RESEARCH ARTICLE
Involvement of xanthine oxidase and hypoxia-inducible factor 1
in Toll-like receptor 7/8-mediated activation of caspase 1
and interleukin-1b
Sally A. Nicholas
•
Vladimir V. Bubnov
•
Inna M. Yasinska
•
Vadim V. Sumbayev
Received: 11 February 2010 / Revised: 18 May 2010 / Accepted: 29 June 2010 / Published online: 15 July 2010
Ó Springer Basel AG 2010
Abstract Inflammatory reactions to ssRNA viruses are
induced by the endosomal Toll-like receptors (TLRs) 7 and
8. TLR7/8-mediated inflammatory reaction results in acti-
vation of the Nalp3 inflammasome via an unknown
mechanism. Here we report for the first time that TLR7/8
mediate activation of xanthine oxidase (XOD) in an HIF-
1a-dependent manner. XOD produces uric acid and reac-
tive oxygen species, which could activate Nalp3 and
therefore induce activation of caspase 1, known to convert
inactive pro-IL-1b into active IL-1b. Specific inhibition of
the XOD activity attenuates TLR7/8-mediated activation of
caspase 1 and IL-1b release. These results were obtained
using human THP-1 myeloid macrophages. The findings
were verified by conducting in vivo experiments on mice.
Keywords Inflammation Á Toll-like receptors Á
Inflammasome Á HIF-1 Á IL-1b Á Caspase 1
Introduction
ssRNA viruses [human immunodeficiency virus 1 (HIV-1),
influenza, hepatitis C, many hemorrhagic fevers, etc.]
cause severe illnesses, resulting in significant morbidity
and mortality in both industrialised and developing coun-
tries [1]. Inflammatory reactions to ssRNA viruses are
induced by the endosomal Toll-like receptors (TLRs) 7 and
8 expressed by various cell types, including myeloid
macrophages and neutrophils [2]. Ligand-induced TLR7/8-
mediated inflammatory reactions result in the production of
pro-inflammatory cytokines including interleukin 1 beta
(IL-1b), interleukin 6 (IL-6), interleukin 12 (IL-12) and
tumour necrosis factor alpha (TNF-a). Among these cyto-
kines only generation of IL-1b is induced indirectly via
activation of the multiprotein complex known as the
inflammasome. Production of other cytokines is controlled
directly by the transcription factors activated by TLR7/8
downstream pathways [1–3]. IL-1b is a highly inflamma-
tory cytokine, which is considered as a major contributor to
further development of the host immune defence [3]. IL-1b
is activated via caspase 1-dependent conversion of the
inactive pro-IL-1b. Caspase 1 is activated in this case by
the inflammasome formed by the Nod-like receptor Nalp3.
Active Nalp3 complexed to the apoptosis-associated speck-
like protein (ASC) interacts with the caspase 1 forming the
inflammasome, which induces the proteolytic caspase
activity of the enzyme [3–5]. However, the biochemical
mechanisms of the cross-talk between TLR7/8 and the
Nalp3 inflammasome remain unknown. It is, however,
known that Nalp3 could be activated by uric acid (UA),
formed by the enzyme xanthine oxidase (XOD), which
converts hypoxanthine into xanthine and further into UA
[6, 7]. This enzyme is activated in response to some TLR
ligands (for example, LPS—the ligand of TLR4 [8, 9]) as
well as to pro-inflammatory cytokines and interferons [8].
These factors promote XOD expression. Both expression
and assembly of XOD require energy [8,
9]. Energy
metabolism is, however, supported during the TLR7/8-
mediated inflammatory reaction by the hypoxia-inducible
factor 1 alpha (HIF-1a), the inducible subunit of the HIF-1
transcription complex, which promotes glycolysis and
S. A. Nicholas Á I. M. Yasinska Á V. V. Sumbayev (&)
Medway School of Pharmacy, University of Kent,
Anson Building, Central Avenue, Chatham Maritime,
Kent ME4 4TB, UK
e-mail: V.Sumbayev@kent.ac.uk
V. V. Bubnov
Odessa State Medical University, Odessa, Ukraine
Cell. Mol. Life Sci. (2011) 68:151–158
DOI 10.1007/s00018-010-0450-3
Cellular and Molecular Life Sciences