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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Hepatology. Author manuscript; available in PMC Sep 1, 2011.
Published in final edited form as:
PMCID: PMC2932863
NIHMSID: NIHMS214401

Pnpla3/adiponutrin deficiency in mice is not associated with fatty liver disease

Abstract

PNPLA3 (Adiponutrin), a novel patatin-like phospholipase domain-containing enzyme, is expressed at high level in fat, but also in other tissues including liver. Polymorphisms in PNPLA3 have been linked to obesity and insulin sensitivity. Notably, a nonsynonymous variant rs738409(G) allele of the PNPLA3 gene was found to be strongly associated with both non-alcoholic and alcoholic fatty liver disease. We have generated Pnpla3−/− mice by gene targeting. Loss of Pnpla3 has no effect on body weight or composition, adipose mass or development, whether the mice were fed regular chow or high-fat diet or bred into Lepob/ob background. Plasma and liver triglyceride content and plasma aspartate aminotransferase and alanine aminotransferase levels were not different between Pnpla3+/+ and Pnpla3−/− mice while they were on regular chow, fed three different fatty liver-inducing diets, or after they were bred into Lepob/ob background. Hepatic Pnpla5 mRNA levels were similar in wild-type and Pnpla3−/− mice, though adipose Pnpla5 mRNA level was increased in Pnpla3−/− mice. A high sucrose lipogenic diet stimulated hepatic Pnpla3 and Pnpla5 mRNA levels to a similar degree, but it did not affect adipose or liver triglyceride lipase (ATGL, aka Pnpla2) mRNA in Pnpla3+/+ and Pnpla3−/− mice. Finally, Pnpla3+/+ and Pnpla3−/− mice displayed similar glucose tolerance and insulin tolerance tests while on regular chow or three different fatty liver-inducing diets. Conclusion: Loss of Pnpla3 does not cause fatty liver, liver enzyme elevation, or insulin resistance in mice.

Keywords: Pnpla3 deficiency, fatty liver disease, insulin resistance

INTRODUCTORY STATEMENT

Fatty liver disease (FLD), especially nonalcoholic fatty liver disease (NAFLD), is characterized by excessive triacylglycerol (TG) accumulation in the liver, and is the most common form of liver disease in western society (1, 2). While alcohol abuse, obesity, insulin resistance and diabetes are known to be associated with FLD, genetic factors causing predisposition to hepatic steatosis are not well understood. Recently, several genome-wide association studies (GWAS) have identified a common genetic variant (rs738409 G allele) in the PNPLA3 gene to be strongly associated with increased liver fat content and susceptibility to nonalcoholic fatty liver disease (NAFLD) (35). A subsequent study extends the association of this single nucleotide polymorphism (SNP) also to alcoholic liver disease (6). PNPLA3 was recently identified as one of the genetic loci influencing plasma levels of liver enzymes (7), and the rs738409 G allele was also associated with elevated serum levels of aspartate aminotransferase (ALT) and alanine aminotransferase (AST) (3, 5). A recent biochemical study showed that this nonsynonymous SNP is associated with an inactive PNPLA3, suggesting a possible direct relation between loss of function of PNPLA3 and an increase in hepatic TG content and inflammation (8).

PNPLA3, also called adiponutrin, belongs to a novel class of patatin-like phospholipase (PNPLA) domain family proteins that also contain a conserved lipase catalytic dyad (Gly-X-Ser-X-Gly and Asp-X-Gly/Ala) (9). Like other PNPLA family proteins such as PNPLA2 (also called adipose triglyceride lipase (ATGL)) and PNPLA5, PNPLA3 possesses both lipase and transacylase activities in vitro (10, 11). Its expression is highest in fat, though it also occurs in other tissues, including liver, in mice, and is markedly upregulated during adipogenesis (12). In both rodents and humans adipose PNPLA3 expression is highly upregulated by glucose and insulin but free fatty acid (FFA) and high fat diet produce no effect (1316). The regulation of PNPLA3 by insulin and glucose is reminiscent of the control of lipogenic enzymes, such as fatty acid synthase and acetyl-CoA reductase (17), and is very different from that of lipolytic enzymes like ATGL (18). Genetic linkage analyses and clinical studies demonstrated that high adipose PNPLA3 expression occurs with obesity though not all reports are consistent (1315, 19). Indeed, Pnpla3 mRNA levels are increased in fa/fa obese Zucker rats that lack leptin receptor (12).

To determine the role of PNPLA3 in fatty liver disease and obesity, we have generated Pnpla3-knockout mice by gene targeting. Loss of Pnpla3 did not affect the liver TG content or serum AST or ALT levels whether the mice were fed normal chow or three different fatty liver-inducing diets, or after they were bred into a genetic obese Lepob/ob background. Furthermore, Pnpla3−/− mice displayed normal body fat and composition and maintained normal glucose homeostasis and insulin sensitivity. We observed, however, an upregulation of Pnpla5 in the fat depot but not liver of Pnpla3−/− mice. These data indicate that inactivation of Pnpla3 does not lead to hepatic TG accumulation or susceptibility to diet-induced hepatic steatosis, nor does it perturb glucose homeostasis, insulin sensitivity, or adipose development in mice.

EXPERIMENTAL PROCEDURES

Animals

Mice were maintained in a temperature-controlled facility with fixed 12-h-light and 12-h-dark cycles and free access to regular chow and water. Some experiments were done on animals fed with a high-fat diet (HFD; 42% of kcal from fat, Harlan Teklad TD88137), high sucrose very low fat diet (HSD, Harlan Teklad TD03045) and Methionine/choline deficient diet (MCD, MP Biomedicals 0296043910) for various periods of time as indicated. Animals of 8 to 22 weeks old were used throughout this study unless otherwise indicated. All animal experiments were done using protocols approved by the IACUC at Baylor College of Medicine.

Plasma biochemistry

Plasma nonesterified fatty acid (NEFA) (Wako), glycerol (Sigma), glucose (Therma Scientific), total cholesterol, total triacylglycerol levels (Therma DMA), ALT and AST (Teco Diagnostics) were measured by enzymatic assay kits for determination of their concentrations. Serum insulin was measured by enzyme-linked immunosorbent assay (Mercodia).

Tissue lipid analysis

Tissues were homogenized in standard PBS buffer. Lipids were extracted according to Bligh and Dyer (20), and dissolved in 5% triton X-100 in PBS. We measured TG concentration with an Infinity triacylglycerol assay kit (Thermo DMA) and normalized to sample weight.

Glucose tolerance and insulin tolerance tests

For glucose tolerance test (GTT), mice were fasted 6 h or overnight and then injected intraperitoneally (i.p.) a bolus of 1.5 g glucose per kg of body weight. Blood was collected before and at 15, 30, 60 and 120 min after injection. We measured glucose levels using a glucometer (FasTake, LifeScan) and serum insulin level by an enzyme-linked immunosorbent assay (Mercodia). For insulin tolerance test (ITT), mice were fasted for 6 h and injected i.p. with humulin at a final concentration of 0.75 U/kg body weight. Blood was collected for glucose measurements before injection and at 15, 30, 60 and 120 min after injection.

Quantitative RT-PCR

Total RNA was isolated from tissues or cultured cells with TRIzol (Invitrogen) and reverse-transcribed using Superscript II reverse transcriptase using random primers (Invitrogen). We performed real-time quantitative RT-PCR on the Strategene MX3000 real time detection system usingiQ SYBR Green PCR reagent kit (Biorad). Primers used were shown in supplemental Table 2.

Statistical analysis

We applied the Student t test for statistical analysis. Differences were considered significant when p values were <0.05. Results were expressed as means ± standard deviation (SD) or standard error (SE) as specified.

RESULTS

Absence of Pnpla3 in mice has no effect on body fat composition and adipose tissue development

The Pnpla3 gene was inactivated by replacing the first 7 exons, including the translation initiation codon and the lipase consensus sequence motif (GXSXG, where G is Glycine, S is Serine, and X is any amino acid), with a neomycin selection cassette (Fig. 1A). Genomic PCR genotyping of tail DNA extracted from wild-type, heterozygous and homozygous littermates are shown in Fig. 1B. Reverse transcription followed by PCR analyses confirmed that there was no Pnpla3 mRNA detectable in the white adipose tissue (WAT) of Pnpla3−/− mice (Fig. 1C).

Fig. 1
Generation of Pnpla3−/− mice

Pnpla3−/− mice were born live with the expected Mendelian mode of inheritance and displayed no overtly abnormal phenotype. They were fertile and nursed their pups normally. The body weights of Pnpla3−/− and Pnpla3+/+ mice showed no difference either while they were fed a normal chow diet (CHD) (Fig. 2A) or a high sucrose very low fat diet (HSD), or high fat diet (HFD) (Fig. 2B), indicating that Pnpla3 deficiency has no effect on body weight. There was also no difference in their 4 h-fasted blood glucose, plasma free fatty acids (NEFA), triacylglycerol (TG), total cholesterol or free glycerol, or overnight fasted plasma insulin level, between Pnpla3−/− mice and wild-type littermates while they were fed regular chow or fed HSD or HFD for 10 weeks (Supplementary Table 1). The adiposity index of mice under the regular chow diet determined by Echo Magnetic resonance imaging (MRI) revealed similar body fat content with the fat making up ~7.62% body weight in male wild-type compared with ~8.18% of body weight in male Pnpla3−/− mice (Fig. 2C). Their lean mass was also similar (Fig. 2D). There was also no difference in the adiposity index in HSD or HFD fed Pnpla3+/+ and Pnpla3−/− mice (data not shown). In addition, we detected no difference in the weights of gonadal, subcutaneous, or brown adipose depots in Pnpla3+/+ and Pnpla3−/− mice (data not shown). Experiments on the in vitro differentiation of stromal vascular cells (SVC) isolated from Pnpla3+/+ and Pnpla3−/− mice revealed no difference in the efficiency of adipocyte differentiation or TG accumulation between the two genotypes (Supplementary Fig. S1, A & B), which agrees with the fact that the adipose depot mass was similar in these mice. Furthermore, the basal and β-adrenergic agonist-stimulated lipolysis in fully differentiated SVC in vitro (Supplementary Fig. S1, C & D) as well as in mice in vivo (Supplementary Fig. S1, E & F) was similar between wild-type and Pnpla3−/− cells or mice, indicating that, unlike ATGL and hormone sensitive lipase, Pnpla3 does not contribute significantly to basal or β-adrenergic agonist-stimulated lipolysis.

Fig. 2
Pnpla3 deficiency has no effect on body weight or body fat

Loss of Pnpla3 does not alter liver TG content or serum ALT and AST levels

The nonsynonymous rs738409 SNP in PNPLA3 was predicted to cause the loss of PNPLA3 enzymatic activity, a consequence functionally similar to the targeted inactivation of Pnpla3 in our mouse model (36). Microscopic examination of Pnpla3−/− mouse liver sections revealed normal histology (data not shown). We analyzed liver TG content in wild-type and Pnpla3−/− mice fed regular chow, and after they had been placed on three different fatty liver-inducing diets. As shown in Table 1, mice in C57BL/6 background fed the different fatty liver-inducing diets [including high sucrose very low fat (HSD), high fat (HFD) and the choline/methionine-deficeint (MCD) diets] displayed varying degrees of increased liver TG content compared with mice fed regular chow (CHD). However, there was no significant difference in the degree of hepatic TG accumulation between wild-type and Pnpla3−/− mice under each type of diet, indicating that loss of Pnpla3 had no direct impact on liver TG accumulation.

Table 1
Liver triacylglycerol content, serum ALT and AST levels in wild-type and Pnpla3−/− mice fed regular chow or different fatty liver-inducing diets or after they were bred into genetic obese Lepob/ob background.

Genetic variations at PNPLA3 have been reported to be associated with increased serum levels of liver enzymes in human populations (3, 5). We found that serum ALT and AST levels varied with the diet conditions (Table 1), being highest in mice fed MCD diet, which may be related to the significant liver damage and inflammation induced by this diet. However, no difference in ALT or AST level was observed between the two genotypes, suggesting that lack of Pnpla3 in mice does not cause an elevated transaminase response in liver either under regular chow or after the mice were fed the different fatty liver-inducing diets.

To further analyze whether loss of Pnpla3 affects fatty liver development associated with a genetic form of obesity, we intercrossed the Lepob/+ mice with Pnpla3−/− mice to produce Lepob/ob/Pnpla3+/+ and Lepob/ob/Pnpla3−/− mice. The obesity phenotype was unchanged in these mice. The Lepob/ob/Pnpla3−/− mice gained similar weight with time as compared to their Lepob/ob/Pnpla3+/+ counterparts (data not shown). Both Lepob/ob/Pnpla3−/− mice and Lepob/ob/Pnpla3+/+ littermates displayed fatty liver, and there was no difference in the hepatic TG content between the two genotypes (Table 1). Moreover, their serum ALT and AST levels were also not different (Table 1). These data indicate that loss of Pnpla3 in mice has no impact on fatty liver development under basal conditions, after they are on different fatty liver-inducing diets or bred into a genetic model associated with obesity and fatty liver.

Loss of PNPLA3 does not cause glucose intolerance or insulin resistance

Lack of PNPLA3 has been postulated to perturb glucose homeostasis and insulin sensitivity in vivo (13, 21). As such, we measured the rate of glucose disposal and insulin sensitivity in wild-type and Pnpla3−/− mice by glucose tolerance test (GTT) and insulin tolerance test (ITT). After administration of an exogenous glucose load, Pnpla3−/− mice and their wild-type littermates showed similar basal and stimulated blood glucose and insulin levels, indicating a normal glucose disposal rate and insulin secretory response to hyperglycemia in the absence of Pnpla3 (Fig. 3A&B). Pnpla3−/− mice and wild-type littermates on a normal chow diet also displayed a similar blood glucose during ITT (Fig. 3C); indicating no significant insulin resistance associated with loss of Pnpla3. We fed these mice a HFD for 15 weeks, and found that the blood glucose levels during the GTT in HFD-fed Pnpla3−/− mice was minimally lower than those in wild-type littermates (Fig. 3D). The plasma insulin was, however, similar during the GTT (Fig. 3E); so was the blood glucose response during an ITT (Fig. 3F) in the two HFD-fed groups. Further examination of a cohort fed a HSD for 12 weeks also revealed no difference in either blood glucose or insulin levels during GTT (Supplementary S2, A & B), or blood glucose levels during an ITT (Supplementary Fig. S2, C), between Pnpla3−/− mice and Pnpla3+/+ mice. Finally, we examined the role of Pnpla3 in mice with genetic obese Lepob/ob background and found that Lepob/ob/Pnpla3−/− and Lepob/ob/Pnpla3+/+ mice displayed similar blood glucose and insulin levels during GTT (Supplementary Fig. S2, D & E), and similar blood glucose levels during ITT (Supplementary Fig. S2, F). Therefore, not only did the absence of Pnpla3 not affect hepatic TG content, it also did not impact the glucose intolerance and insulin resistance that often accompany hepatic steatosis.

Fig. 3
Glucose tolerance test (GTT) and insulin tolerance test (ITT) in Pnpla3−/− and Pnpla3+/+ mice fed regular chow or high fat diet

Upregulated Pnpla5 mRNA level in the WAT but not liver of Pnpla3−/− mice

Thus far, our data indicate that there was no evident change in hepatic TG content or whole body glucose homeostasis between Pnpla3−/− and Pnpla3+/+ mice under 4 dietary conditions (CHD, HFD, HSD, and MCD) and two genotypes (C57BL/6J Lepob/ob and C57BL/6J Lep+/+). Since the PNPLA gene family encompasses 3 paralogous gene products in mice, we next examined the dynamics of the three paralogs in the liver under various dietary conditions. We found that the hepatic Pnpla3 mRNA in wild-type mice was markedly upregulated (~32 fold) by HSD feeding but only moderately by HFD (~5 fold) (Fig. 4A, left panel). Interestingly, HSD and HFD both stimulated hepatic Pnpla5 mRNA expression to the same degree in Pnpla3−/− and Pnpla3+/+ mice (Fig. 4A, middle panel). In contrast, as compared with regular chow (CHD), none of the fatty liver-inducing diets (HSD, HFD and MCD) affected the level of ATGL mRNA expression (Fig. 4A, right panel). It is noteworthy that although MCD diet induced the largest TG accumulation in the liver compared with feeding with other diets (Table 1), it did not have any effect on the mRNA expression of the three different patatin-like family members (Fig. 4A). In any case, there was no evidence of compensatory adjustment in hepatic Pnpla5 or ATGL expression in the absence of Pnpla3 in the liver (Fig. 4A, middle and right panels).

Fig. 4
Quantitative analysis of mRNA expression of PNPLA family members in the liver and WAT of Pnpla3−/− and Pnpla3+/+ mice fed different diets

We next examined the mRNA expression of PNPLA family genes in perigonadal WAT in wild-type and Pnpla3−/− mice. As reported previously (22), and confirmed by us, Pnpla3 expression in the WAT of wild-type mice was significantly induced by HSD diet (~2.5 fold) and slightly upregulated by HFD diet (~1.5 fold, not significant; Figure 4B, left panel). Under the same conditions, the expression of Pnpla5 was not significantly affected (Fig. 4B, open bars, middle panel). The mRNA expression of ATGL was not altered under the different diets in the wild-type WAT; furthermore, the diets did not affect ATGL mRNA in WAT in the two genotypes (Fig. 4B, right panel). Interestingly, the mRNA level of Pnpla5, normally expressed in WAT at very low level compared with the other two paralogs ((23) and our own data), was upregulated by ~5 fold in Pnpla3−/− mice fed regular chow. This upregulation of Pnpla5 was also observed in the gonadal fat of Pnpla3−/− mice fed HSD or HFD, though a little less in the HFD group (Fig. 4B, black bars, middle panel). It thus appears that increased mRNA expression of another patatin-like family member, Pnpla5, may partly compensate for the loss of Pnpla3 in mice, specifically in WAT, but not in liver.

DISCUSSION

GWAS have identified the Pnpla3/adiponutrin gene to be associated with obesity and insulin sensitivity (13, 21, 24), and, more recently, with non-alcoholic (35), as well as alcoholic, fatty liver disease (6) and elevated AST and ALT (3, 5), implicating PNPLA3 in the control of body fat, liver fat and whole-body glucose and lipid homeostasis. However, to our surprise, we found that loss of Pnpla3 in mice does not have any effect on body weight, adiposity or plasma lipid or glucose levels (Fig 1 and Supplemental Table 1), nor does it cause detectable alterations in hepatic TG content or serum ALT and AST levels (Table 1). Furthermore, the whole body glucose homeostasis and insulin sensitivity remained normal. These were evident whether the Pnpla3-null mice were fed regular chow, high fat, high sucrose, or a MCD diet or in mice bred into a genetic obesity Lepob/ob background. We conclude that Pnpla3 appears dispensable for liver TG metabolism and normal adipose development in mice.

Recently, interest in PNPLA3 gene was kindled by genetic epidemiologic analyses by multiple groups showing a strong association between a nonsynonymous variant (rs738409) in the PNPLA3 gene and hepatic fat content, suggesting the involvement of PNPLA3 in the pathogenesis of hepatosteatosis (36, 25). This rs738409 variant leads to an I148M substitution which has been shown to inactivate the enzyme by blocking substrate access to the catalytic site (8). If PNPLA3 mediates lipolysis in the liver, one would expect to see increased TG content in the liver of Pnpla3-deficient mice. However, there was no excess hepatic TG in Pnpla3−/− mice (Table 1). We next fed mice with different fatty liver-inducing diets to test whether loss of Pnpla3 engenders increased susceptibility to fatty liver development under dietary stress. However, none of the diets tested produced a detectable difference in hepatic TG content in Pnpla3−/− mice as compared with the wild-type counterparts in any of the cohorts (Table 1). We also did not detect any significant association of serum AST or ALT in Pnpla3−/− mice fed regular chow, or three different fatty liver-inducing diets, or after they were bred into a genetic obesity Lepob/ob background, though the PNPLA3 locus, and specifically the enzymatically inactive rs738409(G) genetic variant, has been shown to be associated with elevated serum liver enzymes in humans (7, 21, 24).

It is unclear whether the lack of phenotype in lipid accumulation in the liver of Pnpla3−/− mice can be explained by an indirect effect resulting from an adipose-specific upregulation of Pnpla5, a paralogous PNPLA family protein, which expression remains very low and unchanged in the liver of Pnpla3−/− mice. PNPLA5 is highly conserved among different species and is located immediately upstream of the PNPLA3 gene in the human, rat and mouse genome. PNPLA5 exhibits both lipase and transacylase activities in vitro and its mRNA in WAT is regulated by changes in energy balance that is not too dissimilar from that seen with Pnpla3 (23). It should be noted that Pnpla3 and Pnpla5 mRNA levels in liver are much lower than those in WAT in mice. Pnpla5 mRNA especially is barely detectable in mouse liver ((23) and our data). On the other hand, we showed that a lipogenic high sucrose diet treatment leads to a marked stimulation of the transcripts of both Pnpla3 and Pnpla5 in the liver (Fig. 4A). A HFD also upregulated these mRNAs but MCD failed to do so, a finding that may be related to the different mechanism whereby MCD induces hepatic TG accumulation (26). Although the regulatory pattern of Pnpla3 by lipogenic diets could suggest an involvement in an anabolic process, whether Pnpla3 normally plays a direct role in hepatic lipogenesis is unclear. It is interesting that its ablation in mice does not affect hepatic TG accumulation under multiple dietary conditions.

He et al. recently reported that adenovirus-mediated overexpression of human PNPLA3 I148M mutant in liver increased hepatic TG content while overexpression of the normal human PNPLA3 failed to lower TG (8). The seemingly paradoxical observation suggests that Pnpla3 may not be a primary liver TG metabolizing enzyme and it is possible that the I148M mutant has a dominant negative action on other liver TG hydrolases. In our study, the upregulation of Pnpla5 mRNA only happens in the WAT but not in the liver of Pnpla3−/− mice. Such adipose tissue-specific upregulation of Pnpla5 mRNA was consistently seen among the different cohorts receiving the different dietary manipulations (Fig. 4B), implicating a dynamic interaction between Pnpla3 and Pnpla5 in WAT. On the other hand, we cannot rule out inter-species differences of PNPLA3 or PNPLA5 action or expression between humans and mice. For example, one very recent study suggested that PNPLA3 expression is higher in the liver than in the WAT of humans (27), in contrast to mice where its expression is significantly higher in adipose tissue than in liver (our data and (23)).

In conclusion, our study constitutes the initial study demonstrating that loss of Pnpla3 in mice has no effect on hepatic TG accumulation. The observation of the upregulation of Pnpla5 specifically in fat but not liver in Pnpla3−/− mice is intriguing. It is tempting to speculate that upregulated adipose Pnpla5 expression may be a confounding factor that underlies, or possibly modulates, the association between the rs738409(G) allele and the presence or absence of fatty liver at the individual level.

Supplementary Material

Supp FigS1-S2 & Tables S1-S2

Acknowledgments

We are indebted to our coworkers in the Chan laboratory, especially Dr. Vijay Yechoor, Dr. Minako Imamura and Dr. Yisheng Yang, for suggestions and discussions. We are also grateful to Dr. Saul J Karpen for a critical review of the manuscript.

Financial Support: This research was supported by grants from the US National Institutes of Health HL-51586 (to L.C.) and P30DK079638 for a Diabetes and Endocrinology Research Center. W. Chen was supported by a postdoctoral fellowship from American Heart Association South Central Affiliate No.0825134F.

List of Abbreviations

PNPLA
patatin-like phospholipase
FLD
fatty liver disease
NAFLD
nonalcoholic fatty liver disease
ALT
aspartate aminotransferase
AST
alanine aminotransferase
CHD
regular chow diet
HSD
high sucrose very low fat diet
HFD
high fat diet
MCD
methionine-choline deficient diet
GTT
glucose tolerance test
ITT
insulin tolerance test

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