Discussion
Macrophages could promote renal fibrosis, a major driver of progression
to end-stage kidney disease, and M2 macrophages is strongly associated
with kidney fibrosis in both human and experimental diseases. In this
study, we report that macrophage ITGAM contributes to kidney fibrosis in
hyperuricemia-related CKD. Mechanistically, ITGAM promotes macrophage M2
polarization through activating FAK/Akt1/β-catenin pathway.
ITGAM, also known as CD11b, was commonly used as a monocyte/macrophage
surface marker(Ross & Lambris, 1982; Wolf et al., 2018). Gradually,
ITGAM diverse functions were reflected by its rapid confirmational
change that increased affinity for its more than 40 ligands, including
ICAM-1(Dunne et al., 2002), fibrinogen(Altieri et al., 1990),
fibronectin(Kanse et al., 2004), GPIbα(Ehlers et al., 2003),
RAGE(Chavakis et al., 2003), JAM-c(Lange-Sperandio et al., 2006), and
others(Fan & Ley, 2015). We examined main ligands frequently reported,
and found ICAM-1 and fibronectin were significantly upregulated
(Figure 1E and Supplementary Figure 4A-B ), which
indicated potential role of ITGAM in cell-ECM and cell-cell interactions
in kidneys of hyperuricemia-related CKD. Further studies are needed to
clarify how ITGAM mediates cross-talk between macrophage and other cells
or compartments in kidney tissue. ITGAM was reported to be involved in
various immune responses but with bidirectional effects(Rosetti &
Mayadas, 2016). Negative regulation of ITGAM could be observed in
systemic lupus erythematous and acute infectious diseases(Hu et al.,
2016; Villanueva et al., 2022). On the contrary, ITGAM positively
regulated chronic inflammatory diseases(Zirlik et al., 2007).
Lange-Sperandio et al reported that high expression of Mac-1 and its
ligands ICAM-1 and JAM-3 in murine unilateral ureteric obstruction (UUO)
model, and knockout of Mac-1 greatly attenuated kidney
fibrosis(Lange-Sperandio et al., 2006). Taking our results together, the
functional role of ITGAM in contributing to kidney fibrosis through
promoting M2 polarization could be inferred.
Both of integrin α and β subunits are single membrane-spanning segments
with short cytoplasmic tails, thus need to interact with downstream
tyrosine kinases to enable “outside-in” signal transduction(Kornberg
et al., 1991; Takada et al., 2007). Binding to ligands (e.g
extracellular matrix) induces integrins clustering at focal adhesions
and connecting to intracellular molecules. FAK is one of the earliest
identified central integrin signaling. The activated FAK undergoes
autophosphorylation and then stimulates difference molecules
corresponding to various pathways, including Src, Akt, Grb7, and
others(Guan, 2010). Numerous studies reported FAK/Akt pathway
participated in chronic inflammation and fibrosis, such as
atherosclerosis(Yamaura et al., 2019), and lung and liver
fibrosis(Gimenez et al., 2017; Lv et al., 2018). Of notes, we
additionally observed the change in Src phosphorylation and found p-Src
were both attenuated after silencing Itgam and inhibiting p-FAK. Our
finding indicated that Src participated in ITGAM/FAK signaling and might
act as the downstream molecular. It’s to be further explored whether Src
together with FAK stimulates Akt1, or acts through other pathways, such
as Stat3 and Erk (Bolos et al., 2010; Jang et al., 2022; Pan et al.,
2019). β-catenin activation, as the canonical pathway of Wnt signaling,
is associated with macrophage M2 polarization in tumor malignant
behaviors and renal fibrosis(Feng et al., 2018; Yang et al., 2018).
Several studies have shown that Akt activation and phosphorylation of
GSK-3β led to β-catenin stabilization. Accumulated β-catenin in cytosol
then translocates into nucleus and stimulates transcription of target
genes that participates in tissue fibrosis(MacDonald et al., 2009;
Niehrs, 2012).
To our best acknowledgement, this is the first paper investigating the
role of integrin and how it regulates macrophage M2 polarization in
kidneys of hyperuricemia-related CKD. Hyperuricemia-related, especially
hyperuricemia-induced CKD is relatively less studied, due to long time
and difficulty when modeling. We firstly fed mice with potassium oxonate
only at high dose (4.8g/kg) for up to 14 weeks with the purpose of
establishing uric acid-induced CKD, but only found moderately increased
serum uric acid (1.5-2 times of baseline) and almost unchanged kidney
tissue histology. Urate oxidase-knockout mouse is an optimal option, but
low survival greatly limited experiment feasibility and
reproducibility(Lu et al., 2018; Zhao et al., 2022). Not only signal
pathway ITGAM regulates, how ITGAM mediates cell-cell or ECM-cell
cross-talk is also potential mechanism of great meaning to be further
explored.
In summary, macrophage integrin ITGAM might play the vital role in
mediating kidney fibrosis in hyperuricemia-related CKD through
activating its downstream pathway FAK/Akt1/β-catenin and promoting
macrophage M2 polarization. This finding provides new insight into the
prevention and treatment of kidney fibrosis induced by uric acid, and
acts as potential therapeutic target, although more well-designed
studies are needed to verify further mechanisms.