Toxic Effects of Indoxyl Sulfate on Osteoclastogenesis and Osteoblastogenesis

Abstract

Uremic toxins, such as indoxyl sulfate (IS) and kynurenine, accumulate in the blood in the event of kidney failure and contribute to further bone damage. To maintain the homeostasis of the skeletal system, bone remodeling is a persistent process of bone formation and bone resorption that depends on a dynamic balance of osteoblasts and osteoclasts. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that regulates the toxic effects of uremic toxins. IS is an endogenous AhR ligand and is metabolized from tryptophan. In osteoclastogenesis, IS affects the expression of the osteoclast precursor nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) through AhR signaling. It is possible to increase osteoclast differentiation with short-term and low-dose IS exposure and to decrease differentiation with long-term and/or high-dose IS exposure. Coincidentally, during osteoblastogenesis, through the AhR signaling pathway, IS inhibits the phosphorylation of ERK, and p38 reduces the expression of the transcription factor 2 (Runx2), disturbing osteoblastogenesis. The AhR antagonist resveratrol has a protective effect on the IS/AhR pathway. Therefore, it is necessary to understand the multifaceted role of AhR in CKD, as knowledge of these transcription signals could provide a safe and effective method to prevent and treat CKD mineral bone disease.

Keywords: aryl hydrocarbon receptor; bone remodeling; indoxyl sulfate; osteoblast; osteoclast.

Figure 1

The bone remodeling cycle. The purpose of constant bone remodeling is to maintain normal bone mass as a dynamic equilibrium between resorption and formation. Mesenchymal progenitors trigger the activation of canonical Wnt/β-catenin signaling to promote the differentiation of osteoblasts. Osteoblasts produce runt-related transcription factor 2 (Runx2) as the earliest marker and various extracellular matrix proteins, such as ALP, type I collagen, osteocalcin, osteopontin, and ostenectin, to accomplish bone formation and mineralization. Mature osteoblasts produce RANKL to achieve osteoclast differentiation. Alternatively, osteoblasts also release OPG to attenuate the osteoclast stimulant pathway. Macrophage colony stimulation factor (M-CSF) is another modulator of osteoclast differentiation produced by osteoblasts. Osteoclasts secrete acid and enzymes to absorb the bone matrix and release transforming growth factor-beta (TGF-β) and insulin-like growth factors (IGFs) to control the activity of osteoblasts. Moreover, osteoclasts express ephrin B2 ligand and sphingosine 1-phosphate (S1P) to bind with the EphB4 receptor on osteoblasts, which can promote osteoblast differentiation and can suppress osteoclast differentiation. Thus, there is a close link between osteoclastic bone resorption and osteoblastic bone formation.

Figure 2

Bone disorders in CKD. When CKD occurs, the accumulation of uremic toxins (indoxyl sulfate) can directly damage bone cells. Early on in CKD, an electrolyte imbalance and decreased vitamin D levels cause low-turnover bone disease despite a slight increase in PTH. However, when the renal function further deteriorates, the serum PTH levels become persistently high and vitamin D levels may decline more, which may cause high-turnover bone disease, as this can override peripheral PTH resistance and other bone-formation inhibitors.

Figure 3

Time and dose dependent of IS on osteoclastogenesis. Short-term exposure in a low IS dose and ARNT are available, and AhR works as a ligand-activated transcription factor, increasing NFATc1 expression, and thus increasing osteoclastogenesis. On the contrary, long-term exposure in high doses of IS and ARNT are inaccessible, and AhR functions as an E3 ubiquitin ligase, leading to the proteasomic degradation of NFATc1 and thereby inhibiting osteoclastogenesis.

Figure 4

Resveratrol (RSV) restores the reduction in ERK and p38 MAPK phosphorylation caused by IS in osteoblasts. IS inhibits the phosphorylation of ERK and p38 MAPK through the AhR pathway in the osteoblast cytoplasm, which then reduces the expression of Runx2 to impede osteoblast differentiation. RSV, an AhR antagonist, restores the reduction in ERK and p38 MAPK phosphorylation due to IS and then improves the expression of Runx2 to promote osteoblast differentiation.