Background Hyperuricemia is the key pathophysiological basis of gout^[1]. The most common reason generating hyperuricemia is verified the obstacle of urate excretion by current researches^[2]. The intestine is known as the most important organ involved the excretion of uric acid besides the kidney^[1]. ABCG2 has an outsized genetic contribution to extra-renal underexcretion and causes a compensatory increase in urinary urate output^[3]. PDZK1 is a kind of an important structural protein regulated ABCG2 function, by potential molecular interaction with ABCG2^[4].

Objectives The present study was undertaken to explore the effect and its related mechanisms of soluble uric acid on the urate excretion PDZK1 and ABCG2 in HT-29 and Caco-2 cell lines

Methods HT-29 and Caco-2 cell lines were used as a well-established model of human intestinal epithelial cells. Cells were pretreated with or without inhibitors and then stimulated with soluble uric acid. siRNA transfection was used to assess the interaction between ABCG2 and PDZK1. qRT-PCR and western blotting were used to measure mRNA and protein levels, respectively. Subcellular fractionation methods and immunofluorescence were used to examine the proteins in different subcellular compartments. Flow cytometry experiments examined the function of ABCG2.

Results Soluble uric acid significantly up regulated the expression of PDZK1 and ABCG2 in human intestinal cell lines. It also activated TLR4/NLRP3/caspase-1 inflammasome and PI3K/AKT signalling pathway through the phosphorylation of the ser473 pAkt. The increased expression of PDZK1 and ABCG2 was suppressed by a block of TLR4 (TAK-242) and caspase-1 inhibitor (acetyl–YVAD–chloromethylketone) and partly reduced by wortmanning, a specific inhibitior of PI3K. Additionally, lipopolysaccharide (LPS), the potent inducer of inflammatory responses mediated through TLR4, activated TLR4/NLRP3/caspase-1 inflammasome and up regulated the expression of ABCG2 and PDZK1. Besides, the stimulation of soluble uric acid facilitated the translocation of ABCG2 from intracellular compartment to plasma membrane and increased the transport activity. Furthermore, PDZK1 knockdown via siRNA significantly inhibited the expression and transport activity of ABCG2 regardless of the activation by soluble uric acid.

Conclusions The PI3K/AKT and TLR4/NLRP3/caspase-1 signaling pathways modulate ABCG2 and PDZK1 expression stimulated by soluble uric acid in human intestinal cells. PDZK1 plays a pivotal role in the regulation of ABCG2.

References

1. Dalbeth N, Merriman TR, Stamp LK. Gout. Lancet. 2016;388(10055):2039–52.

2. Perez-Ruiz F, Calabozo M, Erauskin GG, Ruibal A, Herrero-Beites AM. Renal underexcretion of uric acid is present in patients with apparent high urinary uric acid output. Arthritis and rheumatism. 2002;47(6):610–3.

3. Matsuo H, Tsunoda T, Ooyama K, Sakiyama M, Sogo T, Takada T, et al. Hyperuricemia in acute gastroenteritis is caused by decreased urate excretion via ABCG2. Scientific reports. 2016;6:31003.

4. Takada Y, Matsuo H, Nakayama A, Sakiyama M, Hishida A, Okada R, et al. Common variant of PDZK1, adaptor protein gene of urate transporters, is not associated with gout. The Journal of rheumatology. 2014;41(11):2330–1.

References

Disclosure of Interest None declared