**2. Vitamin D metabolism in kidney disease: pathophysiology**

#### **2.1. Chronic kidney disease: definition**

According to the widely accepted definition by the international foundation for Kidney Disease/Improving Global Outcomes (KDIGO), chronic kidney disease (CKD) is defined as abnormalities of kidney structure or function, present for more than 3 months, with implications for health [19] (**Table 1**).


albumin:creatinine ratio.

**Table 1.** Criteria for CKD (either of the following for more than 3 months) [19].

CKD is a global health problem, affecting up to 10% of the population [20]. As the glomerular filtration rate (GFR) declines, especially below 60 ml/min/1.73 m2 , the ability of the kidney to excrete phosphate is diminished, leading to disruption of calcium-phosphorus homeostasis, pathological changes in hormone levels (PTH, FGF-23), and decrease in the level of VD metabolites. Subsequently, changes in bone morphology and extraskeletal calcifications occur. The changes in biochemical indicators, bone morphology, and extraskeletal calcium deposits are defined as chronic kidney disease-mineral bone disorder (CKD-MBD), **Table 2**. This is a new definition that clearly states the difference from renal ostheodystrophy, taking into consideration a broader problem in CKD patients [21].

A systemic disorder of mineral and bone metabolism due to CKD manifested by either one or a combination of the following:


Definition of renal osteodystrophy

**Figure 3.** Role of FGF-23 in phosphate homeostasis.

24 A Critical Evaluation of Vitamin D - Clinical Overview

early marker of CKD [17, 18].

**2.1. Chronic kidney disease: definition**

tions for health [19] (**Table 1**).

In healthy subjects, FGF-23 suppresses PTH secretion [12]. In addition, extrarenal effects have been described on cardiovascular system and brain [13]. Alfa-klotho is a protein cofactor for FGF-23 signaling, as it forms complexes with FGF-23 receptor, thus increasing its affinity for the hormone [14]. A soluble klotho was also detected, functioning as humoral factor. Soluble klotho downregulates insulin-like growth factor I, thus exerting antiaging properties [15]. It also potentiates 1,25VD-associated renal calcium absorption [16]. Furthermore, soluble klotho causes hypophosphatemia and phosphaturia independently of FGF-23 and is regarded as an

In summary, the kidney is closely linked to the VD axis and calcium-phosphorus homeostasis. Early changes in renal function are associated with significant changes in VD metabolism. We shall start with VD pathology in patients with renal disease and at the end of our review the

According to the widely accepted definition by the international foundation for Kidney Disease/Improving Global Outcomes (KDIGO), chronic kidney disease (CKD) is defined as abnormalities of kidney structure or function, present for more than 3 months, with implica-

topic vitamin D metabolism after kidney transplantation will be discussed.

**2. Vitamin D metabolism in kidney disease: pathophysiology**


**Table 2.** KDIGO classification of CKD-MBD and renal ostheodystrophy [21].

#### **2.2. Changes in vitamin D and its metabolites**

Changes in vitamin D metabolism are detected in the early stages of CKD in patients with GFR below 60 ml/min/1.73 m2 [22]. Furthermore, the expression of the vitamin D receptor in CKD patients is suppressed [23]. These abnormalities are part of the biochemical component of CKD-related mineral bone disease, together with changes in PTH, bone alkaline phosphatase, serum levels of calcium, and phosphate.

#### *2.2.1. Change in 1,25-dihydroxyvitamin D*

The classical theory stated that the fall in the active VD metabolite is due to the initial kidney damage, thus leading to reduced calcium and phosphorus intestinal absorption and rise in PTH. With the discovery of FGF-23 and alfa-klotho axis however new explanation of the biochemical abnormalities appeared. Kidney damage leads to reduced ability of the tubules to eliminate phosphorus. This leads to rise in FGF-23 level in order to keep the phosphate level within normal limits. The rise of FGF-23, however, is the initial signal for suppressing renal 1 α hydroxylase and reducing 1,25VD. In addition, it leads to increased catabolism due to activation of 24-hydroxilase. FGF-23 starts to rise in patients GFR below 60 ml/min/1.73 m2 , keeping phosphate serum levels within normal limits well below this cut-off value [24].

To sum up, changes in hormones (PTH and FGF-23) and 1,25VD occur in the early stages of CKD, whereas deviations in calcium and phosphate are characteristic for the advanced CKD cases.

#### *2.2.2. 25-Hydroxyvitamin D (25VD)*

25-Hydroxyvitamin D is generally accepted marker for assessing vitamin D status due to its stable serum level and long half-life. Though there is no clear consensus on the definition of VD insufficiency, most of the studies define VD deficiency as 25VD level below 25 nmol/l, whereas insufficiency is defined as 25VD level between 25 and 80 nmol/l. Unfortunately, no clear definition for optimal 25VD level exists though some researchers define it as VD associated with normal PTH value in the general population or VD value above which there is no decrease in PTH [25–27]. Suboptimal levels are widely spread in CKD with prevalence peaking up to 92% in patients on hemodialysis [28]. Several factors can explain the low 25VD level in CKD (**Table 3**).

Poor VD status has been associated with a lot of complications and diseases, apart from its link to the calcium-phosphate homeostasis. Higher mortality was detected in the general population and in CKD patients with low 25VD [28]. Poor 25VD was also associated with higher risk for cancer, diabetes mellitus, hypertension, and depression in humans [29]. VDR was detected in malignant cells too. Activation of VDR in these cells was found to block the cell cycle or cause cell apoptosis [30]. Increased sun exposition had inverse correlation with prevalence of several malignancies [31]. Vitamin D increases insulin secretion and improves insulin resistance in diabetes. In addition, insulin receptor synthesis is improved, as well as systemic inflammation is reduced, which probably explains the positive effect of VD in animal models

and human studies. Vitamin D supplementation in early infancy/or prior to birth was found effective in reducing the prevalence of diabetes type 1 [32]. Several mechanisms have been proposed for the influence of VD on blood pressure—suppression of renin-angiotensin system, calcium ion influx control in smooth muscle cells of the vessels, and improved activity of nitric oxide (NO). Indeed, several cross-sectional studies show that poorer VD status is associated with higher blood pressure values and higher prevalence of hypertension [33]. Several studies indicate that vitamin D insufficiency is linked with higher incidence of depression, without any data for the severity of the disease. There are several possible mechanisms for this relationship—VD may play important role in brain signaling and neuroimmunomodulation, as brain VDR were detected; in addition, vitamin D takes part in serotonin synthesis [34].


**Table 3.** Determinants for lower 25-hydroxyvitamin D levels in CKD.

Further studies are needed to clarify the potential extraskeletal effects of VD in CKD, including larger randomized controlled trials (RTC). The clinical implications of impaired VD status and the possible treatment options in renal patients will be discussed later in this chapter.

#### *2.2.3. The vitamin D receptor in CKD*

**2.2. Changes in vitamin D and its metabolites**

26 A Critical Evaluation of Vitamin D - Clinical Overview

serum levels of calcium, and phosphate.

*2.2.1. Change in 1,25-dihydroxyvitamin D*

*2.2.2. 25-Hydroxyvitamin D (25VD)*

below 60 ml/min/1.73 m2

cases.

CKD (**Table 3**).

Changes in vitamin D metabolism are detected in the early stages of CKD in patients with GFR

patients is suppressed [23]. These abnormalities are part of the biochemical component of CKD-related mineral bone disease, together with changes in PTH, bone alkaline phosphatase,

The classical theory stated that the fall in the active VD metabolite is due to the initial kidney damage, thus leading to reduced calcium and phosphorus intestinal absorption and rise in PTH. With the discovery of FGF-23 and alfa-klotho axis however new explanation of the biochemical abnormalities appeared. Kidney damage leads to reduced ability of the tubules to eliminate phosphorus. This leads to rise in FGF-23 level in order to keep the phosphate level within normal limits. The rise of FGF-23, however, is the initial signal for suppressing renal 1 α hydroxylase and reducing 1,25VD. In addition, it leads to increased catabolism due to activation of 24-hydroxilase. FGF-23 starts to rise in patients GFR below 60 ml/min/1.73 m2

keeping phosphate serum levels within normal limits well below this cut-off value [24].

To sum up, changes in hormones (PTH and FGF-23) and 1,25VD occur in the early stages of CKD, whereas deviations in calcium and phosphate are characteristic for the advanced CKD

25-Hydroxyvitamin D is generally accepted marker for assessing vitamin D status due to its stable serum level and long half-life. Though there is no clear consensus on the definition of VD insufficiency, most of the studies define VD deficiency as 25VD level below 25 nmol/l, whereas insufficiency is defined as 25VD level between 25 and 80 nmol/l. Unfortunately, no clear definition for optimal 25VD level exists though some researchers define it as VD associated with normal PTH value in the general population or VD value above which there is no decrease in PTH [25–27]. Suboptimal levels are widely spread in CKD with prevalence peaking up to 92% in patients on hemodialysis [28]. Several factors can explain the low 25VD level in

Poor VD status has been associated with a lot of complications and diseases, apart from its link to the calcium-phosphate homeostasis. Higher mortality was detected in the general population and in CKD patients with low 25VD [28]. Poor 25VD was also associated with higher risk for cancer, diabetes mellitus, hypertension, and depression in humans [29]. VDR was detected in malignant cells too. Activation of VDR in these cells was found to block the cell cycle or cause cell apoptosis [30]. Increased sun exposition had inverse correlation with prevalence of several malignancies [31]. Vitamin D increases insulin secretion and improves insulin resistance in diabetes. In addition, insulin receptor synthesis is improved, as well as systemic inflammation is reduced, which probably explains the positive effect of VD in animal models

[22]. Furthermore, the expression of the vitamin D receptor in CKD

,

1,25-Dihydroxyvitamin D mediates its effects via the vitamin D receptor (VDR). It is a nuclear peptide, belonging to a superfamily of nucleotide receptors, like the receptors for retinoic acid and the thyroid hormones. As 1,25VD is the active VD metabolite, VDR has almost 1000 times higher affinity for it than for other VD metabolites. However, the receptor can be activated by 25VD too in cases of toxic VD levels above 370 nmol/l. VDR is expressed in almost all the tissues in human body, with highest expression, however, in intestines, parathyroid gland, and bones. Once 1,25VD binds to VDR, the complex forms a heterodimer with the receptor for retinoid X (RXR) within the nucleus. The 1,25VD-VDR-RXR complex binds to vitamin D reacting elements, activating or suppressing genes.

Activation of VDR leads to increased calcium intestinal absorption, suppression of PTH synthesis in parathyroid gland, and modulation of osteoblast and osteoclast activity. However, due to its wide distribution, it is believed that VDR plays a more complicated role in human health, apart from controlling mineral homeostasis. Furthermore, VDR can be located in the cellular membrane, thus placing the VDR-VD axis not only in the middle of genomic effects but also in activation of rapid transmembrane pathways [35].

In uremia, significant changes in VDR function occur. Low levels of 1,25VD lead to downregulation of VDR expression [36]. In addition, in areas of nodular growth in the parathyroid gland reduced VDR content was detected [37]. In CKD, there is a significant decrease in VDR-RXR binding to vitamin D reacting elements, as well as reduced RXR content in the parathyroid glands of uremic animal models, explaining increased PTH levels without the presence of hypocalcemia and hyperphosphatemia [38]. Hypocalcemia in advanced renal failure increases the parathyroid levels of calreticulin, a cytosolic protein that binds the DNAbinding domain of nuclear receptors, thus blocking VDR-mediated transactivation [39]. Higher levels of inflammatory cytokines were found to be associated with impaired binding of VDR-RXR to vitamin D reacting elements, contributing to vitamin D resistance in patients on hemodialysis [40]. Finally, hypocalcemia in CKD suppresses the calcium-sensing receptor (CaSR) in the parathyroid glands, which in turn downregulates parathyroid VDR expression. Stimulating CaSR by increasing extracellular calcium or by using calcimimetics upregulated VDR expression in rat models [41].
