A furan fatty acid and indoxyl sulfate are the putative inhibitors of thyroxine hepatocyte transport in uremia

Abstract

We studied the effects of 3-carboxy-4-methyl-5-propyl-2-furan-propanoic acid (CMPF), indoxyl sulfate, and hippuric acid on iodide production by rat hepatocytes in primary cultures. We questioned whether these substances could explain the alteration of serum thyroid hormone parameters observed in renal failure. Iodide production from [125I]T4 by rat hepatocytes was significantly inhibited in the presence of serum from uremic patients. Serum concentrations of CMPF, indoxyl sulfate, and hippuric acid were markedly elevated in uremic patients. The minimum concentration that inhibited iodide production, when expressed as a molar ratio of the inhibitor to BSA, was 0.13 for CMPF, 0.53 for indoxyl sulfate, and 1.33 for hippuric acid. This molar ratio was lower than the corresponding mean molar ratio in uremic sera for CMPF (0.38) and indoxyl sulfate (0.63), while it was higher than that found for hippuric acid (0.85). The inhibition was reproduced when the inhibitors were added to normal human serum. The decreased iodide production was not due to the inhibition of deiodinase activity. The deiodination of rT3 by rat liver microsomes was unaffected by these inhibitors. Charcoal adsorption of uremic serum normalized the iodide production by hepatocytes. This normalization coincided with almost complete removal of CMPF and indoxyl sulfate, with a concomitant reduction of the free T4 fraction. Dialysis of uremic serum only partially restored iodide production. Even though indoxyl sulfate and hippuric acid were no longer detectable, a high concentration of CMPF remained in the serum. The serum free T4 fraction remained elevated in uremic patients after dialysis. Our studies indicate that CMPF and indoxyl sulfate in concentrations normally present in the serum of uremic patients inhibit cellular transport and subsequent deiodination of T4. These substances may account for the low total T3 level in uremic patients.

Thyroid Hormone Metabolism and Thyroid Diseases in Chronic Renal Failure

“…Free fractions of T 4 by tracer equilibrium dialysis are normal (32,33) or increased in ESRD patients (34), T 4 -binding globulin (TBG) concentrations are normal (33)(34)(35)(36)(37)(38) or increased (32,39), and transthyretin concentrations are normal (34), while serum albumin levels may be reduced (32,34,35). Inhibitors of T 4 binding to serum carrier proteins in euthyroid uremic patients may include elevated serum levels of 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF), indoxyl sulfate, and hippuric acid in uremic serum (17,40), as well as increased serum levels of interleukin-l|3 (IL-1/3), tumor necrosis factor (TNF-a), and their respective specific inhibitors (41). In nonuremic patients, elevated levels of oleic acid (42), interleukin-6 (IL-6) (43), and TNF-a (44) may reduce T 4 binding to serum carrier proteins, as may elevated bilirubin and nonesterified fatty acids in association with hypoalbuminemia in hepatic failure (45).…”

“…In addition, exogenous inhibitors of T 4 binding to serum carrier proteins, such as furosemide, nonsteroidal antiinflammatory drugs, and heparin, may play a role (46). Sera from sick patients also inhibit in vitro T 4 binding to solid matrices in assays (47) In addition to inhibiting T 4 binding to serum carrier proteins, CMPF, hippuric acid, and indoxyl sulfate in uremic sera and bilirubin and nonesterified fatty acids in nonuremic sera inhibit T 4 uptake by rat hepatocytes in vitro (17,40,45,49). Circulating inhibitors from sera of other sick patients also interfere with uptake of T 4 by rat hepatocytes (47) and cultured human hepatoma cells (50).…”

Effects of Indoxylsulfate on the in vitro Hepatic Metabolism of Various Compounds Using Human Liver Microsomes and Hepatocytes

“…Because the plasma concentrations of indoxylsulfate in patients with CRF were considerably high (417 ± 217 µ M ) compared to those in normal subjects (16 ± 11 µ M ) [30] and highly variable (74–940 µ M ) [3,31,32], relatively high concentrations (500–800 µ M ) of uremic substances were applied in this study. 3-Indoxylsulfate inhibited the formation of resorufin and 6β-hydroxytestosterone in a noncompetitive manner (fig.…”

“…3-Indoxylsulfate may be taken up into the hepatocytes actively. Although the transport of thyroxine into hepatocytes has been shown to be inhibited by indoxylsulfate [30], there has been no reported study supporting the active transport of indoxylsulfate into hepatocytes. Although the plasma concentration of indoxylsulfate is highly variable, the results of microsome and hepatocytes suggest that relatively high concentration of indoxylsulfate in the liver is required to cause clinically significant inhibition of metabolism.…”

Production of antiserum for sensitive enzyme‐linked immunosorbent assay of 3‐carboxy‐4‐methyl‐5‐propyl‐2‐furanpropanoic acid by chemiluminescence

“…In particular, 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF) (Scheme 1) in an endogenous, ligand is tightly bound to serum protein (17,18) and, there-fore, the concentration of CMPF in uremic plasma is not significantly decreased by hemodialysis (19). Recently, it was suggested that CMPF inhibits transport of thyroxine into hepatocytes in uremia (20), inhibits the active uptake of thyroxine into anterior pituitary cells (21), and enhances drug inhibition of thyroxine binding in serum (22). Analysis of CMPF has previously been performed by gas chromatographic (23), and gas chromatographic-mass spectrometric techniques (24), as well as by high-performance liquid chromatography (19) but these methods require sample preparation and can be inconvenient for routine screening of many samples.…”