Excess of
uric acid is mainly treated with
xanthine oxidase (XO) inhibitors, also called uricostatics because they block the conversion of
hypoxanthine and
xanthine into
urate. Normally, accumulation of upstream metabolites is prevented by the
hypoxanthine-guanine phosphoribosyltransferase (
HPRT)
enzyme. The recycling pathway, however, is impaired in the presence of
HPRT deficiency, as observed in
Lesch-Nyhan disease. To gain insights into the consequences of
purine accumulation with
HPRT deficiency, we investigated the effects of the XO inhibitor
allopurinol in
Hprt-lacking (
HPRT-/-) mice.
Allopurinol was administered in the
drinking water of E12-E14 pregnant mothers at dosages of 150 or 75 μg/ml, and mice sacrificed after weaning. The drug was well tolerated by wild-type animals and heterozygous
HPRT+/- mice. Instead, a profound alteration of the renal function was observed in the
HPRT-/- model. Increased
hypoxanthine and
xanthine concentrations were found in the blood. The kidneys showed a yellowish appearance, diffuse
interstitial nephritis, with dilated tubules, inflammatory and fibrotic changes of the interstitium. There were numerous
xanthine tubular crystals, as determined by HPLC analysis.
Oil red O staining demonstrated
lipid accumulation in the same location of
xanthine deposits.
mRNA analysis showed increased expression of adipogenesis-related molecules as well as profibrotic and proinflammatory pathways. Immunostaining showed numerous monocyte-macrophages and overexpression of alpha-smooth muscle actin in the tubulointerstitium. In vitro, addition of
xanthine to tubular cells caused diffuse
oil red O positivity and modification of the cell phenotype, with loss of epithelial features and appearance of mesenchymal characteristics, similarly to what was observed in vivo. Our results indicate that in the absence of
HPRT, blockade of XO by
allopurinol causes rapidly developing
renal failure due to
xanthine deposition within the mouse kidney.
Xanthine seems to be directly involved in promoting
lipid accumulation and subsequent phenotype changes of tubular cells, with activation of
inflammation and
fibrosis.