Andrew P. Feranchak

Evidence-Based Practice Recommendations for Nutrition-Related Management of Children and Adults with Cystic Fibrosis and Pancreatic Insufficiency: Results of a Systematic Review

The Cystic Fibrosis Foundation established a process of systematic review of evidence to inform the development of clinical care guidelines and encourage evidence-based practice. The Subcommittee on Growth and Nutrition reviewed the evidence in two areas: energy intake and dosing for pancreatic enzyme replacement therapy. Evidence-based recommendations are presented here. Also, an ad hoc working group conducted a review of the literature and performed new analyses using the Cystic Fibrosis Foundation Patient Registry to update the recommendations for growth and weight-status monitoring. These Registry data-based recommendations are presented.

Hepatocellular ATP-binding Cassette Protein Expression Enhances ATP Release and Autocrine Regulation of Cell Volume

In a model liver cell line, recovery from swelling is mediated by a sensitive autocrine pathway involving conductive release of ATP, P2 receptor stimulation, and opening of membrane Cl− channels (Wang, Y., Roman, R. M., Lidofsky, S. D., and Fitz, J. G. (1996)Proc. Natl. Acad. Sci. U. S. A. 93, 12020–12025). However, the mechanisms coupling changes in cell volume to ATP release are not known. Based on evidence that certain ATP-binding cassette (ABC) proteins may function as ATP channels or channel regulators, we evaluated the potential role of ABC proteins by comparing ATP release and volume regulation in rat HTC and HTC-R hepatoma cells, the latter of which overexpress Mdr proteins. In both cell types, Cl−current activation (I Cl-swell) and volume recovery following swelling were dependent on conductive ATP efflux. The rate of volume recovery was ∼6-fold faster in HTC-R cells compared with HTC cells. This effect is likely due to enhanced ABC protein-dependent ATP release since (i)I Cl-swell and cell volume recovery were eliminated by inhibition of P-glycoprotein transport (20 μm verapamil and 15 μm cyclosporin A); (ii) swelling-induced Cl− current density was similar in both cell types (approximately −50 pA/pF; not significant); and (iii) ATP conductance measured by whole-cell techniques was increased ∼3-fold in HTC-R cells compared with HTC cells. Moreover, HTC-R cells exhibited enhanced survival during hypotonic stress. By modulating ATP release, hepatic ABC proteins may play a key role in the cellular pathways coupling changes in cell volume to ion permeability and secretion.

Prospective, long-term study of fat-soluble vitamin status in children with cystic fibrosis identified by newborn screen☆☆☆★

OBJECTIVE To prospectively evaluate the biochemical status of vitamins A, D, and E in children with cystic fibrosis (CF). SUBJECTS A total of 127 infants identified by the Colorado CF newborn screening program. DESIGN Vitamin status (serum retinol, 25-hydroxy vitamin D, ratio of alpha-tocopherol/total lipids) and serum albumin were assessed at diagnosis (4 to 8 weeks), ages 6 months, 12 months, and yearly thereafter, to age 10 years. RESULTS Deficiency of 1 or more vitamins was present in 44 (45.8%) of 96 patients at age 4 to 8 weeks as follows: vitamin A 29.0%, vitamin D 22.5%, and vitamin E 22.8%. Of these patients with initial deficiency, the percent that was deficient at 1 or more subsequent time points, despite supplementation, was vitamin A 11.1%, vitamin D 12.5%, and vitamin E 57.1%. Of the initial patients with vitamin sufficiency, the percent who became deficient at any time during the 10-year period was as follows: vitamin A 4.5%, vitamin D 14.4%, and vitamin E 11.8%. The percent of patients deficient for 1 or more vitamins ranged from 4% to 45% for any given year. CONCLUSIONS Despite supplementation with standard multivitamins and pancreatic enzymes, the sporadic occurrence of fat-soluble vitamin deficiency and persistent deficiency is relatively common. Frequent and serial monitoring of the serum concentrations of these vitamins is therefore essential in children with CF.

Phosphatidylinositol 3-kinase contributes to cell volume regulation through effects on ATP release

Regulated changes in cell volume represent a signal that modulates a broad range of cell and organ functions. In HTC hepatoma cells, increases in volume are coupled to membrane ion permeability through a pathway involving (i) ATP efflux, (ii) autocrine stimulation of P2 receptors, and (iii) increases in anion permeability and Cl− efflux, contributing to recovery of volume toward basal values. Based on recent evidence that cell volume increases also stimulate phosphoinositide kinases, the purpose of these studies was to determine if phosphatidylinositol 3-kinase (PI 3-kinase) modulates these pathways. Exposure of cells to hypotonic buffer (20 or 40% less NaCl) caused an initial increase in cell volume and stimulated a rapid increase in ATP release. Subsequent opening of Cl− channels was followed by recovery of cell volume toward basal values, despite the continuous presence of hypotonic buffer. Inhibition of PI 3-kinase with wortmannin (K i = 3 nm) significantly inhibited both the rate of volume recovery and activation of Cl−currents; similar results were obtained with LY294002 (10 μm). Additionally, current activation was inhibited by intracellular dialysis with antibodies specific for the 110-kDa catalytic subunit of PI 3-kinase. Since release of ATP is a critical element in the volume-regulatory pathway, the role of PI 3-kinase on volume-stimulated ATP release was assessed. Both wortmannin and LY294002 decreased basal and volume-stimulated ATP permeability but had no effect on the current response to exogenous ATP (10 μm). These findings indicate that PI 3-kinase plays a significant role in regulation of cell volume and suggest that the effects are mediated in part through modulation of cellular ATP release.

Endogenous ATP release regulates Cl- secretion in cultured human and rat biliary epithelial cells.

P2Y receptor stimulation increases membrane Cl- permeability in biliary epithelial cells, but the source of extracellular nucleotides and physiological relevance of purinergic signaling to biliary secretion are unknown. Our objectives were to determine whether biliary cells release ATP under physiological conditions and whether extracellular ATP contributes to cell volume regulation and transepithelial secretion. With the use of a sensitive bioluminescence assay, constitutive ATP release was detected from human Mz-ChA-1 cholangiocarcinoma cells and polarized normal rat cholangiocyte monolayers. ATP release increased rapidly during cell swelling induced by hypotonic exposure. In Mz-ChA-1 cells, removal of extracellular ATP (apyrase) and P2 receptor blockade (suramin) reversibly inhibited whole cell Cl- current activation and prevented cell volume recovery during hypotonic stress. Moreover, exposure to apyrase induced cell swelling under isotonic conditions. In intact normal rat cholangiocyte monolayers, hypotonic perfusion activated apical Cl-currents, which were inhibited by addition of apyrase and suramin to bathing media. These findings indicate that modulation of ATP release by the cellular hydration state represents a potential signal coordinating cell volume with membrane Cl- permeability and transepithelial Cl-secretion.P2Y receptor stimulation increases membrane Cl- permeability in biliary epithelial cells, but the source of extracellular nucleotides and physiological relevance of purinergic signaling to biliary secretion are unknown. Our objectives were to determine whether biliary cells release ATP under physiological conditions and whether extracellular ATP contributes to cell volume regulation and transepithelial secretion. With the use of a sensitive bioluminescence assay, constitutive ATP release was detected from human Mz-ChA-1 cholangiocarcinoma cells and polarized normal rat cholangiocyte monolayers. ATP release increased rapidly during cell swelling induced by hypotonic exposure. In Mz-ChA-1 cells, removal of extracellular ATP (apyrase) and P2 receptor blockade (suramin) reversibly inhibited whole cell Cl- current activation and prevented cell volume recovery during hypotonic stress. Moreover, exposure to apyrase induced cell swelling under isotonic conditions. In intact normal rat cholangiocyte monolayers, hypotonic perfusion activated apical Cl- currents, which were inhibited by addition of apyrase and suramin to bathing media. These findings indicate that modulation of ATP release by the cellular hydration state represents a potential signal coordinating cell volume with membrane Cl- permeability and transepithelial Cl- secretion.

The lipid products of phosphoinositide 3-kinase contribute to regulation of cholangiocyte ATP and chloride transport.

ATP stimulates Cl− secretion and bile formation by activation of purinergic receptors in the apical membrane of cholangiocytes. The purpose of these studies was to determine the cellular origin of biliary ATP and to assess the regulatory pathways involved in its release. In Mz-Cha-1 human cholangiocarcinoma cells, increases in cell volume were followed by increases in phophoinositide (PI) 3-kinase activity, ATP release, and membrane Cl− permeability. PI 3-kinase signaling appears to play a regulatory role because ATP release was inhibited by wortmannin or LY294002 and because volume-sensitive current activation was inhibited by intracellular dialysis with antibodies to the 110 kDa-subunit of PI 3-kinase. Similarly, in intact normal rat cholangiocyte monolayers, increases in cell volume stimulated luminal Cl− secretion through a wortmannin-sensitive pathway. To assess the role of PI 3-kinase more directly, cells were dialyzed with the synthetic lipid products of PI 3-kinase. Intracellular delivery of phosphatidylinositol 3,4-bisphosphate, and phosphatidylinositol 3,4,5-trisphosphate activated Cl− currents analogous to those observed following cell swelling. Taken together, these findings indicate that volume-sensitive activation of PI 3-kinase and the generation of lipid messengers modulate cholangiocyte ATP release, Cl− secretion, and, hence, bile formation.

Fluid flow induces mechanosensitive ATP release, calcium signalling and Cl− transport in biliary epithelial cells through a PKCζ-dependent pathway

ATP in bile is a potent secretogogue, stimulating cholangiocyte Cl− and fluid secretion via binding to membrane P2 receptors, though the physiological stimuli involved in biliary ATP release are unknown. The goal of the present studies was to determine the potential role of fluid flow in biliary ATP release and secretion. In both human Mz‐Cha‐1 biliary cells and normal rat cholangiocyte monolayers, exposure to flow increased relative ATP release which was proportional to the shear stress. In parallel studies, shear was associated with an increase in [Ca2+]i and membrane Cl− permeability, which were both dependent on extracellular ATP and P2 receptor stimulation. Flow‐stimulated ATP release was dependent on [Ca2+]i, exhibited desensitization with repetitive stimulation, and was regulated by PKCζ. In conclusion, both human and rat biliary cells exhibit flow‐stimulated, PKCζ‐dependent, ATP release, increases in [Ca2+]i and Cl− secretion. The finding that fluid flow can regulate membrane transport suggests that mechanosensitive ATP release may be a key regulator of biliary secretion and an important target to modulate bile flow in the treatment of cholestatic liver diseases.

Identification and Functional Characterization of TMEM16A, a Ca2+-activated Cl− Channel Activated by Extracellular Nucleotides, in Biliary Epithelium

Cl− channels in the apical membrane of biliary epithelial cells (BECs) provide the driving force for ductular bile formation. Although a cystic fibrosis transmembrane conductance regulator has been identified in BECs and contributes to secretion via secretin binding basolateral receptors and increasing [cAMP]i, an alternate Cl− secretory pathway has been identified that is activated via nucleotides (ATP, UTP) binding apical P2 receptors and increasing [Ca2+]i. The molecular identity of this Ca2+-activated Cl− channel is unknown. The present studies in human, mouse, and rat BECs provide evidence that TMEM16A is the operative channel and contributes to Ca2+-activated Cl− secretion in response to extracellular nucleotides. Furthermore, Cl− currents measured from BECs isolated from distinct areas of intrahepatic bile ducts revealed important functional differences. Large BECs, but not small BECs, exhibit cAMP-stimulated Cl− currents. However, both large and small BECs express TMEM16A and exhibit Ca2+-activated Cl− efflux in response to extracellular nucleotides. Incubation of polarized BEC monolayers with IL-4 increased TMEM16A protein expression, membrane localization, and transepithelial secretion (Isc). These studies represent the first molecular identification of an alternate, noncystic fibrosis transmembrane conductance regulator, Cl− channel in BECs and suggest that TMEM16A may be a potential target to modulate bile formation in the treatment of cholestatic liver disorders.

Evidence for multidrug resistance-1 P-glycoprotein-dependent regulation of cellular ATP permeability.

Abstract. The mechanisms responsible for regulating epithelial ATP permeability and purinergic signaling are not well defined. Based on the observations that members of the ATP-binding cassette (ABC)1 family of proteins may contribute to ATP release, the purpose of these studies was to assess whether multidrug resistance-1 (MDR1) proteins are involved in ATP release from HTC hepatoma cells. Using a bioluminescence assay to detect extracellular ATP, increases in cell volume increased ATP release ∼3-fold. The MDR1 inhibitors cyclosporine A (10 μm) and verapramil (10 μm) inhibited ATP release by 69% and 62%, respectively (p < 0.001). Similarly, in whole-cell patch-clamp recordings, intracellular dialysis with C219 antibodies to inhibit MDR1 decreased ATP-dependent volume-sensitive Cl− current density from −33.1 ± 12.5 pA/pF to −2.0 ± 0.3 pA/pF (−80 mV, p≤ 0.02). In contrast, overexpression of MDR1 in NIH 3T3 cells increased ATP release rates. Inhibition of ATP release by Gd3+ had no effect on transport of the MDR1 substrate rhodamine-123; and alteration of MDR1-substrate selectivity by mutation of G185 to V185 had no effect on ATP release. Since the effects of P-glycoproteins on ATP release can be dissociated from P-glycoprotein substrate transport, MDR1 is not likely to function as an ATP channel, but instead serves as a potent regulator of other cellular ATP transport pathways.

Initiation of purinergic signaling by exocytosis of ATP-containing vesicles in liver epithelium

Extracellular ATP represents an important autocrine/paracrine signaling molecule within the liver. The mechanisms responsible for ATP release are unknown, and alternative pathways have been proposed, including either conductive ATP movement through channels or exocytosis of ATP-enriched vesicles, although direct evidence from liver cells has been lacking. Utilizing dynamic imaging modalities (confocal and total internal reflection fluorescence microscopy and luminescence detection utilizing a high sensitivity CCD camera) at different scales, including confluent cell populations, single cells, and the intracellular submembrane space, we have demonstrated in a model liver cell line that (i) ATP release is not uniform but reflects point source release by a defined subset of cells; (ii) ATP within cells is localized to discrete zones of high intensity that are ∼1 μm in diameter, suggesting a vesicular localization; (iii) these vesicles originate from a bafilomycin A1-sensitive pool, are depleted by hypotonic exposure, and are not rapidly replenished from recycling of endocytic vesicles; and (iv) exocytosis of vesicles in response to cell volume changes depends upon a complex series of signaling events that requires intact microtubules as well as phosphoinositide 3-kinase and protein kinase C. Collectively, these findings are most consistent with an essential role for exocytosis in regulated release of ATP and initiation of purinergic signaling in liver cells.