Diversity of effects of fructose loads on different parts of the nephron

Transcriptome signature for dietary fructose-specific changes in rat renal cortex: A quantitative approach to physiological relevance

Fructose consumption causes metabolic diseases and renal injury primarily in the renal cortex where fructose is metabolized. Analyzing gene differential expression induced by dietary manipulation is challenging. The effects may depend on the base diet and primary changes likely induce secondary or higher order changes that are difficult to capture by conventional univariate transcriptome analyses. We hypothesized that dietary fructose induces a genetic program in the kidney cortex that favors lipogenesis and gluconeogenesis. To test this, we analyzed renal cortical transcriptomes of rats on normal- and high-salt base diets supplemented with fructose. Both sets of data were analyzed using the Characteristic Direction method to yield fructose-induced gene vectors of associated differential expression values. A fructose-specific "signature" of 139 genes differentially expressed was extracted from the 2 diet vectors by a new algorithm that takes into account a gene's rank and standard deviation of its differential expression value. Of these genes, 97 were annotated and the top 34 accounted for 80% of the signal in the annotated signature. The genes were predominantly proximal tubule-specific, coding for metabolic enzymes or transporters. Cosine similarity of signature genes in the two fructose-induced vectors was >0.78. These 139 genes of the fructose signature contributed 27% and 38% of total differential expression on normal- and high- salt diet, respectively. Principal Component Analysis showed that the individual animals could be grouped according to diet. The fructose signature contained a greater enrichment of Gene Ontology processes related to nutrition and metabolism of fructose than two univariate analysis methods. The major feature of the fructose signature is a change in metabolic programs of the renal proximal tubule consistent with gluconeogenesis and de-novo lipogenesis. This new "signature" constitutes a new metric to bridge the gap between physiological phenomena and differential expression profile.

Dietary Fructose Enhances the Ability of Low Concentrations of Angiotensin II to Stimulate Proximal Tubule Na⁺ Reabsorption

Fructose-enriched diets cause salt-sensitive hypertension. Proximal tubules (PTs) reabsorb 70% of the water and salt filtered through the glomerulus. Angiotensin II (Ang II) regulates this process. Normally, dietary salt reduces Ang II allowing the kidney to excrete more salt, thereby preventing hypertension. We hypothesized that fructose-enriched diets enhance the ability of low concentrations of Ang II to stimulate PT transport. We measured the effects of a low concentration of Ang II (10⁻¹² mol/L) on transport-related oxygen consumption (QO₂), and Na/K-ATPase and Na/H-exchange (NHE) activities and expression in PTs from rats consuming tap water (Control) or 20% fructose (FRUC). In FRUC-treated PTs, Ang II increased QO₂ by 14.9 ± 1.3 nmol/mg/min (p < 0.01) but had no effect in Controls. FRUC elevated NHE3 expression by 19 ± 3% (p < 0.004) but not Na/K-ATPase expression. Ang II stimulated NHE activity in FRUC PT (Δ + 0.7 ± 0.1 Arbitrary Fluorescent units (AFU)/s, p < 0.01) but not in Controls. Na/K-ATPase activity was not affected. The PKC inhibitor Gö6976 blocked the ability of FRUC to augment the actions of Ang II. FRUC did not alter the inhibitory effect of dopamine on NHE activity. We conclude that dietary fructose increases the ability of low concentrations of Ang II to stimulate PT Na reabsorption via effects on NHE.

Urine glyceraldehyde excretion is elevated in the renal Fanconi syndrome

We analyzed urinary constituents using GC/MS in 16 children with the renal Fanconi syndrome and 13 normal individuals. Urine glyceraldehyde levels were strikingly elevated in the renal Fanconi syndrome group (mean 5.1 +/- 4.8 mg/mg creatinine) compared to levels in the normal group (mean 0.04 +/- 0.04 mg/mg creatinine, P less than 0.001). Urine lactate levels were also elevated in the renal Fanconi syndrome group (mean 2.3 +/- 2.6 mg/mg creatinine) compared to normals (mean 0.01 +/- 0.01 mg/mg creatinine, P less than 0.003). Only small elevations of glyceraldehyde and lactate were found in urine from children with other renal disorders. Serum levels of glyceraldehyde and lactate were no greater in individuals with the Fanconi syndrome than in the normals. The fractional reabsorption of both glyceraldehyde and lactate was virtually complete in the normals, but was markedly impaired in the Fanconi syndrome patients where, in some cases, glyceraldehyde excretion greatly exceeded the excretion of creatinine. We conclude that marked glyceraldehyde excretion is a previously unrecognized feature of the renal Fanconi syndrome which may result from disordered proximal tubular glycolytic metabolism. Further studies will be required to determine the role of glyceraldehyde loss in the pathogenesis of this generalized disturbance of proximal tubular function.

Isolation of cells from rabbit renal proximal tubules by using a hyperosmolar intracellular-like solution

A novel method of isolation of cells from rabbit kidney proximal tubules by using an intracellular-like solution (ICS) and gentle mechanical agitation in the absence of enzymes or chelators is described. Metabolic and functional characteristics of these cells were studied after washing and resuspension in modified Hanks medium, and the results were compared with those obtained in cells similarly prepared in extra-cellular-like solution (ECS). Trypan Blue exclusion and protein content were not different between the two preparations. However, oxygen consumption, ATP content and time- and concentration-dependent rates of uptake of phosphate, alpha-methyl glucoside and L-alanine were severalfold higher in cells prepared in ICS. Na+-dependent uptake of these solutes was 95% and 80% of total uptake in cells prepared in ICS and ECS respectively. Maximum transport rates (Tmax.) of phosphate, alpha-methyl glucoside and L-alanine were significantly higher in cells prepared in ICS. We propose that the use of ICS in the isolation procedure would yield a functionally more viable cell preparation, and therefore provides an ideal model for transport and metabolic studies at a cellular level.