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Abstract

  1. Top of page
  2. Abstract
  3. Patients and Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
  8. Supporting Information

Previous studies examining the relationship between hepatic iron deposition and histological severity in nonalcoholic fatty liver disease (NAFLD) have been inconclusive. The goal of this study was to examine the relationship between hepatic iron deposition and liver histology in 849 patients enrolled in the Nonalcoholic Steatohepatitis Clinical Research Network. Hepatic iron staining was performed in a central laboratory, and the stains were scored for grade and cellular and parenchymal localization by a central pathology committee; the relationship between the grade and pattern of iron deposition and the clinical, laboratory, and histological variables was examined with univariate and multivariate analyses. Stainable hepatic iron was present in 293 of 849 patients (34.5%) in one of three histological patterns: a hepatocellular (HC) pattern [63/849 (7.4%)], a reticuloendothelial system (RES) cell pattern [91/849 (10.7%)], or a mixed RES/HC pattern [139/849 (16.4%)]. Patients with the RES iron-staining pattern were more likely to have advanced fibrosis compared to those with those with HC iron (P = 0.01). Patients with RES iron were also more likely to have advanced histological features such as fibrosis (P = 0.049), portal inflammation (P = 0.002), HC ballooning (P = 0.006), and definite nonalcoholic steatohepatitis (P = 0.007) compared to those with patients with HC or mixed iron patterns. The presence of RES iron (odds ratio = 1.60, 95% confidence interval = 1.10-2.33, P = 0.015) was independently associated with advanced hepatic fibrosis on multiple regression analysis after adjustments for age, gender, diabetes status, and body mass index. Conclusion: The presence and pattern of hepatic iron deposition are associated with distinct histological features in patients with NAFLD and may have implications for pathophysiology and therapy. (HEPATOLOGY 2011;53:448-457)

Increased deposition of iron within the liver may contribute to liver disease via the production of reactive oxygen species (ROS), which may lead to lipid peroxidation, dysfunction of mitochondria and other organelles, cell injury, and death.1 In addition to hemochromatosis, hepatic iron accumulation may occur in patients with a variety of chronic liver diseases, including chronic hepatitis C, alcoholic fatty liver disease, nonalcoholic fatty liver disease (NAFLD), cryptogenic cirrhosis, and end-stage liver disease.2-5 Hepatic iron deposition in the setting of chronic liver disease may be present in one of three different patterns: exclusively in hepatocytes, exclusively in cells of the reticuloendothelial system (RES), or in a mixed pattern involving both hepatocytes and RES.2-5 In hereditary hemochromatosis types 1, 2, and 3, iron preferentially accumulates in hepatocytes because of mutations in the hemochromatosis gene (HFE), the hemojuvelin (HJV) or hepcidin genes, and the transferrin receptor 2 (TFR2) gene, respectively.2-5 In contrast, hepatic iron deposition in the setting of cirrhosis and secondary iron overload occurs primarily in RES cells and usually begins with sinusoidal lining cells in an azonal pattern.2-5 Iron deposition in patients with alcoholic fatty liver disease, NAFLD, or a chronic hepatitis C infection may occur in any of these three patterns.2-5

The contribution of hepatic iron accumulation to the severity or progression of chronic liver diseases other than hemochromatosis remains unclear. A number of studies have assessed the relationship between hepatic iron loading and disease stage in chronic hepatitis C; the majority of these studies (but not all) support an association between advanced fibrosis and the presence of iron deposition in the nonparenchymal RES cells (i.e., sinusoidal, endothelial, and portal tracts).6, 7 In contrast, parenchymal iron deposition is a feature of alcoholic liver disease, although RES iron is more prevalent in the advanced stages of disease.8

NAFLD is the most common liver disease in the United States and may be present in up to 30% of the general population.9 A subset of patients with NAFLD have nonalcoholic steatohepatitis (NASH), a more severe form of this disease associated with hepatocellular (HC) injury, inflammation, and varying levels of fibrosis. A number of previous studies have investigated the role of iron stores in NAFLD by assessing the presence of stainable hepatic iron deposits, the biochemical hepatic iron content, or both. However, the findings thus far have been conflicting, with some studies finding hepatic iron deposition to be associated with increased disease severity10-12 and others not finding such an association.13-16 One previous study examining the distribution of iron in 157 patients with NASH-related cirrhosis, including 51 with hepatocellular carcinoma (HCC), demonstrated that patients with HCC were more likely to have mild to moderate RES cell iron deposits than patients without HCC.17 However, most previous studies examining the relationship between hepatic iron deposition and histological features of NASH have had several limitations, including small sample sizes, a lack of uniform criteria for the diagnosis of NASH, and a lack of a standardized liver histological scoring system for NASH and iron deposition. Most importantly, previous studies have not examined the relationship between each of the three distinct patterns of hepatic iron deposition and histological severity among patients with NASH.

The goal of the current study was to analyze the relationships between the pattern of hepatic iron deposition and liver histology in liver biopsy specimens from an unselected cohort of NAFLD patients prospectively enrolled in the National Institutes for Health–funded Nonalcoholic Steatohepatitis Clinical Research Network (NASH CRN) from eight participating centers in the United States.

Patients and Methods

  1. Top of page
  2. Abstract
  3. Patients and Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
  8. Supporting Information

Participants were enrolled in the NASH CRN studies from October 2005 to February 2008 according to inclusion criteria described elsewhere.18, 19 Briefly, NASH CRN study participants at least 18 years of age constituted the patient population for this study. Patients with known hemochromatosis (defined as a hepatic iron index ≥ 1.9 or the removal of >4 g of iron by phlebotomy), C282Y homozygosity for the HFE gene, or unexplained hepatic iron overload (≥3+ stainable iron on liver biopsy) were excluded from all NASH CRN studies. Demographic information such as age, gender, ethnicity, and race was obtained. A medical history was obtained for all subjects; it included a menstrual history for women, the presence of comorbid conditions, and medication usage. The total dietary consumption of iron, vitamin C, tea, and coffee was determined with the Block 98 food frequency questionnaire; alcohol consumption was determined with the Alcohol Use Disorders Identification Test–Consumption questionnaire during the NASH CRN studies closest to the time of biopsy. A physical examination, which included body weight and height measures, was performed for all subjects. The histological evaluation was based on 849 liver biopsy samples with hepatic iron staining results, which were read centrally by the pathology committee of NASH CRN. In addition, clinical and laboratory data obtained within 6 months of liver biopsy were compared between iron stain–positive subjects and iron stain–negative subjects if they were available (n = 573).

Histological Assessment.

Histological features of fatty liver disease and iron-staining patterns were assessed by the pathology committee of NASH CRN in a centralized consensus review format using criteria previously described.20 Pathologists were blinded to all clinical, laboratory, and demographic information. Iron stains were performed by a central laboratory with Perls' iron stain; iron stains were scored prospectively by a method decided by the pathology committee. Only granular iron deposition was scored, and this was based on the agreement that only discernible hemosiderin granules represent significant iron deposition.3, 4 HC iron was scored from 0 to 4 with the method of Rowe et al.,21 except that a 20× objective was used in place of the 25× objective. Non-HC iron (RES) was scored on a three-point scale as none, mild, or more than mild.

Statistical Analysis.

Baseline demographic, clinical, and laboratory characteristics were recorded as numbers and percentages, means and standard deviations, or medians and interquartile ranges. Laboratory measures were not normally distributed and therefore were analyzed with the Wilcoxon rank-sum test or Kruskal-Wallis test for continuous variables. Categorical variables, including histological features such as steatosis grade and location, fibrosis stage, and lobular inflammation grade, were analyzed with either Fisher's exact test or the chi-square test. Multiple logistic regression analysis was used to examine the relationship between advanced fibrosis and the presence and grade of HC and RES iron. Controlling for age at biopsy, gender, presence of diabetes, and body mass index (BMI), we used stepwise conditional logistic regression to determine the effects of the following variables selected a priori on the presence of iron staining: ethnicity, history of gastrointestinal bleeding or iron overload, menstrual history, alcohol consumption, tea and coffee consumption, and dietary or supplemental iron and vitamin C consumption. All variables not independently associated with iron with a threshold P value of ≤0.20 were removed from the model. All analyses were performed with SAS 9 (SAS Institute, Cary, NC) or Stata 9 (Stata Corp., College Station, TX). Nominal, two-sided P values were used and were considered to be statistically significant if P < 0.05; no adjustments for multiple comparisons were made.

Results

  1. Top of page
  2. Abstract
  3. Patients and Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
  8. Supporting Information

Patient Characteristics.

Eight hundred forty-nine subjects (a subset of the 1525 patients enrolled in the NASH CRN database study, the Pioglitazone or Vitamin E for NASH study, and the Treatment of Nonalcoholic Fatty Liver Disease in Children study) were included in this analysis of hepatic iron deposition. The reasons for the exclusion of the remaining 676 subjects were as follows: (1) the subject was less than 18 years old (n = 368; iron overload was rare in children in our cohort), (2) a liver biopsy sample was not available (n = 167), and (3) iron staining was not performed on a liver biopsy sample (n = 141).

A comparison of clinical and demographic data for subjects with positive hepatic iron staining and the entire cohort is shown in Table 1. Stainable hepatic iron was present in 293 of 849 patients (34.5%); the majority were men (57%, P < 0.0001). There was no significant difference in age between subjects with stainable iron and those without stainable iron. Subjects with positive iron staining had a lower mean BMI than subjects without iron (33.2 versus 34.9 kg/m2, P = 0.0002). Iron staining was more common in non-Hispanics (36%) versus Hispanics (25%, P = 0.04); otherwise, no racial differences were identified between subjects with stainable iron and those without stainable iron.

Table 1. Patient Characteristics
CharacteristicAll PatientsIron Stain– Positive PatientsP Value*
  • Values are presented as numbers and percentages or as means and standard deviations. Bolded P values are statistically significant.

  • *

    From Fisher's exact test for categorical variables and from the Wilcoxon rank-sum test for continuous variables.

Total (n)849293 (35) 
Male sex [n (%)]309175 (57)<0.0001
Age category [n (%)]  0.34
 <40 years198 (23)60 (30) 
 40-60 years505 (60)179 (35) 
 ≥60 years146 (17)54 (37) 
Mean age (years)48.4 ± 11.749.0 ± 11.30.49
BMI category [n (%)]  0.004
 <25 kg/m234 (4)18 (53) 
 25-30 kg/m2197 (23)80 (41) 
 ≥30 kg/m2616 (73)194 (31) 
Mean BMI (kg/m2)34.3 ± 6.333.2 ± 6.00.0002
Race [n (%)]  0.08
 White696 (82)236 (34) 
 Black24 (3)9 (38) 
 Asian43 (5)23 (53) 
 American Indian or Alaska Native25 (3)7 (28) 
 Other61 (7)18 (30) 
Ethnicity [n (%)]  0.04
 Non-Hispanic750 (88)268 (36) 
 Hispanic99 (12)25 (25) 

Laboratory Data for Subjects With or Without Stainable Hepatic Iron.

The results of laboratory tests for subjects with or without stainable hepatic iron are shown in Table 2. Subjects with a liver biopsy sample showing a positive iron stain tended to have evidence of more active and advanced disease, as shown by higher serum alanine aminotransferase (ALT) levels (P = 0.004), total bilirubin levels (P < 0.0001), and prothrombin times (P = 0.09) and lower platelet counts (P < 0.0001). In contrast, metabolic abnormalities, including fasting insulin and glucose levels, homeostasis model assessment of insulin resistance (HOMA-IR), and lipid levels, were slightly worse among subjects without stainable iron, but with the exception of total cholesterol (P = 0.02), these were not statistically significant. The high-density lipoprotein (HDL) level was higher in subjects without iron (P = 0.004). As might be expected, patients with stainable hepatic iron had higher serum iron studies [iron, total iron-binding capacity (TIBC), ferritin, and transferrin saturation (TS) percentage; for all, P < 0.0001].

Table 2. Laboratory Values for Patients With NAFLD With or Without Stainable Hepatic Iron
CharacteristicIron Stain–Negative (n = 556)Iron Stain–Positive (n = 293)P Value
  • Values are presented as medians and interquartile ranges.

  • Abbreviation: LDL, low-density lipoprotein.

  • *

    Only laboratory values collected within 6 months of liver biopsy were included (n = 573).

  • From the Wilcoxon rank-sum test.

Laboratory values*
 ALT (U/L)63 (42-91)75 (50-111)0.004
 AST (U/L)45 (32-68)48 (33-67)0.39
 AST/ALT0.74 (0.59-0.92)0.66 (0.51-0.84)0.0002
 Total bilirubin (mg/dL)0.6 (0.5-0.8)0.8 (0.6-1.0)<0.0001
 Prothrombin time (seconds)11.8 (10.3-12.7)12.0 (10.6-13.1)0.09
 Total cholesterol (mg/dL)200 (176-224)190 (166-222)0.02
 Triglycerides (mg/dL)159 (114-232)152 (105-214)0.26
 LDL (mg/dL)121 (93-148)116 (90-141)0.10
 HDL (mg/dL)43 (36-51)39 (34-49)0.004
 Glucose (mg/dL)97 (86-110)95 (85-108)0.54
 Fasting insulin (μU/mL)18.8 (13-29)17.7 (11.8-26.0)0.10
 HOMA-IR4.6 (3.0-7.5)4.4 (2.5-6.5)0.10
 Platelets (K/cmm)255 (220-293)216 (179-261)<0.0001
 Hemoglobin (g/dL)14.1 (13.4-15.0)15.1 (14.3-15.9)<0.0001
Serum iron studies
 Iron (μg/dL)81 (61-103)103 (80-123)<0.0001
 TIBC (μg/dL)380 (337-424)336 (301-384)<0.0001
 Ferritin (ng/mL)106 (62-192)328 (221-526)<0.0001
 TS (iron/TIBC)0.21 (0.16-0.27)0.29 (0.23-0.39)<0.0001

Potential Dietary and Clinical Factors Affecting Hepatic Iron Deposition.

We examined the effects of factors potentially influencing body iron stores, such as diet (i.e., iron consumption, vitamin C, coffee, and tea), alcohol, and other factors (e.g., a history of gastrointestinal bleeding, iron overload, and menstruation in the past 5 years). In a multivariate stepwise logistic regression analysis using these a priori selected variables and adjusting for age, gender, BMI, ethnicity, and diabetes, male sex [odds ratio (OR) = 5.08, 95% confidence interval (CI) = 3.67-7.02, P < 0.0001], older age (OR = 1.02, 95% CI = 1.01-1.04, P = 0.001), and lower BMI (OR = 0.967, 95% CI = 0.941-0.991, P = 0.009) were independently associated with the presence of hepatic iron. Among women, rare or no periods (in the past 5 years) were also strongly associated with iron deposition (OR = 1.57, 95% CI = 1.28-1.94, P < 0.0001).

Relationship Between Patterns of Hepatic Iron Staining and Clinical and Laboratory Differences.

Three distinct patterns of hepatic iron staining were observed as follows: iron was localized solely in hepatocytes in 63 of 849 subject biopsy samples (7.4%), iron was localized solely in RES cells (mainly Kupffer cells) in 91 of 849 biopsy samples (10.7%), and a mixed pattern of HC/RES staining was present in 139 of 849 biopsy samples (16.4%).

Clinical and laboratory values that were significantly different among the various iron-staining groups and subjects without stainable hepatic iron are shown in Table 3. Subjects with RES iron had the highest serum ALT, aspartate aminotransferase (AST), and HOMA-IR values among all the groups. Subjects with HC iron generally had values (e.g., ALT, AST, total bilirubin, and platelet values) similar to those of the no-iron group. The mixed iron group tended to be intermediate or closer to the RES group versus the HC or no-iron groups for most values. Subjects in the mixed iron group had the highest iron stores according to serum iron, TS, and ferritin levels. This group also had a greater proportion of men (70%) versus the other groups.

Table 3. Comparison of Clinical and Laboratory Values for Subjects With Different Hepatic Iron Phenotypes
CharacteristicNo Iron Stain (n = 556)HC Iron Only (n = 63)RES Iron Only (n = 91)Mixed HC/RES Iron (n = 139)P Value*
  • Values are presented as medians and interquartile ranges, except for BMI (means and standard deviations) and male sex (numbers and percentages). Only laboratory values collected within 6 months of liver biopsy were used in analyses (n = 573).

  • *

    From the Kruskal-Wallis test for continuous variables, except for male sex (chi-square test).

Male sex [n (%)]134 (24)31 (49)47 (52)97 (70)0.0001
BMI (kg/m2)34.9 ± 6.432.7 ± 6.833.5 ± 5.733.2 ± 5.80.0015
ALT (U/L)63 (42-91)62 (50-92)88 (58-128)70 (44-117)0.002
AST (U/L)45 (32-68)40 (31-56)58 (39-77)45 (33-63)0.03
AST/ALT0.74 (0.59-0.92)0.62 (0.53-0.90)0.64 (0.49-0.87)0.66 (0.52-0.79)0.003
Total bilirubin (mg/dL)0.6 (0.5-0.8)0.6 (0.5-0.9)0.7 (0.6-1.1)0.9 (0.6-1.1)0.0001
HDL (mg/dL)43 (36-51)41 (33-50)39 (33-49)40 (34-49)0.04
Insulin (μU/mL)19 (13-29)16 (11-24)23 (12-32)18 (12-23)0.05
HOMA-IR4.6 (3.0-7.5)3.7 (2.8-5.3)5.2 (2.8-8.9)4.2 (2.5-5.8)0.0001
Platelets (K/cmm)255 (220-293)247 (198-286)215 (178-258)206 (174-258)0.0001
Hemoglobin (g/dL)14.1 (13.4-15.0)15.1 (14.2-15.5)14.9 (14.2-15.6)15.5 (14.6-16.0)0.0001
Serum iron (μg/dL)81 (61-103)91 (71-116)94 (76-114)107 (89-133)0.0001
TIBC (μg/dL)380 (337-424)348 (295-378)356 (324-406)320 (286-365)0.0001
Ferritin (ng/mL)106 (62-192)218 (126-283)344 (227-497)378 (263-673)0.0001
TS (iron/TIBC)0.21 (0.16-0.27)0.27 (0.22-0.37)0.26 (0.21-0.32)0.33 (0.27-0.43)0.0001

Relationship Between the Hepatic Iron-Staining Pattern and the Histological Severity.

A comparison of the histological grade and stage for the various hepatic iron-staining groups is shown in Table 4. There were significant differences across all groups in the proportions of subjects in different categories of severity for portal inflammation, HC ballooning, and NASH diagnosis but not for steatosis grade, lobular inflammation, or fibrosis stage. Mixed iron subjects had higher grades of HC and RES iron deposits than the HC or RES groups, respectively. The NAFLD activity score (NAS) was significantly different across all groups (Fig. 1A; P = 0.0007, Kruskal-Wallis test) and was highest among subjects with RES iron staining (NAS = 4.8) and lowest in the group with HC iron staining (NAS = 4.0). The NAS was also significantly higher among subjects without iron versus those with an HC pattern (P = 0.005) or a mixed pattern (P = 0.006). As shown in Fig. 1B, the mean fibrosis score (0-4 scale) was also significantly different across all groups (P = 0.0012, Kruskal-Wallis test). The RES iron group had a significantly higher mean fibrosis stage (mean score = 1.9) versus each of the other groups, including the no-iron group (P = 0.006). In contrast, the HC group had a significantly lower mean fibrosis stage than the other three groups (mean score = 1.1).

thumbnail image

Figure 1. Comparison of the mean NAS values and the mean fibrosis scores of NAFLD subjects with different iron-staining patterns. (A) The mean NAS values and (B) the mean histological fibrosis scores (scale = 0-4) are shown for each study group. Significant differences between groups are indicated by arrows (Wilcoxon rank-sum test). Standard deviations are indicated by the error bars.

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Table 4. Histological Differences Between Iron-Staining Groups
Histological FeatureNo Iron Stain (n = 556)HC Iron Only (n = 63)RES Iron Only (n = 91)Mixed HC/RES Iron (n = 139)P Value*
  • *

    From either Fisher's exact text or the chi-square test as appropriate.

Steatosis grade [n (%)]    0.170
 5%-33%220 (40)27 (43)35 (38)71 (51) 
 34%-66%190 (34)24 (38)29 (32)35 (25) 
 >66%146 (26)12 (19)27 (30)33 (24) 
Fibrosis stage [n (%)]    0.092
 None144 (26)25 (40)11 (12)33 (24) 
 Mild to moderate, zone 3, perisinusoidal, or portal/periportal only155 (28)18 (28)27 (30)41 (29) 
 Zone 3 and periportal, any combination104 (19)8 (13)19 (21)25 (18) 
 Bridging108 (19)10 (16)24 (26)27 (19) 
 Cirrhosis45 (8)2 (3)10 (11)13 (9) 
Lobular inflammation [n (%)]
 00 (0)0 (0)0 (0)2 (1)0.110
 <2 (20×)275 (50)36 (57)37 (41)73 (53) 
 2-4 (20×)213 (38)22 (35)44 (48)53 (38) 
 >4 (20×)68 (12)5 (8)10 (11)11 (8) 
Chronic portal inflammation [n (%)]
 None81 (15)9 (14)10 (11)23 (17)0.017
 Mild355 (64)48 (76)52 (57)77 (55) 
 More than mild120 (21)6 (10)29 (32)39 (28) 
HC ballooning [n (%)]     
 None166 (30)30 (48)25 (28)50 (36)0.005
 Mild136 (24)18 (28)22 (24)42 (30) 
 More than mild254 (46)15 (24)44 (48)47 (34) 
NASH diagnosis [n (%)]
 No NASH106 (19)19 (30)12 (13)33 (24)0.049
 Suspicious/borderline105 (19)16 (25)17 (19)30 (21) 
 Definite345 (62)28 (44)62 (68)76 (55) 
HC iron grade [n (%)]
 Absent or barely discernable (40×)556 (100)0 (0)91 (100)0 (0)0.001
 Barely discernable granules (20×)0 (0)52 (83)0 (0)80 (58) 
 Discrete granules resolved (10×)0 (0)11 (17)0 (0)49 (35) 
 Discrete granules resolved (4×)0 (0)0 (0)0 (0)10 (7) 
RES iron grade [n (%)]
 None556 (100)63 (100)0 (0)0 (0)<0.001
 Mild0 (0)0 (0)74 (81)79 (57) 
 More than mild0 (0)0 (0)17 (19)60 (43) 

We also examined the relationship between the hepatic iron pattern and advanced histological features. The RES iron group had the highest proportion of subjects in the most severe category for each histological feature (Fig. 2). In contrast, the HC iron group had the lowest proportion of subjects in the most severe categories for each histological feature, and the mixed iron group had intermediate proportions of subjects in the most severe categories (Fig. 2). The no-iron group had proportions similar to those of the mixed iron group (data not shown). The differences in the proportion of subjects with advanced histological features across all iron groups was statistically significant for advanced fibrosis (P = 0.049), portal inflammation (P = 0.002), HC ballooning (P = 0.006), and definite NASH (P = 0.007). In comparison with the HC iron group, the RES iron group had a significantly higher proportion of subjects with each advanced histological feature: stage 3-4 fibrosis (37% versus 19%, P = 0.01), grade 2-3 lobular inflammation (59% versus 43%, P = 0.04), grade 2 portal inflammation (32% versus 10%, P = 0.001), grade 2 ballooning (48% versus 24%, P = 0.002), and a definitive diagnosis of NASH (68% versus 44%, P = 0.003). Subjects with HC iron alone (19%) were least likely to have advanced (stage 3-4) fibrosis in comparison with the no-iron (27%), mixed iron (29%), and RES iron groups (37%). Advanced hepatic fibrosis was significantly more common in subjects with RES iron versus those with HC iron (χ2 = 5.96, P = 0.01). A similar trend was observed in comparison with the no-iron group (χ2 = 3.69, P = 0.055). On multiple regression analysis, both the presence (OR = 1.60, 95% CI = 1.10-2.33, P = 0.015) and grade (OR = 2.15, 95% CI = 1.21-3.84, P = 0.021) of RES iron were independently associated with advanced fibrosis after adjustments for age at biopsy, gender, diabetes status, and BMI (Fig. 3). Neither the presence nor grade of HC iron was associated with advanced fibrosis.

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Figure 2. Relationship between the histological features and the iron-staining patterns of subjects with stainable iron. The proportions of biopsy samples within each iron-staining group that scored in the highest category of each histological feature and were definitively diagnosed with NASH are compared. The most severe category for each histological feature is shown in black, and the remaining categories are combined to add up to a total of 100%. P values were calculated with Fisher's exact test as shown.

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thumbnail image

Figure 3. Multiple logistic regression analysis of the independent risk of iron-staining parameters for advanced fibrosis. Multiple logistic regression analysis was used to model the independent risk of the presence and grade of HC and RES iron for the occurrence of advanced fibrosis (yes versus no). Each of the predictors in the table was modeled individually after adjustments for age at biopsy, gender, diabetes status, and BMI.

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Discussion

  1. Top of page
  2. Abstract
  3. Patients and Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
  8. Supporting Information

We examined the relationship between the pattern of hepatic iron distribution and the clinical and histological findings in 849 unselected adult NAFLD patients from a total of 1525 subjects enrolled in NASH CRN.

This study identified novel relationships between the pattern of hepatic iron deposition and the histological features of NAFLD. RES iron was associated with more severe disease; this was shown by the greater proportion of subjects with advanced histological features, a higher mean NAS, a higher mean fibrosis stage, higher AST, ALT, and total bilirubin values, and lower platelet counts in comparison with the other study groups. In contrast, HC iron was associated with milder histological features in comparison with the other groups, whereas the mixed iron group showed intermediate findings. Similar relationships between iron distribution and disease severity have been observed in chronic hepatitis C virus6, 7 and alcoholic liver disease.8

Previous studies have explored the relationship between hepatic iron deposition and disease severity in NAFLD; however, our study is unique in its examination of the relationship between histological severity and each of the three distinct patterns of hepatic iron deposition observed in NAFLD. The strengths of the present study include the utilization of a centralized pathology committee review, a multicenter design, and a standardized histological scoring system and the largest sample size to date for the exploration of this issue. A recent study by Valenti et al found that predominantly hepatocellular iron was associated with an increased likelihood of fibrosis stage >1 in 587 Italian NAFLD patients, while predominantly nonparenchymal iron was not.12 Differences in the patient population between the current study and the report by Valenti et al may explain these seemingly discordant data.22 These include a higher proportion of subjects with stage 3-4 fibrosis (28% versus 14% in Valenti et al.'s study), a higher mean BMI, and greater ethnic diversity. In addition, 60% of the subjects in the present study had definitive NASH; Valenti et al. did not report the proportion of patients with NASH.

The mechanism of differential iron deposition in liver cells for patients with NAFLD and NASH is unclear but is likely multifactorial; gender, age, menstruation, and a lower BMI but not diet, alcohol, or previous gastrointestinal bleeding likely contribute to overall hepatic iron deposition. We also speculate that the degree and pattern of hepatic iron deposition in NAFLD may be related to the dual regulatory mechanism (iron stores and inflammation) of the key body iron regulator hepcidin. Hepcidin plays a central role in iron regulation by binding to and internalizing the cellular iron export protein ferroportin and thus down-regulating iron efflux from enterocytes, macrophages, and hepatocytes.23, 24 Hepcidin is regulated in response to iron stores via the bone morphogenetic protein/HJV/SMAD pathway25 or via the HFE/TFR1/TFR2 complex in response to plasma transferrin levels.26-28 Thus, increased HC iron in patients with NAFLD may be due to increased iron absorption as a result of decreased hepcidin activity, possibly via mutations in hepcidin regulatory genes such as HFE, TFR1 or TFR2, HJV, ferroportin, and bone morphogenetic proteins. We recently reported that over half of 126 NASH patients with hepatic iron staining carried common mutations in the HFE gene.29 Moreover, because hepcidin is expressed in adipose tissue, our observation that subjects with HC iron had a lower BMI is consistent with the hypothesis that decreased serum hepcidin levels from less adipose mass result in increased iron absorption.30 Hepcidin expression is also induced during inflammation by activation of the transcription factor signal transducer and activator of transcription 3 by the inflammatory cytokines interleukin-6 (IL-6) and IL-131, 32 and has also recently been shown to be up-regulated by endoplasmic reticulum stress.33 RES cell iron accumulation in NAFLD may be due to an increased systemic inflammatory state and/or other as yet undefined stimuli that increase hepatic necroinflammation and erythrocyte fragility and result in increased iron uptake by Kupffer and other hepatic RES cells.34 Iron may then subsequently be retained within Kupffer cells and adjacent sinusoidal lining cells because of inflammatory mediated up-regulation of hepcidin expression. Up-regulation of hepcidin via IL-6 is the mechanism responsible for the anemia of inflammation often observed with chronic disease and associated with iron sequestration in Kupffer cells and other macrophages.35

Our data are consistent with numerous studies suggesting that the consequences of iron overload in the liver are related to the role of iron in catalyzing the production of ROS, which cause lipid peroxidation and stimulate a variety of proinflammatory, profibrogenic, and cytotoxic pathways via the induction of the redox-sensitive transcription factor nuclear factor κB in Kupffer cells (the main component of RES).36-41 Hepatic iron deposition also leads to activation of hepatic stellate cells and deposition of extracellular matrix components such as collagen types I and III.34, 42-46 A number of studies suggest that this process may be mediated by iron-induced oxidative stress, particularly in Kupffer cells.1, 34, 42-44 Further support for this concept comes from a recent study by Otogawa et al.,34 who showed that iron depletion by phlebotomy in a rabbit model of NAFLD was associated with significant reductions in Kupffer cell iron deposition, serum levels of lipid peroxidation and hydroxyproline (a marker of fibrosis), deposition of collagen and α-smooth muscle actin (a marker of hepatic stellate cell activation), and apoptosis. Thus, it is likely that the localized effects of iron, particularly in Kupffer cells and other RES cells, may play a role in the progression of NASH.

A novel finding of this study is the inverse association between HC iron and phenotypic features of metabolic syndrome (including lower BMI and HOMA-IR) as well as milder histological findings among NAFLD patients. We speculate that these subjects may represent a novel form of NAFLD independent of the presence of metabolic syndrome and instead related to the localized pathophysiology of iron, such as direct cytotoxicity and ROS formation. It is also possible that in contrast to Kupffer cells, ROS may not be as pathogenic when they are present in hepatocytes, and this results in the milder phenotype of these patients. In agreement with our hypothesis that HC iron deposition and RES iron deposition result from separate cellular processes resulting in divergent hepcidin signaling, the presence of RES iron in mixed patients likely appears after the establishment of HC iron and thus exacerbates the mild HC phenotype; this results in intermediate disease severity for these patients.

Our study has practical clinical implications for the management of NASH. First, we found that hepatic iron deposition was common in this unselected population of patients with NAFLD. Furthermore, RES cell iron was found to be an independent predictor of advanced fibrosis and to be associated with histological severity. Therefore, these data provide support for the implementation of clinical trials examining iron depletion as a treatment for NASH. Phlebotomy is safe and well tolerated, has been shown to lower serum ferritin and ALT levels, and may improve insulin sensitivity as measured by HOMA-IR in NAFLD subjects.47-50

We recognize that the current study has limitations. We did not have data on hepatic hepcidin gene expression or serum hepcidin levels and did not have information on HFE mutation status or biochemical hepatic iron measurements for our cohort. We also recognize that longitudinal follow-up studies will be required to definitively establish that RES cell iron causes more rapid disease progression and increased fibrosis in NAFLD.

In summary, our results have demonstrated novel relationships between the presence and pattern of hepatic iron staining and histological severity in a large, systematic, unselected multicenter national cohort of patients with NAFLD. Further studies are warranted to define the mechanisms of hepatic iron deposition in NASH, the contribution of hepatic iron to disease severity and progression, and the possible role of iron depletion as a treatment for this common disorder.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Patients and Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
  8. Supporting Information

The authors thank Pat Belt (NASH CRN Data Coordinating Center) for her assistance with the data preparation and Jay H. Hoofnagle, M.D. (National Institute of Diabetes and Digestive and Kidney Diseases), for his careful review of and contributions to the final manuscript.

References

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  2. Abstract
  3. Patients and Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
  8. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. Patients and Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
  8. Supporting Information

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