doi:10.1152/ajpregu.91021.2008 297:960-967, 2009. First published Jul 22, 2009;Am J Physiol Regulatory Integrative Comp Physiol

Glatz, Joost J. F. P. Luiken, Mary-Ellen Harper and Arend Bonen Graham P. Holloway, Swati S. Jain, Veronic Bezaire, Xiao Xia Han, Jan F. C.

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, October 1, 2009; 297 (4): R1202-R1212. Am J Physiol Regulatory Integrative Comp PhysiolLuiken A. Bonen, G. P. Holloway, N. N. Tandon, X.-X. Han, J. McFarlan, J. F. C. Glatz and J. J. F. P.

fatty acid oxidation in lean and Zucker diabetic fatty ratsCardiac and skeletal muscle fatty acid transport and transporters and triacylglycerol and

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FAT/CD36-null mice reveal that mitochondrial FAT/CD36 is required toupregulate mitochondrial fatty acid oxidation in contracting muscle

Graham P. Holloway,1 Swati S. Jain,1 Veronic Bezaire,2 Xiao Xia Han,1 Jan F. C. Glatz,3

Joost J. F. P. Luiken,3 Mary-Ellen Harper,2 and Arend Bonen1

1Department of Human Health & Nutritional Sciences, University of Guelph, Guelph, Ontario, Canada; 2Department ofBiochemistry, Microbiology and Immunology, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada; and3Department of Molecular Genetics, Maastricht University, Maastricht, The Netherlands

Submitted 16 December 2008; accepted in final form 15 July 2009

Holloway GP, Jain SS, Bezaire V, Han XX, Glatz JF, LuikenJJ, Harper ME, Bonen A. FAT/CD36-null mice reveal that mito-chondrial FAT/CD36 is required to upregulate mitochondrial fattyacid oxidation in contracting muscle. Am J Physiol Regul Integr CompPhysiol 297: R960–R967, 2009. First published July 22, 2009;doi:10.1152/ajpregu.91021.2008.—The plasma membrane fatty acidtransport protein FAT/CD36 is also present at the mitochondria,where it may contribute to the regulation of fatty acid oxidation,although this has been challenged. Therefore, we have comparedenzyme activities and rates of mitochondrial palmitate oxidation inmuscles of wild-type (WT) and FAT/CD36 knockout (KO) mice, atrest and after muscle contraction. In WT and KO mice, carnitinepalmitoyltransferase-I, citrate synthase, and !-hydroxyacyl-CoA de-hydrogenase activities did not differ in subsarcolemmal (SS) andintermyofibrillar (IMF) mitochondria of WT and FAT/CD36 KOmice. Basal palmitate oxidation rates were lower (P " 0.05) in KOmice (SS #18%; IMF #13%). Muscle contraction increased fattyacid oxidation ($18%) and mitochondrial FAT/CD36 protein($16%) in WT IMF but not in WT SS, or in either mitochondrialsubpopulation in KO mice. This revealed that the difference in IMFmitochondrial fatty acid oxidation between WT and KO mice can beincreased %2.5-fold from 13% under basal conditions to 35% duringmuscle contraction. The FAT/CD36 inhibitor sulfo-N-succinimidyloleate (SSO), inhibited palmitate transport across the plasma mem-brane in WT, but not in KO mice. In contrast, SSO bound tomitochondrial membranes and reduced palmitate oxidation rates to asimilar extent in both WT and KO mitochondria (%80%; P " 0.05).In addition, SSO reduced state III respiration with succinate as asubstrate, without altering mitochondrial coupling (P/O ratios). Thus,while SSO inhibits FAT/CD36-mediated palmitate transport at theplasma membrane, SSO has undefined effects on mitochondria. Nev-ertheless, the KO animals reveal that FAT/CD36 contributes to theregulation of mitochondrial fatty acid oxidation, which is especiallyimportant for meeting the increased metabolic demands during musclecontraction.

subsarcolemmal; intermyofibrillar; muscle contraction

THE REGULATION OF FATTY ACID oxidation at the level of mito-chondria has long been attributed to the activity of the malonyl-CoA (M-CoA) carnitine palmitoyltransferase-I (CPTI) axis(32, 33). However, the IC50 of CPTI for M-CoA is belowphysiological concentrations of this biological inhibitor (22,32), and reductions in muscle M-CoA concentrations duringexercise in humans cannot account for the observed increase infatty acid oxidation (24, 25, 28). Collectively, these studieshave begun to suggest there are likely additional proteins that

contribute to the regulation of mitochondrial fatty acid oxida-tion.

Recently, fatty acid translocase (FAT)/CD36, a plasmamembrane fatty acid transport protein, has been found onskeletal muscle mitochondrial membranes (4, 10). Administra-tion of sulfo-N-succinimidyl oleate (SSO), a putative specificinhibitor of FAT/CD36, decreased fatty acid oxidation rates(%80%) in isolated mitochondria (4, 10), implicating FAT/CD36 in the regulation of mitochondrial fatty acid oxidation.Further support that FAT/CD36 regulates mitochondrial fattyacid oxidation comes from a number of other observations,including the following: 1) FAT/CD36 coimmunoprecipateswith CPTI (10, 29, 30), and 2) there is a concomitant increasein mitochondrial fatty acid oxidation and the amount ofmitochondrial FAT/CD36 protein in exercising muscle (10,17), such that mitochondrial FAT/CD36 is positively corre-lated with mitochondrial fatty acid oxidation rates duringexercise (17).

A recent report (18) has challenged the interpretation thatFAT/CD36 has a role in mitochondrial fatty acid oxidation,since 1) there were no differences in mitochondrial fatty acidoxidation in wild-type (WT) and FAT/CD36 null (KO) mice,and 2) the reactive ester SSO inhibited mitochondrial fatty acidoxidation to a similar extent in both WT and FAT/CD36 KOmice (18). However, this conclusion may not be warranted,since these authors did find a %25% reduction in mitochondrialfatty acid oxidation in KO mice (18). Failure to detect statis-tical significance in this 25% reduction appeared to be relatedto the low statistical power in some aspects of that study (18).Moreover, the metabolic challenge studies were conducted inmitochondria obtained from resting muscle (18), in whichmitochondrial FAT/CD36 content was kept static in WT mice.This likely limited the differences that could be observed in therespiration rates between WT and KO mice, as we (10, 17) andothers (29), have suggested that an increase in mitochondrialFAT/CD36 is essential for upregulating the rates of mitochon-drial fatty acid oxidation. Thus, there is controversy as towhether FAT/CD36 is central to increasing mitochondrial fattyoxidation, particularly during muscle contraction.

In the present study, we have examined mitochondrial fattyacid oxidation in relation to FAT/CD36. Specifically, we have1) determined whether FAT/CD36 KO mice have a reducedrate of palmitate oxidation in isolated subsarcolemmal (SS) andintermyofibrillar (IMF) mitochondria, 2) examined the speci-ficity of SSO for FAT/CD36, and 3) determined whether ametabolic challenge (muscle contraction) differentially altersmitochondrial fatty acid oxidation in WT and FAT/CD36 KOanimals.

Address for reprint requests and other correspondence: G. Holloway, Hu-man Health & Nutritional Sciences, Univ. of Guelph, Guelph, Canada (e-mail:ghollowa@uoguelph.ca).

Am J Physiol Regul Integr Comp Physiol 297: R960–R967, 2009.First published July 22, 2009; doi:10.1152/ajpregu.91021.2008.

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METHODS

Animals. FAT/CD36 KO mice were obtained from Dr. MariaFebbraio (Cleveland Clinic, Cleveland OH) (14). Breeding of WT andKO mice was conducted on site at the University of Guelph. Age-matched (%8 wk) female WT (n & 40, weighing 22.4 ' 0.8 g) andKO (n & 40, weighing 23.8 ' 0.9 g) mice were used in this study.Animals were housed in a climate- and temperature-controlled room,on a 12:12-h light-dark cycle, with standard chow and water providedad libitum. This study was approved by the University of GuelphAnimal Care Committee.

Genotyping. Genotyping was confirmed using standard reversetranscription methods as outlined by Qiagen (DNAse easy kit) using thefollowing primer sets; WT forward, 5(-CAG CTC ATA CAT TGCTGT TTA TGC ATG-3(; reverse, 5(-GGT ACA ATC ACA GTG TTTTCT ACG TGG-3(; KO forward, 5(-CAG CTC ATA CAT TGC TGTTTA TGC ATG-3(; and reverse, 5(-CCG CTT CCT CGT GCT TTACGG TAT C-3( (targeted to the PGKneo cassette).

Giant vesicles and fatty acid transport. Giant vesicles were pre-pared from murine hindlimb muscles as previously described (8, 9, 20,21). Briefly, muscle tissues were cut into thin layers (1–3 mm thick)and incubated for 1 h at 34°C in 140 mM KCl-10 mM MOPS (pH7.4), aprotinin (30 )g/ml), and collagenase type VII (150 U/ml)(Sigma-Aldrich, St. Louis, MO) in a shaking water bath. At the end ofthe incubation, the supernatant fraction was collected, and the remain-ing tissue was washed with KCl-MOPS and 10 mM EDTA (Sigma-Aldrich), which resulted in a second supernatant fraction. Both su-pernatant fractions were pooled, and Percoll (Sigma-Aldrich), KCl,and aprotinin were added to final concentrations of 3.5% (vol/vol), 28mM and 10 )g/ml, respectively. The resulting suspension was placedat the bottom of a density gradient consisting of a 3-ml middle layerof 4% Nycodenz (wt/vol) (Sigma-Aldrich) and a 1-ml KCl-MOPSupper layer. This sample was centrifuged at 60 g for 45 min at roomtemperature. Subsequently, the vesicles were harvested from theinterface of the upper and middle layers, diluted in KCl- MOPS, andrecentrifuged at 12,000 g for 5 min.

Giant vesicles from WT and KO animals were incubated for 15 minin the presence of 200 )M SSO [dissolved in dimethylsulfoxide(DMSO)], or in the same volume of DMSO (control purposes).Following this preincubation, vesicles were washed to remove excessSSO/DMSO, and palmitate transport rates were measured as de-scribed above (11–13). Briefly, 40 )l of 0.1% BSA in KCl-MOPS,containing unlabeled (15 )M) and radiolabeled 0.3 )Ci [3H]-palmi-tate, and 0.06 )Ci [14C]-mannitol, were added to 40 )l of vesiclesuspension. The incubation was carried out for 15 s. Palmitate uptakewas terminated by the addition of 1.4 ml of ice-cold KCl-MOPS, 2.5mM HgCl2, and 0.1% BSA. The sample was then quickly centrifuged,and the supernatant fraction was discarded. Thereafter, radioactivitywas determined in the remaining pellet. Nonspecific uptake wasmeasured by adding the stop solution before the addition of theradiolabeled palmitate solution.

Isolation of mitochondria from skeletal muscle. Differential cen-trifugation was used to obtain both SS and IMF mitochondrial frac-tions (2, 5, 10) from hindlimb muscles. All procedures were identicalto those previously published by our group (4, 10, 17). Briefly, musclewas homogenized and centrifuged at 800 g for 10 min to separate theSS and IMF mitochondria. The IMF mitochondria were treated witha protease (Subtilisin A, 0.25 )g protease/mg starting wet weightmuscle) (Sigma-Aldrich) for exactly 5 min to digest the myofibrils.Further centrifugation was used to remove the myofibrils, and mito-chondria were finally recovered by centrifuging twice at 10,000 g for10 min.

Mitochondrial palmitate oxidation. Labeled CO2 production frompalmitate oxidation and acid-soluble trapped 14C was measured in asealed system, as we have previously described (4, 17). Briefly,mitochondria (100 )l) were added to a system containing a pregassedmodified Krebs Ringer buffer, and the reaction was initiated by a 6:1

palmitate:BSA complex (containing 150 nCi of [1-14C] palmitate, anda final palmitate concentration of 75 )M). The reaction continued for30 min at 37°C and was terminated with the addition of ice-cold 12 Nperchloric acid.

A fraction of the reaction medium was removed and analyzed forisotopic fixation, while gaseous CO2 produced from oxidation of[1-14C] palmitate was measured by acidifying the remaining reactionmixture. Liberated 14CO2 was trapped by benzethonium hydroxideover a 90-min incubation period at room temperature, and the radio-activity was determined.

Inhibition studies were performed by preincubating mitochondriawith 200 )M SSO dissolved in DMSO for 15 min. Following thepreincubation, mitochondria were washed twice to remove SSO/DMSO before being resuspended in their original volume. For controlpurposes the same volume (1 )l) of DMSO (final concentration 0.1%)was added to mitochondria that were not supplemented with SSO.

Mitochondrial oxygen consumption measurements. Oxygen con-sumption was measured in isolated mitochondria (0.2 mg/ml) at 37°Cusing a Clark-type oxygen electrode (Hansatech, Norfolk, UK) andincubated in standard incubation medium (IM: 120 mM KCl, 1 mMEGTA, 5 mM KH2PO4, 5 mM MgCl2, and 5 mM HEPES; pH 7.4)containing 0.3% defatted BSA and assumed to contain 406 nmolO2/ml at 37°C (27). State III (maximum phosphorylating) respirationwas determined with 100 )M ADP, using the following substrates: 10mM succinate $ 5 )M rotenone, or 40 )M palmitoylcarnitine $ 0.5mM malate. State IV (nonphosphorylating) respiration was deter-mined following the addition of oligomycin (12 )g/mg protein).

Submitochondrial distribution of SSO. To determine the submito-chondrial location of SSO, isolated mitochondria were preincubatedfor 15 min with radiolabeled SSO (200 )M, 130 nCi [3H]-SSO) andwashed twice to remove SSO. Thereafter, mitochondria were lysed byrepeated freeze thawing, and repelleted using centrifugation (10,000g * 10 min). The subsequent supernatant fraction was removed, andthe radioactivity was determined in both the supernatant fraction(representing SSO within the mitochondrial matrix) and the pellet(representing SSO bound to mitochondrial membranes).

Mitochondrial enzymatic activities. Isolated SS and IMF mitochon-dria were used to determine the activities of citrate synthase (CS) and!-hydroxyacyl-CoA dehydrogenase (!-HAD). CS activity was as-sayed spectrophotometrically at 37°C at 412 nm (31), and !-HADactivity was measured at 340 nm (37°C) (3), after lysing the mito-chondria with 0.04% Triton X-100 and repeated freeze-thawing. Theforward radioisotope assay was used for the determination of CPTIactivity, as described by McGarry et al. (22) with minor modifications,as we have previously reported (1, 2). Briefly, the assay was con-ducted at 37°C and initiated by the addition of mitochondrial standardreaction medium containing 75 )M P-CoA (L-[3H]carnitine Amer-sham Bioscience, Buckinghamshire, England). The reaction wasstopped after 6 min with the addition of ice-cold HCl. Palmitoyl-[3H]carnitine was extracted in water-saturated butanol in a process involv-ing three washes with distilled water and subsequent recentrifugationsteps to separate the butanol phase, in which the radioactivity wascounted. Data were normalized to mitochondrial protein.

Muscle contraction and mitochondrial fatty acid oxidation. Ratesof mitochondrial fatty acid oxidation and contents of mitochondrialFAT/CD36 were compared in resting and contracting muscles fromWT and FAT/CD36 KO mice. Following anesthetization (intraperi-toneal injections of pentobarbital sodium, 6 mg/100 g body wt; MTCPharmaceuticals, Cambridge, ON, Canada), the femoral artery of thecontrol limb was ligated, and the resting hindlimb muscles wereexcised. For muscle contraction studies, the sciatic nerve of thecontralateral limb was exposed, and stimulating electrodes were placedaround the nerve. Electrical stimulation was applied for three repetitionsof 5 min (train delivery, 100 Hz/3 s at 6–8 V; train duration, 200 ms;pulse duration 10 ms), with 2 min of recovery between stimulationbouts. Following stimulation, the hindlimb muscles were excised.

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Because of tissue limitations, muscles from two animals were pooledprior to isolating mitochondria.

Western blot analysis. Three WT and KO animals were used toconfirm contraction-induced increases in FAT/CD36. These sampleswere analyzed for total protein (BCA protein assay), and 5 )g ofdenatured isolated mitochondrial protein were used for Western blotanalysis. All samples were separated by electrophoresis on SDS-polyacrylamide gels and transferred to polyvinylidene difluoridemembranes. Commercially available antibodies were used to detectcytochrome c oxidase complex IV (COXIV; Invitrogen, Burlington,ON, Canada) and FAT/CD36 (Santa Cruz Biotechnology, Santa Cruz,CA). Blots were quantified using chemiluminescence and the Chemi-Genius 2 Bioimaging system (SynGene, Cambridge, UK).

Statistics. All data are presented as the mean ' SE. Two-wayANOVA was used, and when significance was obtained, a Fisher’sLSD post hoc analysis was employed. Statistical significance wasaccepted at P " 0.05.

RESULTS

WT and KO animals were not different in mean age orweight (P + 0.05). Genotyping confirmed that homozygousWT and FAT/CD36 KO mice were used in this study.

Isolated mitochondrial enzymatic activities. Enzymatic ac-tivities of mitochondrial CPTI, CS, or !HAD did not differ inWT and KO mice (Fig. 1, A–C). Compared with SS mitochon-dria, IMF mitochondria displayed higher rates of CPTI(%70%; P " 0.05, Fig. 1A), CS (%35%), and !HAD (%80%)activities (P " 0.05, Fig. 1, B and C, respectively) in both WTand KO mice.

Palmitate oxidation and FAT/CD36 content in isolated mi-tochondria at rest and after muscle contraction. Under basalconditions, KO animals had lower (P " 0.05) rates of palmitateoxidation in both SS (#18%) and IMF (#13%) mitochondria(Fig. 2). Compared with SS mitochondria, palmitate oxidationrates were higher (P " 0.05) in IMF mitochondria (WT $55%;KO $65%) (Fig. 2).

In a separate group of animals, we compared the effects ofmuscle contraction on mitochondrial fatty acid oxidation. Inthese animals, we again observed a lower rate of fatty acidoxidation in control muscle SS and IMF mitochondria ofFAT/CD36 KO mice (Fig. 3). Muscle contraction increasedpalmitate oxidation in IMF, but not in SS mitochondria, of WTmice ($18%). In contrast, muscle contraction failed to alterpalmitate oxidation in either SS or IMF mitochondria in KOmice (Fig. 3). Hence, the difference in palmitate oxidationbetween WT and KO animals was amplified %2.5-fold ($35%) inIMF mitochondria, when presented with a metabolic challenge(i.e., muscle contraction). FAT/CD36 protein was not present inKO animals, and under basal conditions, WT SS mitochondriacontained more (P " 0.05) FAT/CD36 than WT IMF mito-chondria (Fig. 3). In WT animals, muscle contraction did notalter FAT/CD36 content in SS mitochondria, whereas theamount of FAT/CD36 in IMF mitochondria increased follow-ing electrical stimulation ($16%, P " 0.05; Fig. 3).

Effects of SSO on fatty acid transport into giant vesicles andon mitochondrial oxidation. As expected, the basal rate ofpalmitate transport into giant vesicles was lower in FAT/CD36KO mice compared with WT animals (#14%, P " 0.05, Fig. 4).SSO inhibited palmitate transport in WT animals (#33%,P " 0.05), but as expected, this inhibitor had no effect onpalmitate transport in FAT/CD36 KO mice (Figs. 4 and 5A).In contrast, SSO inhibited palmitate oxidation by %80% in

SS and IMF mitochondria in both WT and KO mice (P "0.05, Fig. 5B).

To examine whether SSO altered mitochondrial respiration,oxygen consumption was measured in isolated mitochondria.SSO reduced state III respiration when both succinate andpalmitoylcarnitine were used as substrates (Table 1). However,SSO did not alter P/O ratios with either substrate (Table 1).Experiments with radiolabeled SSO showed that in both WTand KO mice, only minimal SSO (%1.5%) was present withinthe mitochondrial matrix (3.9 ' 0.6 nmol/mg mitochondrial

Fig. 1. Enzymatic activities of carnitine palmitoyltransferase I (CPTI) (A),citrate synthase (CS) (B), and !-hydroxyacyl-CoA dehydrogenase (!-HAD) (C) in subsarcolemmal (SS) and intermyofibrillar (IMF) mitochon-dria in wild-type (WT) and FAT/CD36 knockout (KO) mice. Data areexpressed as the means ' SE; n & 4 for each of CPTI, !-HAD, and CS inSS and IMF mitochondria. †Significantly different (P " 0.05) from SSmitochondria.

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protein), while the vast quantity of SSO (%98.5%) was boundto mitochondrial membranes (290 ' 24, nmol/mg mitochon-drial protein; P " 0.05).

DISCUSSION

The present studies have shown that FAT/CD36 contributesto the regulation of mitochondrial fatty acid oxidation, both atrest and when challenged metabolically. Specifically, we havefound the following: 1) basal rates of palmitate oxidation arelower in isolated SS and IMF mitochondria of FAT/CD36 KO

mice, and 2) muscle contraction increased mitochondrial fattyacid oxidation in WT mice, whereas this stimulus failed toupregulate mitochondrial fatty acid oxidation in FAT/CD36KO mice. As expected, 3) we confirmed that SSO inhibitsFAT/CD36-mediated fatty acid transport across the plasmamembrane, but 4) in marked contrast, in mitochondria, SSOinhibited mitochondrial fatty acid oxidation and respiration inan unknown manner.

Fig. 2. Palmitate oxidation rates in SS and IMF mitochondria in WT andFAT/CD36 KO mice. Data are expressed as the means ' SE; n & 13independent experiments for WT and KO mice. To obtain sufficient tissue,muscles from two mice were pooled for each independent experiment. *Sig-nificantly different (P " 0.05) different from WT. †Significantly different(P " 0.05) from SS mitochondria.

Fig. 3. The effect of muscle contraction on palmitate oxidation rates and FAT/CD36 protein (C) in SS and IMF mitochondria in WT (A) and FAT/CD36 KO(B) mice. Data are expressed as means ' SE. In A and B, n & 5 independent experiments for WT and KO. To obtain sufficient tissue, muscles from two micewere pooled for each independent experiment. *Significantly different (P " 0.05) from control. †Significantly different (P " 0.05) from SS mitochondria.

Fig. 4. Fatty acid transport into giant sarcolemmal vesicles under basalconditions in WT and FAT/CD36 KO mice in the absence and presence ofSSO. Data are expressed as the means ' SE; n & 4 independent experimentsfor WT and KO. To obtain sufficient tissue, muscles from two mice werepooled for each independent experiment. SSO concentration, 200 )M. *Sig-nificantly different (P " 0.05) from control. †Significantly different (P " 0.05)from WT.

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Basal rates of fatty acid oxidation are reduced in FAT/CD36KO mice. A key finding in the present study is that basal ratesof palmitate oxidation are reduced in SS and IMF mitochondriaof FAT/CD36 KO mice. We have confirmed this finding in thecurrent study, as basal rates of palmitate oxidation were alsoreduced in both SS (#15%) and IMF (#15%) mitochondria ina separate group of animals used for the muscle contractionexperiments. These data in muscles of FAT/CD36 mice pro-vide direct evidence that FAT/CD36 participates in the regu-lation of mitochondrial fatty acid oxidation in this tissue. Thereduction in basal mitochondrial palmitate oxidation in FAT/CD36 KO mice observed in the current study (%15%) wasslightly less than that previously reported at the whole musclelevel (#26%) (7). This greater difference in whole muscle

oxidation likely reflects the ability of FAT/CD36 to affect bothplasma membrane fatty acid transport and mitochondrial fattyacid oxidation rates (see Figs. 2 and 4). The reduction in basalmitochondrial fatty acid oxidation in the current study is also,however, slightly less than previously reported with isolatedmitochondria (%25%) (18). The degree of attenuation in fattyacid oxidation in mitochondria isolated from FAT/CD36 KOmay reflect sex differences, as only female animals were usedin the current study, whereas King et al. (18) used male mice.However, while King et al. (18) concluded that FAT/CD36 isnot important in regulating mitochondrial fatty acid oxidation(18), we interpret their results to indicate that there likely wasa reduction in mitochondrial fatty acid oxidation in FAT/CD36KO mice. Specifically, examination of their study (18) indi-cated that there was a large variance in some of their data.From this, we calculated that the statistical power was at timesquite low and seemed to be insufficient to conclude thatmitochondrial fatty acid oxidation is not impaired in FAT/CD36 KO mice. We have been careful to ensure that statisticalpower in our oxidation data was sufficiently robust to detectdifferences in fatty acid oxidation between WT and FAT/CD36KO mice. Taken altogether, we conclude that the small reduc-tions in the basal rates of mitochondrial fatty acid oxidation inFAT/CD36 KO mice in the present study, and likely in thestudy by King et al. (18), suggest that mitochondrial FAT/CD36 has perhaps a limited role in regulating fatty acidoxidation rates under basal conditions.

Because the basal rates of plasma membrane palmitatetransport and mitochondrial oxidation are both reduced by%15% in FAT/CD36 KO animals, it would appear that FAT/CD36 has only a minor role in regulating these processes underbasal conditions. Compensatory upregulation by other proteinsmay be counteracting the effect of ablating FAT/CD36. In thecurrent study, we have not observed changes in the activities ofselected enzymes involved in fatty acid oxidation, namelyCPTI, CS, or !HAD. However, it is possible that compensa-tory changes in fatty acid transport protein 1 (FATP1) mini-mized the effect of ablating FAT/CD36. We have previouslyshown that both FATP1 and FATP4 are increased in the soleusmuscle of these animals (7), which may mask, in part, thecontribution of FAT/CD36 to basal plasma membrane fattyacid transport. Similar logic may hold true at the mitochondria,as we have detected a %30% increase in FATP1 in both SS andIMF mitochondria isolated from FAT/CD36 KO animals (Hol-loway GP and Bonen A, unpublished data). It is currently notknown what role FATP1 may play in regulating mitochondrialfatty acid oxidation in mature mammalian muscle. We havepreviously shown that acute overexpression of FATP1 canincrease whole muscle fatty acid oxidation rates (23); however,it is unclear whether this occurs exclusively in response to an

Fig. 5. Relative inhibition (%) of fatty acid transport (A) and mitochondrialfatty acid oxidation (B) induced by SSO in WT and FAT/CD36 KO mice. Dataare expressed as means ' SE; n & 4 or 5 independent experiments for WT andKO. To obtain sufficient tissue, muscles from two mice were pooled for eachindependent experiment. SSO concentration, 200 )M. *Significantly different(P " 0.05) from WT.

Table 1. The effect of SSO on oxygen consumption in isolated mitochondria

Substrate GroupState III, nmol

O2 ! mg#1 ! min#1State IV, nmol

O2 ! mg#1 ! min#1 RCR P/O

40 )M Palmitoylcarnitine $0.5 )M Malate Control 100.5'6.4 23.1'1.9 4.4'0.4 2.98'0.16SSO 41.5'9.5* 16.5'0.4* 2.5'0.6 2.69'0.03

10 )M Succinate $5 )M Rotenone Control 281.9'15.3 117.5'7.1 2.5'0.2 1.44'0.11SSO 186.9'18.7* 140.5'11.4 1.4'0.1* 1.38'0.14

Values are presented as means ' SE (n & 3 or 4). SSO, sulfo-N-succinimidyl oleate; RCR, respiratory control ratio; P/O, xxx. *Significantly different (P "0.05) from DMSO control experiment.

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increase in plasma membrane fatty acid transport, orwhether FATP1 also contributes at the level of the mito-chondria in a similar fashion to FAT/CD36. A recent elo-quent series of studies by Sebastian and colleagues (30) hasshown that CPTI, FAT/CD36, and FATP1 coimmunopre-cipitate, and overexpression of these proteins in L6E9 myo-tubes increase rates of palmitate oxidation. These datasuggest that compensatory changes in mitochondrial FATP1may attenuate the response of ablating mitochondrial FAT/CD36. Nevertheless, both gain-of-function (30) and loss-of-function (current data) studies have now suggested thatFAT/CD36 contributes to mitochondrial fatty acid oxida-tion.

Muscle contraction fails to increase mitochondrial fattyacid oxidation in FAT/CD36 KO mice. In the present study,we have demonstrated that the contraction-induced upregu-lation of mitochondrial fatty acid oxidation requires thepresence of mitochondrial FAT/CD36, because in the ab-sence of mitochondrial FAT/CD36, there was no increase incontraction-induced fatty acid oxidation. This is fully con-sistent with the conclusions drawn in our previous studies(10, 17). Consequently, with muscle contraction, the differ-ences between WT and KO IMF mitochondrial palmitateoxidation rates were amplified further ($35% in WT) be-yond those observed under basal conditions ($13% in WT).The present results also parallel differences in a previousreport in WT and FAT/CD36 KO mice at the whole musclelevel, in which basal palmitate oxidation rates were slightlyhigher in WT mice ($26%), while AICAR stimulated fattyacid oxidation much more in muscles of WT mice than inFAT/CD36 KO mice (7). Taken altogether, the present studyand others (10, 17, 29) strongly support the notion thatmitochondrial FAT/CD36 is required to upregulate, at leasta portion of, mitochondrial fatty acid oxidation in responseto metabolic challenges such as AICAR and muscle con-traction. In support of this interpretation, WT IMF mito-chondrial FAT/CD36 protein content increased ($16%)proportionately with the increase in WT IMF fatty acidoxidation ($18%), while neither FAT/CD36 protein norpalmitate oxidation rates were altered in WT SS mitochon-dria.

It is perhaps not surprising that King and colleagues (18)have previously concluded that FAT/CD36 is not important inthe regulation of mitochondrial fatty acid oxidation. All of theirmeasurements were obtained under conditions in which mito-chondrial FAT/CD36 remained static in WT animals during ametabolic challenge (state III respiration). This presumablylimited the magnitude of the differences in fatty acid oxidationthat could be achieved between WT and KO animals. How-ever, in our studies (10, 17) and others (29), an increase in fattyacid oxidation has always been associated with a concurrentincrease in mitochondrial FAT/CD36. Thus, an increase inmitochondrial FAT/CD36 appears to be central for upregulat-ing mitochondrial fatty acid oxidation, given 1) that there is apositive relationship between mitochondrial FAT/CD36 andmitochondrial fatty acid oxidation during exercise (r & 0.63)(17), and 2) that with muscle contraction mitochondria com-pletely failed to upregulate mitochondrial fatty acid oxidationin FAT/CD36 KO mice (present study).

Inhibition studies with SSO in isolated mitochondria. Previ-ous inhibition studies using SSO in human (4, 17) and rat (10)

skeletal muscle suggested a role for FAT/CD36 in regulatingmitochondrial fatty acid oxidation. Essential to this logic wasthe notion that SSO was specific to FAT/CD36. In the currentstudy, we have shown that SSO does not affect plasma mem-brane fatty acid transport in FAT/CD36 KO animals, indicatingSSO specifically blocks FAT/CD36 at the plasma membrane.This confirms a previous study in the hearts of FAT/CD36 KOmice (15) and other reports showing that 3H-SSO specificallybinds to FAT/CD36 at the plasma membrane (26). Despite this,we have confirmed a recent report (18), which found that SSOreduces palmitolycarnitine oxidation rates similarly in mito-chondria isolated from either WT or FAT/CD36 KO mice. Wehave also observed that SSO alters mitochondrial respirationrates without altering P/O ratios. These data suggest that SSOdoes not uncouple the mitochondria (i.e., unaltered P/O ratios,with either palmitoylcarnitine or succinate). While it is unclearhow SSO is affecting mitochondria, the current data showingthat 1) SSO binds to mitochondrial membranes and 2) reducesstate III respiration with succinate suggest that the inhibitoryeffect of SSO may be occurring at the level of the electrontransport chain.

While the present study suggests that FAT/CD36 partici-pates in mitochondrial fatty acid oxidation, the modest reduc-tion observed in the KO animals suggests it is likely not theonly regulator of overall mitochondrial fatty acid oxidation,particularly as CPTI activity is seen to be unaltered. A numberof other observations also support this proposition, as 1) thecontent of FAT/CD36 under basal conditions is higher in theSS mitochondria and the fatty acid oxidation rates in SSmitochondria are lower; 2) with aerobic training in rodentsthere is a disproportionate increase in SS mitochondrial palmi-tate oxidation (twofold) rates compared with changes in CPTIactivity (%50%) (19), while in IMF mitochondria, fatty acidoxidation rates increased without alterations in CPTI activity(19); 3) in some of our other work, mitochondrial CPTI activityand FAT/CD36 combined, but not independently, provided astrong prediction of mitochondrial fatty acid oxidation [r &0.90, multiple regression, (4)], and 4) an increase in FAT/CD36 that coimmunoprecipitated with CPTI was stronglycorrelated with the concurrent increase in whole body fatoxidation (r & 0.93) (29). Lastly, 5) FATP1 in L6E9 myotubeshas been shown to have a collaborative effect with CPTI inregulating fatty acid oxidation rates (30). Collectively, theseforegoing observations, in agreement with results from thepresent study, suggest strongly that a complex of proteins at themitochondrial membrane appears to regulate mitochondrialfatty acid oxidation. These proteins remain to be fully identi-fied.

Perspectives and Significance

The current study provides evidence that FAT/CD36participates in regulating fatty acid oxidation at the level ofmitochondria. We propose that the mobilization of FAT/CD36 from an intracellular depot to the mitochondrialmembrane may be particularly important for increasingmitochondrial fatty acid oxidation during exercise. Addi-tionally, we have previously suggested that FAT/CD36 islocated distal to CPTI as a direct result of the observationthat SSO inhibited palmitoylcarnitine oxidation withoutaltering CPTI activity (4, 17). However, if SSO directly

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affects the electron transport chain as we have currentlyhypothesized (reduced state III respiration with succinatewithout altering P/O ratios), in the presence of SSO, oxida-tion of all substrates would be impaired without compro-mising CPTI activity. Therefore, the exact location of FAT/CD36 on mitochondrial membranes must be revisited. Wespeculate that FAT/CD36 is located on the outer mitochon-drial membrane, proximal to CPTI. FAT/CD36 has a largecytoplasmic loop (16), and in this location could increase therate of substrate delivery to either acyl-CoA synthetase (palmitate)or CPTI (palmitoyl-CoA). This is similar to the speculated role ofFAT/CD36 on the plasma membrane, where the increased deliv-ery of fatty acids to the sarcolemma, increases the rate ofmembrane insertion of fatty acids and ’flip-flop’ across themembrane (reviewed in Ref. 6). Regardless of the exact mech-anism of action at the level of the mitochondria, evidence ismounting to suggest that during exercise FAT/CD36 representsa highly regulated protein with the unique capability of increas-ing both substrate delivery into the muscle cell via sarcolem-mal fatty acid transport and mitochondrial fatty acid oxidationin an as yet unknown manner.

GRANTS

This work was funded by the Canadian Institutes of Health Research (to A.Bonen and M.-E. Harper) and the Natural Sciences and Engineering ResearchCouncil of Canada (to A. Bonen and M.-E. Harper); the Netherlands HeartFoundation Grant 2002.T049, the Netherlands Organization for Health Re-search and Development (NWO-ZonMw grant 40–00812-98–03075), and theEuropean Commission (Integrated Project LSHM-CT-2004–005272, Exgen-esis). J. Luiken was the recipient of a VIDI-Innovational Research Grant fromthe Netherlands Organization of Scientific Research (NWO-ZonMw Grant016.036.305). J. F. C. Glatz is the Netherlands Heart Foundation Professor ofCardiac Metabolism. A. Bonen is the Canada Research Chair in Metabolismand Health.

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