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Determination of Acute Lethal and Chronic Lethal Thresholds of Valproic Acid using 3D Spheroids Constructed from the Immortal Human Hepatocyte Cell Line Hepg2/C3A Fey, Stephen John; Wrzesinski, Krzysztof

Published in: Valproic Acid

Publication date: 2013

Document version Accepted author manuscript

Citation for pulished version (APA): Fey, S. J., & Wrzesinski, K. (2013). Determination of Acute Lethal and Chronic Lethal Thresholds of Valproic Acid using 3D Spheroids Constructed from the Immortal Human Hepatocyte Cell Line Hepg2/C3A. In A. Boucher (Ed.), Valproic Acid. (pp. 141-165). Chapter V.Nova Science Publishers, Incorporated.

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Chapter V

Determination of Acute Lethal and Chronic Lethal Dose Thresholds of Valproic Acid Using 3D Spheroids Constructed from the Immortal Human Hepatocyte Cell Line HepG2/C3A

Stephen J. Fey,† and Krzysztof Wrzesinski† Department of Biochemistry and Molecular Biology, University of Southern Denmark, Denmark

Abstract

Valproic acid (VPA) is a broad-spectrum antiepileptic drug that is now used commonly for several other neurological and psychiatric indications. While VPA is usually well tolerated, on rare occasions, it has

 To whom correspondence should be addressed at Department of Biochemistry and Molecular Biology, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark. Tel: +45 6550 2440; Fax: +45 6550 2467; Email: [email protected]. † These authors contributed equally to the work presented here. 142 Stephen J. Fey and Krzysztof Wrzesinski

been associated with severe, and sometimes, fatal liver injuries. These complications may also arise due to acute VPA overdose. As a branched chain carboxylic acid, VPA is readily metabolized in the liver via glucuronic acid conjugation, mitochondrial β- and cytosolic ω-oxidation to produce multiple metabolites, and it is probably some of these metabolites that are involved in its toxicity. While the actual mechanism of VPA hepatotoxicity is still not understood, two pathways have been in focus: the first involves the formation of reactive metabolites of VPA (and their subsequent covalent binding to cellular proteins) and the second is the development of oxidative stress in the cell. We describe here a culture system based on 3D spheroid culture of immortal hepatocytes which can determine the toxicity of valproic acid (or structurally or functionally related molecules) in vitro. The spheroids were used to follow changes in ATP production, glucose uptake and adenylate kinase following treatment and can be treated repeatedly with VPA to determine both acutely-lethal and chronically-lethal doses. This system is homeostatic and has been shown to be metabolically stable over at least 24 days and can therefore be used to follow recovery after treatment.

Keywords: 3D Spheroid culture; drug toxicity; valproic acid; acute lethal threshold; chronic lethal threshold

Introduction

When cells are grown in three dimensional (3D) environments (i.e. as multilayers or clusters in suspension, on scaffolds etc.) they have been shown to possess a number of physiological characteristics which makes them resemble more closely the native tissue from which they originated than the same cells grown in classical two dimensional (2D – i.e. as monolayers) conditions in culture flasks or microtitre plates. This may be due to the fact that cells grown in 3D cell culture are allowed to develop a more elaborate extracellular matrix and better intercellular communication [1], [2], [3], and this leads to a recovery or maintenance of in vivo function [4]. For this reason, we recently investigated how long it takes cells to recover from trypsinisation. Our studies revealed that the hepatocyte cell line HepG2- C3A went through some form of transition after 18 days, after which several key metabolic traits (growth rate, ATP levels, urea and cholesterol production) all stabilised[5]. A review of the literature showed that this was not a unique Determination of Acute Lethal and Chronic Lethal Dose … 143 phenomenon and there were several well-known cell lines (including the cell lines Caco-2, HT 29, MDCK, MCF-10A and HepG2 used to model the small and large intestine, kidney, breast acini and liver respectively) which all needed similar periods of time to fully establish themselves. We therefore suggested that these changes represent a ubiquitous recovery process after trypsinisation rather than differentiation as is often stated. This would help to explain why three dimensional structure has been proposed as the missing link which will provide in vitro models that sufficiently mimic in vivo conditions [6], [7] and [8]. In this study we have exploited the potential offered by 3D spheroid culture to investigate drug treatment over extended periods of time. After the 18 days needed to recover from trypsin, 3D spheroid cultures are essentially homeostatically stable from 18 days to at least 42 days of culture [9]. They grow slowly, have a high survival percentage; their physiological capabilities (glycogenesis, ATP, cholesterol and urea synthesis and drug metabolism) are constant; and they have a stable gene expression of several key genes related to the main liver pathways. Furthermore, they appear to more predicitive than primary hepatocytes or similar cells when grown in classical 2D cultures of the in vivo toxicity in man, at least for a small number of drugs [10]. All in all, this makes 3D spheroids ideal for long term studies – for example to investigate drug induced liver injury (DILI). Valproic acid (VPA) has become one of the most widely prescribed anticonvulsant and mood-stabilizing drug worldwide. Despite the pharmacological importance and the effectiveness of VPA, its potential hepatotoxicity is still a major concern. The recommended VPA starting dose is between 750 and 1500 mg daily taken in divided doses and then dosages are adjusted based on response and blood level. The usual effective dose range is between 1000 and 2000 mg daily. VPA may have its beneficial neurological effects by increasing the concentration of the inhibitory neurotransmitter Gamma Amino Butyrate (GABA). VPA acts as a double edged sword causing partial creatine deficiency in the brain by stimulating the GABA-forming enzyme glutamate decarboxylase (GAD65/67); and inhibiting 2-oxoglutarate dehydrogenase (a rate-limiting mitochondrial Kreb’s cycle enzyme) and the GABA-catabolizing enzymes GABA transaminase (GABA T) and succinic semialdehyde dehydrogenase (SSADH)[11][12]. This becomes toxic if GABA levels are raised too far. However, treatment with VPA can also lead to liver toxicity, or more specifically drug-induced liver injury (DILI). This is an unresolved problem 144 Stephen J. Fey and Krzysztof Wrzesinski which has significant clinical and economic impacts [13][14]. While the exact mechanisms of DILI remain largely unknown [15] there are several areas under investigation [16]. In the liver, the disturbance of mitochondrial function (including the inhibition of mitochondrial β-oxidation) leads to microvesicularfatty liver [17] and the impairment of oxidative phosphorylation and reduction of ATP synthesis [18][19][20]. Finally, valproic acid can also cause epigenetic effects due to a histone deacetylase inhibitor activity (which would then stimulate gene expression in a more or less specific manner) [21]. Using the two key advantages of hepatic spheroids, namely their long term stability and the fact that they are homeostatic and metabolically ‘in equilibrium’ we describe here the effects of VPA treatment and define two different lethal treatment thresholds, (one for a single treatment (acute) and one for multiple treatments (chronic)) by measuring basic physiological parameters of glucose uptake, cellular ATP levels and the release of adenylate kinase. The results suggest that cell death by single high doses of VPA (acute) are different to cell death caused by multiple low doses of VPA (chronic).

Materials and Methods

Standard 2D Cell Culture Conditions

The immortal human hepatocyte cell line, HepG2/C3A (ATCC CRL- 10741, third passage after receipt from ATCC, Manassas, Virginia.), were thawed from liquid nitrogen storage and cultured in standard tissue culture conditions in a customised D-MEM growth medium (87.5% D-MEM (1 g Glucose/L) (Gibco (Carlsbad, California) Cat. no. 31885-023); 1% Non Essential Amino Acids (Gibco Cat. no. 11140-035); 10% FCS (Foetal calf serum) (Sigma (St. Louis) Cat. no. F 7524); 0.5% Penicillin / Streptomycin (Gibco Cat. no. 15140-122) 1% GlutaMAX (Gibco Cat. no. 35050-038), 37⁰C,

5% CO2 95% air). In order to generate cells for spheroid construction, the C3A cells were initially propagated using traditional 2D tissue culture. Cells were trypsinised (0.05% Trypsin / EDTA Cat. no.: Gibco 15400-054) for 3 minutes and sown out into falcon flasks/microtitre plates and cultivated at 37⁰C, 5% CO2 95% air in a humidified incubator. Determination of Acute Lethal and Chronic Lethal Dose … 145

To eliminate any effects of storage in liquid nitrogen or thawing, cells were grown for at least three passages before starting the experiments, exchanging the medium every two to three days. Cells were used between passages 4 and 15. The doubling time for C3A cells grown under these conditions was 76 hours.

3D Spheroid Culture Conditions Preparation of Spheroids Using AggreWell™Plates

The C3A cell spheroids have been prepared with use of AggreWellTM 400 plates (Stemcel Technologies (Grenoble, France) Cat. no. 27845). Each AggreWell™ plate has 6 wells and each of these contains approximately 4,700 microwells measuring 400 µm in diameter. These plates are used to form cell aggregates. Before use, the plates were washed twice with growth medium (customised D-MEM). In order to remove all residual air bubbles from the well surface the plates were prefilled with 0.5 ml of growth medium and centrifuged (3 min with 3,000 x g). Cells (1.2 x 106) were added to each well and the plates were centrifuged (3 min with 100 x g) and left in the AggreWellTM plate overnight to form spheroids.

Spheroid Culture in ProtoTissue™ Bioreactors

The spheroids were detached from the AggreWellTM plates by gently washing the wells with prewarmed growth medium. The detached spheroids were collected in to a Petri dish and the quality of the spheroids checked by microscopy. Compact spheroids were introduced into ProtoTissue™ bioreactors (MC2 Biotek, (Hørsholm, Denmark) Cat. no. 010). These bioreactors are specially constructed to be easy to open and close and designed to maintain 100% humidity around the growth chamber. The spheroids

(approximately 300 per bioreactor) were then cultivated at 37⁰C, 5% CO2 95% air in a humidified incubator for a minimum of 21 days, exchanging the medium every two to three days [22]. The day when the cells were transferred into the bioreactor is defined as day 0. An estimated 90% of the medium was changed on day 1 and thereafter three times a week during the recoveryperiod. To achieve a stable suspension of spheroids, the rotation speed of the bioreactors was initially set between 21- 23 rpm and it was adjusted to compensate for the growth in size of the 146 Stephen J. Fey and Krzysztof Wrzesinski spheroids (and reached 27-30 rpm on day 21). Speed adjustments were made so that inter-spheroid contact was minimised and these adjustments were made more frequently at the beginning of the culture than at the end. Because the spheroids get larger with time, the population density of the spheroids was regulated by opening the bioreactor and ‘splitting the population’ byremoving excess spheroids (this usually needed to be performed between day 11 and 14). Before use, the spheroid batch quality was assessed by staining for 3 min. with 0.4% Trypan Blue (Gibco Cat. no. 15250-061). Batches showing greater than 90% viability were accepted.

Microscopy and Planimetry

Photomicrographs were taken using an Olympus IX81 motorised microscope, and an Olympus DP71 camera. Images were transferred to the Olympus AnalySiS®Docu program (Soft Imaging System) and the ‘shadow’ area of spheroids measured using the ‘Fitted Polygon Area’ function which calculates the planar surface of the spheroids in µm².

Drug Treatment

Valproic acid (VPA) was purchased from Sigma (Cat no. P4543). Stock solutions were prepared just before use by dissolving the compound in either growth medium or DMSO (Sigma Cat. no. D2650) and diluted into growth media. A control media was prepared which contained the same concentration of DMSO vehicle without any compound. The maximum final DMSO concentration was 0.01%. Three hundred 21-day old spheroids (corresponding to approximately 25 million cells or 3.5 mg protein) were gently pipetted (using a cut-tip) into each 10ml bioreactor before treatment to provide ample material for analysis. To initiate drug treatment, the bioreactor rotation was stopped for 30 s to allow the spheroids to settle to the bottom of the bioreactor. Ninety percent of the media volume was exchanged with media containing the compound at concentrations which gave the final treatment concentrations given in the figures. Experiments to determine the LD50 were carried out initially using a broad range of drug concentrations (e.g. varying by a factor of ten) followed by two experiments using a narrow concentration range (varying typically by a factor of four). Determination of Acute Lethal and Chronic Lethal Dose … 147

The data from the broad range of VPA concentrations were used to identify the final narrow range used, and so these data are not presented. Each narrow range experiment was carried out in duplicate and ATP was measured in technical triplicates for each sample. Media was exchanged at 48, 96, 168, 216 and 264 h after the start of the experiment.

ATP Assay

Cell viability was measured based on their ability to produce ATP [23]. Samples of the hepatocytes grown as spheroids were collected (usually 2 - 6 spheroids per assay point depending on the size of the spheroids) at appropriate times and transferred to white opaque microtitre plates (Nunc, (Roskilde, Denmark) Cat. no. 165306), the volume of growth medium was adjusted to 100 µl. The cells were then lysed with 100 µl of lysis buffer (CellTiter-Glo luminescent cell viability assay, Promega (Fitchburg, WI) Cat. no. G7571) and shaken in the dark for 20 min. before the luminescence was measured in a FluoStar OPTIMA (BMG Labtech, Ortenberg, Germany) using the following parameters: one kinetic window, 10 measurement cycles with 0.3 s of measurements interval time, 2 s delay per measurement, additional 0.5 s delay per position change, (repeated twice for each measured plate). Replicates were performed as described under ‘Drug Treatment’. The data was normalised with reference to a standard curve for ATP and to the untreated control.

Adenylate Kinase Assay for Cell Viability

Adenylate kinase was measured using the bioluminescent cytotoxicity assay kit (LonzaToxilight assay kit, cat. no. LT07-117). Spent media from spheroid cultures were collected in triplicate in connection with refreshing the growth media (typically at 48 h intervals), diluted with five volumes of adenlyate kinase detection reagent, allowed to incubate for five minutes and then read the bioluminescence in a Fluostar Omega® (BMG Labtech) using 1 second measurement interval time per each measured well. A standard curve was prepared in the same way for each assay plate using a cell standard (4.29 million C3A cells per mL lysed in lysis buffer). Gain was adjusted based on the highest standard curve values. 148 Stephen J. Fey and Krzysztof Wrzesinski

Protein Determination

In order to correlate the shadow area of the spheroid to its protein content, the latter was determined using the fluorescence based ProStain Protein Qantification Kit (Active Motif, Inc (La Hulpe, Belgium) Cat. no. 15001) according to the manufacturer’s instructions. All data was expressed as a function of the amount of protein present.

Glucose Determination

Samples for glucose determination were collected from every culture at: 0, 4, 24, 48, 52, 72, 96, 100, 168, 174, 192, 216, 222, 240, 264 and 270 h and frozen until needed. Glucose was measured using a ‘Onetouch Vita glucose meter’ and test strips, (MediqDanmark, Cat No. 6407078 Cat. No. 6407079 respectively). The instrument was calibrated using a 1 mg/ml D-glucose standard solution, (Sigma-Aldrich Cat. No. G3285-5ML) and Onetouch Vita control solution (MediqDanmark Cat.No. 64-07-081). Standards and samples were warmed to room temperature and shaken on a rotary shaker at 450rpm for 1 h. For each measurement a test strip was inserted into the instrument and 2-3 µl of each sample were spotted onto it. The glucose concentration is read immediately.

Results

This study was set up to investigate the effects of repeatedly treating hepatocyte spheroids grown in rotating bioreactors with various doses of VPA, to mimic the repeated treatment of patients in vivo and thus to investigate the molecular changes occurring during the development of drug-induced liver injury (DILI). When spheroids are formed using Aggrewell™ plates and grown in the bioreactors (figure 1) using the procedures described here, they are quite uniform in size (± 21% [10]). A few spheroids are shown in figure 2 to illustrate this (a lower power image of many spheroids can be seen in [10]). They can be expected to respond more uniformly than spheroids of different sizes (for example in large spheroids the outer cells might metabolise the drug Determination of Acute Lethal and Chronic Lethal Dose … 149 and thus inner cells would be shielded from the full drug concentration, whereas cells in a small spheroid will all receive the full dose).

Figure 1. A bioreactor used in this study. Spheroids are cultivated in the 10ml chamber in front of the white membrane and the air in the chamber behind the membrane is humidified. The spheroid cultivation chamber can be opened to give a petri-dish like access to the spheroids. The two round plugs measure 1.5 cm and give access to either the growth chamber or to refill the humidification reservoir. The ‘tall’ white plug is removed when changing the media.

Figure 2. 21 day-old spheroids. Photograph of a small number of 21 day-old spheroids to illustrate their uniformity. Note that there is very little cellular contamination in the media. Scale bar = 1mm. 150 Stephen J. Fey and Krzysztof Wrzesinski

Determination of Acute Lethal and Chronic Lethal Dose … 151

Figure 3. Relationship between spheroid shadow area, protein content and number of cells. The spheroid shadow area was measured in four separate experiments and the data combined, averaged and smoothed. A) Averaged spheroid shadow area (on average 180 spheroids were measured per data point). B) the averaged protein content per mm2 of the shadow area of the spheroid. Error bars in A) and B) represent the standard deviation of the unsmoothed data. C) Calculated protein content of a spheroid depending on it’s age; D) calculated number of cells in a spheroid depending on it’s age. The data have been smoothed so that the graphs can be used as look up tables for the actual experiments.

The relationship between the size of a spheroid (it’s shadow area), their protein content and the calculated number of cells contains is shown in figure 3 for spheroids from their day of formation (day 0) to day 42. In order to determine the acute and chronic lethal concentrations at which valproic acid kills human C3A spheroids, twenty one day old spheroids were treated with different concentrations of valproic acid. The spheroids were repeatedly treated with VPA by exchanging ninety percent of the growth media (containing the appropriate VPA concentration) after 48, 96, 168, 216 and 264 h (2, 4, 7, 9 and 11 days). Only ninety percent of the media was exchanged to reduce the risk that the spheroids agglomerated. Measurements of cellular ATP levels have been suggested to be one of the best indicators of cellular viability [23]. A small number of spheroids (so as not to substantially affect the total number in the bioreactor) were collected from each culture during each media refreshment, in order to determine ATP levels. The actual number collected depended on the size of the spheroids and was typically between six and two depending on the age of the culture. All data have been normalised with respect to the amount of ATP present in the control, mock treated culture at the same age. 152 Stephen J. Fey and Krzysztof Wrzesinski

The results (figure 4) show that a dose of 20 milligram VPA per milligram total cellular protein (20 mg VPA/mg TCP) was sufficient to reduce cellular ATP levels to lower than 30% after 24 hours and lower than 11% after 48 hours. The second treatment killed the remaining few cells. Treatment of the spheroids with the first dose of 4 mg VPA/mg TCP resulted in a fall of ATP levels to about 75% of the control levels (figure 4). After the second, fourth and fifth doses cells appeared to be able to recover their ATP levels to normal. However there was a trend that, at each subsequent dose, the ATP levels are supressed lower and lower and by extrapolation, one could anticipate that if treatment was repeated a few times more that the cells would die (after the sixth treatment the ATP levels were down to that seen with one treatment of 20 mg VPA/mg TCP. Lower doses (0.8 and 0.16 mg VPA/mg TCP) perturbed the ATP levels but each time the ATP levels recovered to essentially the same level as seen in the untreated controls. ATP levels above that seen in the controls were frequently measured. In order to investigate the system further, we investigated glucose consumption of the cultures because VPA has been shown to have a profound effect on glucose metabolism [24]. Fresh growth media contains 1 mg/mL glucose (ca. 5.5 mM). The glucose levels of the cultures were measured each time the media was changed (at 48, 96, 168, 216 and 264 h) and again 4 or 6 h after the media change. Thus glucose levels were measured at 0, 4, 24, 48, 52, 72, 96, 100, 168, 174, 192, 216, 222, 240, 264 and 270 h for every culture. Figure 5 reflects the actual amount of glucose in the growth media, and shows for example typically that most of the glucose in the media had be consumed 24 h after its refreshment even at moderate VPA doses. Interpreting the data becomes easier if one views selected measurements. Figure 6A shows only the glucose measurements made 4 or 6 h after refreshing the growth media and figure 6B shows the glucose concentrations in the spent media before refreshment. These selected data show that there is a clear correlation between the VPA dose and ability to remove glucose from the media. At high VPA doses the spheroids rapidly lose the ability to import glucose while spheroids treated with low doses and the mock-treated control became increasing proficient at doing so. figure 3Exactly the same interpretation can be reached by viewing the glucose concentrations in the spent media (figure 6B). Spheroid cultures treated with the highest VPA (20 mg VPA/mg TCP) managed to remove about 70% of the glucose from the media in the first 48 hrs (long dashes in figure 6B). Thereafter they showed no ability to remove glucose and were essentially dead. Again spheroids treated with the 4 mg Determination of Acute Lethal and Chronic Lethal Dose … 153

VPA/mg TCP dose initially can cope with the treatment but at later stages (after 168 hrs) have almost lost the ability to import glucose (medium length dashed line in figure 6B).

Figure 4. ATP content of cells in VPA treated spheroids. The spheroids were treated with 0 (____ solid line), 0.16 (……. Dotted line), 0.8 (--- short dash), 4 (- - - medium dash) and 20 (______long dash) mg VPA/mg total cellular protein (TCP) at the timepoints indicated by the hollow arrows on the abscissa. The data has then been normalised to the mock-treated control at the same timepoint. Error bars (only positive bar shown for clarity) show the standard deviation calculated from the average of triplicate technical replicates for two independent biological replicates.

The final question we investigated was whether the changes in cell reactivity was related to apoptosis or necrosis [25][26]. In apoptosis, cell death is an ATP-driven, carefully programmed process whereas in necrosis, cell death is a result of a cellular trauma (e.g. infection or poisoning) that the cell cannot cope with. Necrosis is an ATP independent event which results in the massive release of cellular contents, including the enzyme adenylate kinase and this results in inflammation of the surrounding cells). Measurements of adenylate kinase release reveal an interesting aspect to what is happening in the cultures (figure 7). Treatment with the high dose of VPA (20 mg VPA/mg TCP) caused a clear release of adenylate kinase, peaking at 48hrs. Thereafter the amount falls as there are fewer living cells left which can release the enzyme. No other VPA dose gave rise to a similar reaction pattern. 154 Stephen J. Fey and Krzysztof Wrzesinski

When compared to the mock treated control, all three of the other VPA doses appear to repress adenylate kinase release in a dose dependent manner, especially at later timepoints. Since low ATP levels do not lead to mitochondrial membrane depolarisation, the spheroid cells are probably dying via necrosis. This is consistent with breach of the plasma membrane and release of adenylate kinase at the high doses of VPA (20 mg VPA/mg TCP) [29]. However, the moderate doses of VPA (i.e. 4 mg VPA/mg TCP) also lead towards a slow ATP depletion, and since apoptosis is ATP dependent then these moderate doses would also be expected to be driving the hepatocytes also towards an ATP-independent necrotic death despite the absence of adenylate kinase release. Thus it appears that there could be differences in the necrotic pathways. This needs to be explored further, for example by measuring cytochrome c release, glutathione, Bcl-2 and caspase 3 activation during these processes [30][31] and [32].

Figure 5. Glucose clearance from the media. The amount of glucose remaining in the growth media was measured during the VPA treatment. All other details as in figure 4, except that the data were not normalised to the control. Determination of Acute Lethal and Chronic Lethal Dose … 155

Figure 6. Glucose clearance from the media. Subsets of data shown in figure 5. Glucose levels remaining in the growth media A) four or six hours after refreshing the media and B) in the spent media immediately before refreshment.

Apoptosis is an energy dependent process and is usually associated with mitochondrial membrane depolarisation and release of cytochrome C [27]and [28].

156 Stephen J. Fey and Krzysztof Wrzesinski

Discussion

The goal of this study was to investigate the suitability of 3D spheroids for long term drug treatment using VPA as the test compound, with the long term aim of using the system to investigate drug-induced liver injury. Our approach was to treat mature 3D spheroids with different concentrations of VPA repeatedly at two or three day intervals. Cell viability and cytotoxicity was then measured by following the total cellular ATP content, the glucose concentration of the media and the release of adenylate kinase.

Figure 7. Adenylate kinase levels in the media. The amount of adenylate kinase released into the growth media during the VPA treatment. All other details as in figure 4.

The data presented here showed that spheroids treated with high levels of VPA (20 mg VPA/mg TCP) rapidly lost their ability to synthesise ATP, import glucose and they released large amounts of adenylate kinase. Moderate levels of VPA (4 mg VPA/mg TCP) caused a slow loss in the ability to synthesise ATP (over a 10 day period) and a corresponding slow loss in the ability to import glucose. Low levels of VPA (0.8 and 0.16 mg VPA/mg TCP) induced an initial increase in ATP levels, despite a lowered glucose consumption. Both moderate and low levels of VPA treatment reduced the amount of adenylate kinase released to below control levels. This is presumably because the number of cells in the spheroids is increasing. From this data it is clear that spheroids treated with the 4 mg VPA/mg TCP dose Determination of Acute Lethal and Chronic Lethal Dose … 157 initially can cope with the treatment but at later stages (after 168 h) are losing the ability to import glucose.

Molecular Basis for VPA Toxicity

Measurement of glucose levels in the 3D cultures were quite revealing because they showed that mock treated cultures cleared the glucose from the media very quickly, probably in about 12 hours and certainly by 24 h. Presumably, the cells are importing the glucose and synthesising glycogen (glycogen granules have been described in 3D spheroid cultured cells [5]). At high oxygen and glucose levels the C3A cells are probably functioning ‘inefficiently’ and exhibit the Crabtree effect and reduce Kreb’s cycle activity [33][34]. When the glucose levels in the media fall, the cells will activate the glycolytic pathway and the theKreb’s cycle so that they can hydrolyse glycogen and metabolise glucose efficiently in order to ‘survive’ until the next media change. It has been shown that VPA causes the inhibition of glycogen synthase kinase-3 (GSK-3) [25], [35] (in a manner analogous to lithium, another mood controlling compound [36]). This would be expected to lead to an increase in glycogen synthesis which in turn would to cause an increase in glucose uptake from the media. This effect was never seen in the studies presented here even at the lowest VPA doses investigated and therefore if VPA is acting through GSK-3 must be occurring via one of GSK-3’s other roles. Another possibility is via a signalling pathway [36]. GSK-3 is a part of the canonical β-catenin/Wnt pathway, which signals the cell to divide and proliferate [37] and thererfore inhibition of GSK-3 would be expected to slow these cellular activities. However, in spheroids, cells are growing very slowly and therefore GSK-3’s inhibition would be expected also to have a minimal effect and so once again, VPA treatment is probably not operating via this pathway. A more probable explanation is that VPA is a simple fatty acid and as such can be a substrate for the fatty acid b-oxidation pathway. This occurs primarily in the mitochondria and is known to result in a cumulative inhibitory effect on long-chain fatty acid b-oxidation [20] [38][39]. Simultaneous inhibition of the mitochondrial pyruvate uptake will impair the pyruvate- driven oxidative phosphorylation and together with the inhibition of 2- oxoglutarate dehydrogenase reduce ATP synthesis [11], [12], [18], [19], [20] and [40]. This is potentially the primal cause of both the observed reduction of 158 Stephen J. Fey and Krzysztof Wrzesinski glucose uptake and the fall in cellular ATP levels (at moderate or high VPA doses).

Acute and Chronic Lethal Thresholds

The treatment of spheroids revealed that there were two different ‘lethal thresholds’. At high VPA doses (20 mg VPA/mg TCP), the cells were killed essentially immediately by one dose and we defined this as the ‘acute lethal threshold’. Above this dose a single dose is sufficient to kill the cells whereas below this dose, the single dose is not sufficient to kill. A VPA dose five times lower than the acute lethal dose resulted in a fall of the ATP level to 75% of the control. At each subsequent dose, the ATP levels were supressed lower and lower and the cells would be expected to die if treatment was repeated a few times more. Exactly analogous results were obtained measuring glucose clearance from the media: the spheroids were increasingly inept following each successive treatment. Thus one can infer that the chronic lethal threshold dose will be about 4 mg VPA/mg TCP. Based on the ATP data, above this dose (but below 20 mg VPA/mg TCP) it would be anticipated that cultures will die if repeatedly treated with VPA, while below this dose they will not die. Lower doses (0.8 and 0.16 mg VPA/mg TCP) perturbed the ATP levels but each time the ATP levels recovered to essentially the same level as seen in the untreated controls. ATP levels above that seen in the controls were frequently measured: this is interpreted to indicate additional cellular activity to repair VPA-induced cellular damage (e.g. the replacement of VPA metabolite-cellular protein covalent adducts [41] or the repair of other damage done to the cell and its membranes [42]. Extrapolation of these data would suggest that these lower doses would be unable to kill the C3A spheroids irrespective of how often the cells were treated. From the data that we obtained, it appears that the acute lethal dose and a chronic lethal dose kill the cells by different mechanisms: in the acute situation, there is a sudden release of adenylate kinase; whereas in the chronic situation (at a VPA dose only 5 times lower), there is an apparent ‘repression’ of adenylate kinase release. The explanation of this is not clear. One possible explanation is that if cells are releasing adenlyate kinase immediately after chronic VPA treatment, then because the enzyme has a short half-life in solution (ca. 6 h) it cannot be detected when the assay is carried out 48hrs after treatment. Another explanation could be that the VPA treatment is merely Determination of Acute Lethal and Chronic Lethal Dose … 159 paralysing cellular activity, including adenylate kinase release, and not resulting in cell death. The lower than control adenylate kinase levels (‘repression’) could then be a reflection of the fact that the control cells are growing whereas the treated cells are presumably not.

Modelling the Treatment Regimen

Currently, the FDA official recommendation for the initiation of VPA treatment using sodium valproate injection is to start therapy at 10 to 15 mg/kg/day. The dosage should be increased by 5 to 10 mg/kg/week to achieve optimal clinical response [43]. Since the liver is almost the exclusive site for VPA metabolism, it could be expected that the apparent chronic ‘cellular paralysis’ (failing glucose import and falling ATP levels) induced by low levels of VPA treatment, might slow the metabolism and removal of VPA. Subsequent doses would then result in a positive feedback situation increasing the ‘effective dose’ because the liver’s capacity to respond is successively impaired. This situation therefore underlines the importance of slowly increasing the dose given to the patient.

Dose Frequency

In the model described here we have used a single constant dose given each 48 or 72hrs. Since the half-life of VPA is 10 – 20 h [44] one could argue that the VPA is given too infrequently. However, in vitro the VPA metabolites are not removed as they are in vivo by the kidneys and therefore the biological half life in vitro would be expected to be longer. 48hr treatment intervals were used to minimise disturbances to the spheroids in the bioreactor cultures. If VPA had been given more frequently, then one would expect that the chronic threshold value identified would be lower. The ideal in vitro situation would probably be a continuous perfusion supply and this would be expected to reveal the lowest threshold values. However this would not mimic the intermittent treatment effect when compounds are administered to patients. The spheroid system described here, because of its stability (extending over weeks) and the relative ease with which small samples can be removed without perturbing the system (i.e. changing the drug cell number ratio) could 160 Stephen J. Fey and Krzysztof Wrzesinski be used to provide in vitro data to match at least part of the theoretical simulations of mathematical models [45].

Conclusion

The 3D spheroid model, because of its homeostatic ability to recover following treatment and it stability over relatively long periods of time, offers an excellent model for studying long term effects of compounds like VPA on liver cells and for studying the effects of repeated insult leading for example to the development of drug-induced liver injury.

Funding Information

This work was supported by MC2 Biotek, Hørsholm, Denmark and by the University of Southern Denmark. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. The authors declare that there are no conflicts of interest.

Acknowledgments

The authors would like to thank Signe Marie Andersen, KiraEydJoensen and Jacob BastholmOlesen for expert technical support in growing and maintaining spheroid cultures and performing some of the assays.

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Reviewed by: Dr Cliff Rowe, Research associate, MRC Centre for Drug Safety Science, Department of Pharmacology and Therapeutics, Sherrington Building, Ashton Street, The University of Liverpool, L69 3GE, U.K., Tel: +44 (0)151 794 6615, Fax: +44 (0)151 794 5540.