The release of EPO by the kidneys, a hormone that is important in the regulation of erythropoiesis, is triggered by tissue hypoxia (22). Under hypoxic conditions, EPO stimulates the differentiation of erythroid progenitor cells and normoblasts to increase the amount of red blood cell (RBCs) (22). Hypoxia-inducible factors (HIFs) are heterodimeric proteins that regulate the physiological response to hypoxia by altering the expression of downstream genes, including EPO, in interstitial cells in the renal cortex near the proximal tubule (23).

There may be a negative feedback between EPO and α-KL such that EPO promotes the expression of α-KL and increasing α-KL suppresses the production of EPO in CKD (22,24). KL deficiency is a characteristic feature of CKD with anemia (22). Hu et al (10) revealed that KL expression was reduced during the progression of kidney disease. In addition, the treatment of anemia with EPO has been demonstrated to enhance renal and extrarenal production of α-KL in patients with CKD (14,24). Furthermore, EPO has been demonstrated to inhibit uremia-induced nuclear factor-κB (NF-κB) production in endothelial cells and prevented a decrease in intracellular KL (24). However, a previous study reported that KL directly regulates hematopoietic stem cell differentiation, and erythroid cell generation and maturation (22). KL deficiency in mice resulted in increased erythropoiesis through activation of the HIF signaling pathway, and subsequent upregulation of renal EPO synthesis and secretion (22). Additionally, the expression of HIF-1α and HIF-2α was significantly upregulated in KL⁻/⁻ bone tissue, resulting in localized overexpression of EPO (22). The molecular mechanism responsible for the KL-induced inhibition of the HIF signaling pathway and EPO expression may be associated with reduced osteoblast numbers and osteopenia (22). The effects of α-KL on EPO and the HIF signaling pathway are summarized in Fig. 1.

Kempe et al (25) demonstrated that a lack of KL expression leads to increased cytosolic Ca²⁺ activity, leading to enhanced scrambling of cell membrane phospholipids and cell shrinkage in erythrocytes. This suggests that KL deficiency accelerates eryptosis, the suicidal death of erythrocytes. Thus, the role of KL in hemopoiesis remains unclear and further studies into this area are warranted.

Iron-deficiency anemia (IDA) is responsible for ~50% of all anemia cases (26). It has been reported that 273,000 individuals succumb to IDA each year worldwide (26). ‘Absolute’ iron deficiency is common in patients with CKD and anemia (27). Different conditions, including inflammation, liver disease and pregnancy, may induce iron-restricted erythropoiesis and aggravate renal anemia (1). The distinction between these conditions is clinically relevant; however, peripheral iron indices are of little help in the differential diagnosis (28).

Iron deficiency may lead to high expression of KL in patients with CKD. A study of 70 patients with stages I–V CKD demonstrated that the FGF23 levels were elevated and KL levels were decreased, and the magnitude of these changes increased as the tumor stage increased (29). In addition, significant correlations were identified between serum KL levels and ferritin levels, and serum KL levels and transferrin saturation percentage, which suggest that KL serves a negative role in iron regulation (29). However, little is known about the underlying molecular mechanisms of these effects. A previous study demonstrated that serum iron overload decreased renal expression of KL at the mRNA and protein levels, and that iron chelation suppressed angiotensin II-induced downregulation of KL (30). These results indicate that the underlying mechanism of the angiotensin II-induced downregulation of KL is the alteration of iron metabolism in the kidney. Therefore, the renin-angiotensin system may serve a role in the regulation of iron and α-KL.

HIFs affect the majority of aspects of iron metabolism (23). HIF-2 enhances iron absorption by small bowel enterocytes and augments iron export from duodenal cells (23). Additionally, HIFs serve a role in the utilization of iron from senescent RBCs via reticuloendothelial system macrophages (31). In this manner HIFs increases iron stores through absorption from the duodenum and the recycling of senescent RBCs (31). The increased efflux of iron to the circulation is achieved via HIF-mediated increased expression of ferroportin in duodenal enterocytes and macrophages (31). KL deficiency in mice results in the activation of the HIF signaling pathway (22). The effects the HIF signaling pathway on iron supplies are summarized in Fig. 1.

Accumulating evidence suggests that vitamin D deficiency increased the progression of CKD (32–34). Vitamin D is important in anemia, with low vitamin D levels being associated with an increased risk of anemia (35,36). Thus, vitamin D deficiency may be a factor in the development of anemia in patients with CKD.

Vitamin D and KL exert negative feedback towards one another in patients with renal anemia. A previous study suggested that the vitamin D/parathyroid hormone (PTH) signaling pathway regulates the KL/FGF23 signaling pathway, and vice versa (37). 1,25-dihydroxyvitamin D [1,25(OH)D2], the metabolically active form of vitamin D, was demonstrated to upregulate KL expression (37). PTH may indirectly upregulate KL via upregulating 1,25(OH)D2 (38). In addition, several recent studies have demonstrated that vitamin D stimulates the expression of FGF23 and KL, and that vitamin D formation is limited by a negative feedback regulation (39–41). This negative feedback is lost in kl⁻/kl⁻ mice and in these mice the formation of 1,25(OH)2D3 is a function of dietary vitamin D even at excessive 1,25(OH)2D3 concentrations (42). The mechanism by which KL inhibits the production of 1,25(OH)2D3 may be via inhibiting 25-hydroxyvitamin D 1α-hydroxylase (39). A summary of the current knowledge of the association between vitamin D and α-KL is presented in Fig. 2.

Inflammation is an essential component to the body's defense system; however, excessive inflammation is considered to be the cause of anemia in patients with CKD (43). Inflammatory cells release large amounts of chemokines and vasoactive factors, including monocyte chemotactic protein-1 (MCP-1) and NF-κB, which induce the production of profibrotic cytokines following kidney injury (44). At the site of injury, proinflammatory factors, including interleukin (IL)-12, MCP-1 and NF-κB, stimulate the generation of myofibroblasts and the deposition of extracellular matrix, eventually leading to renal dysfunction and potentially to the indirect development of anemia (45). Notably, anemia is a common complication in patients with infections, autoimmune disorders, malignancies, CKD and other inflammation-associated disorders (46). Inflammatory cytokines impair erythropoiesis by inhibiting the production and function of EPO, and inhibiting erythroid progenitor cell proliferation and differentiation. Notably, inflammation induces the expression of iron regulatory hormone hepcidin and suppresses the iron exporter ferroportin, restricting the supply of iron for erythropoiesis (46).

KL is known to be an inhibitor of several inflammatory cytokines (47). The transcription factor NF-κB is an important stimulator of inflammation in CKD (48). A recent study reported that intracellular KL diminished the DNA binding ability of NF-κB and stabilized the NF-κB/NF-κB inhibitor α complex, thus preventing uremia-induced NF-κB expression (24). KL serves as an anti-inflammatory modulator through negatively regulating the production of NF-κB-associated inflammatory proteins (47). A potential mechanism for this effect is that KL inhibits the Ser⁵³⁶ phosphorylation of the NF-κB p65 subunit and its subsequent recruitment to the promoter sites of multiple cytokines (49). Additionally, KL may attenuate the glucose-stimulated activation of the NF-κB by downregulating the expression of toll-like receptor 4 (50), which is associated with CKD (51). Conversely, NF-κB regulates the activity of exogenous and intracellular KL in endothelial cells (52). NF-κB also suppresses the activity of KL, potentially by promoting the production of reactive oxygen species (52). Finally, NF-κB also inhibits the proinflammatory effect of IL-12 (20). These results demonstrate the anti-inflammatory effects of KL (Fig. 2).