediate toxicity in renal epithelial cells.

Intracellular events in the initiation of calcium oxalate stones.

This review summarizes our current understanding of intracellular events in the initiation of kidney stone formation, focusing on results from studies using renal epithelial cells in vitro. Such studies have shown that oxalate - either in crystalline or in soluble form - triggers a spectrum of responses in renal cells that favor stone formation, including alterations in membrane surface properties that promote crystal attachment and alterations in cell viability that provide debris for crystal nucleation. Activation of cytosolic PLA2 appears to play an important role in oxalate actions, triggering a signaling cascade that generates several lipid mediators (arachidonic acid, AA; lysophosphatidylcholine, Lyso-PC; ceramide) that act on key intracellular targets (mitochondria, nucleus). The net effect is increased production of reactive oxygen molecules (that in turn affect other cellular processes), an increase in cell death and an induction of a number of genes in surviving cells, some of which may promote proliferation for replacement of damaged cells, or may promote secretion of urinary macromolecules that serve to modulate crystal formation. A scheme is provided that explains how such oxalate-induced alterations could initiate stone formation in vivo.

How elevated oxalate can promote kidney stone disease: changes at the surface and in the cytosol of renal cells that promote crystal adherence and growth.

The present review assesses the mechanisms by which oxalate-induced alterations in renal cell function may promote stone disease focusing on 1) changes in membrane surface properties that promote the attachment of nascent crystals and 2) changes in the expression/secretion of urinary macromolecules that alter the kinetics of crystal nucleation, agglomeration and growth. The general role of renal cellular injury in promoting these responses and the specific role of urinary oxalate in producing injury is emphasized, and the signaling pathways that lead to the observed changes in cell surface properties and in the viability and growth of renal cells are discussed. Particular attention is paid to evidence linking oxalate-induced activation of cytosolic phospholipase A2 to changes in gene expression and to the activation of a second signaling pathway involving ceramide. The effects of the lipid signals, arachidonic acid, lysophosphatidylcholine and ceramide, on mitochondrial function are considered in some detail since many of the actions of oxalate appear to be secondary to increased production of reactive oxygen molecules within these organelles. Data from these studies and from a variety of other studies in vitro and in vivo were used to construct a model that illustrates possible mechanisms by which an increase in urinary oxalate levels leads to an increase in kidney stone formation. Further studies will be required to assess the validity of various aspects of this proposed model and to determine effective strategies for countering these responses in stone-forming individuals.

Mechanisms mediating oxalate-induced alterations in renal cell functions.

Oxalate is a major component of the most common form of kidney stones--calcium oxalate stones. High concentrations of oxalate promote stone formation in two ways: (1) by providing urinary conditions favorable to the formation of calcium oxalate crystals, and (2) by inducing renal injury that generates cellular debris and promotes crystal nucleation and attachment. Oxalate toxicity is mediated in part by activation of lipid signaling pathways that produce arachidonic acid, lysophospholipids, and ceramide. These lipids disrupt mitochondrial function by increasing reactive oxygen species (ROS), decreasing mitochondrial membrane potential, and increasing mitochondrial permeability. The net response is cytochrome C release, activation of caspases, and apoptosis or necrosis. Not all cells succumb to oxalate toxicity, however, in those cells that don't, ROS and lipid-signaling molecules induce changes in gene expression that allow them to survive and adapt to the toxic insult. The increased expression of immediate early genes (IEGs), osteopontin, extracellular matrix (ECM) proteins, crystallization inhibitors, and chemokines orchestrates a group of cellular responses--including cell proliferation, secretion of kidney stone inhibitory proteins, recruitment of immune cells, and tissue remodeling--that limit accumulation of cell debris or increase the production of inhibitors of calcium oxalate crystallization, thereby limiting stone formation.