There are two major isoforms of vitamin D, vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) [29,30]. Both vitamin D2/3 need exposure to sunlight’s UVB radiation to be synthesized from ergosterol and 7-dehydrocholesterol, respectively. Vitamin D (both vitamin D2 and D3, calciol) originating from diet or endogenous skin synthesis is delivered to the liver by vitamin D-binding protein (VDBP). There, vitamin D is metabolized by vitamin D 25-hydroxylase (CYP2R1 and CYP27A1) to 25(OH)D (calcidiol), which is the major circulating form of vitamin D in the serum, and its circulating concentration is accepted as an indicator of vitamin D status in a human organism  [31,32]. 25(OH)D is further metabolized by 25(OH)D 1α-hydroxylase (CYP27B1) mainly in the proximal tubule of the kidney to 1α,25-dihydroxyvitamin D (1α,25(OH)2D, calcitriol), which is the recognized biologically active form of vitamin D (Figure 1) [31,32]. Calcitriol then enters the circulation and, after binding to VDBP, is delivered to target tissues such as the intestine, bone, and kidney, where vitamin D is known to regulate absorption, mobilization, and reabsorption, respectively, of calcium and phosphate [29]. After being produced, the levels of both calcidiol and calcitriol are tightly regulated by 25(OH)D 24-hydroxylase (CYP24A1), which is the primary vitamin D inactivating enzyme catalyzing hydroxylation at C-24 and C-23 of both calcidiol and calcitriol [31,32]. This 24-hydroxylation pathway produces the biologically inactive calcitroic acid excreted in the bile [33]. The importance of this inactivation step, mediated by CYP24A1, was highlighted in CYP24A1 knockout mice showing impaired intramembranous bone mineralization and hypercalcemia, leading to a lethal phenotype in 50% of the mice [34,35]. However, this defect was rescued in CYP24A1 and VDR double-knockout mice, which suggested that it is the increased calcitriol levels and not the absence of downstream metabolites that were responsible for the flawed phenotype [35].

Alternatively, vitamin D metabolism is mediated by CYP11A1 (known as a cytochrome P450 side-chain cleavage (P450scc) enzyme) [36]. Vitamin D serves as an alternative substrate for CYP11A1 instead of cholesterol and is sequentially hydroxylated, predominantly at C-20 or C-22, without the cleavage of the side chain producing a multitude of metabolites [37]. Overall, it is estimated that this alternative path produces more than 21 hydroxy-metabolites of vitamin D [36]. Summarily, CYP11A1 products exhibit: (i) antiproliferative, differentiating, and anti-inflammatory abilities in skin cells comparable to that of calcitriol [38,39], (ii) are involved in defense pathways against UVB-induced damage and oxidative stress, and (iii) elicit anticancer abilities in a cell-specific manner [40]. As a point of interest, these alternative metabolites and normal 1,24,25-(OH)3 vitamin D3 do not activate VDR. Thus, the calcemic effects or expression of CYP24A1 can be seen only in response to calcitriol.

As a result of its diverse function, calcitriol is tightly regulated in a negative feedback mechanism [33,41]. Calcitriol inactivation primarily involves modification by CYP24A1, which is among the most prominent targets of the calcitriol–VDR–RXR complex (Figure 1) [42]. In addition, calcitriol can also induce CYP24A1 expression by recruiting histone H4 acetyltransferases and RNA polymerase II to a site approximately 50–70 kb downstream of the human CYP24A1 gene [43]. So, calcitriol signaling levels are tightly kept in control by calcitriol-driven expression of CYP24A1.

Independently, vitamin D metabolism is regulated by two hormones, parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF-23), both of which maintain the calcium and phosphate homeostasis [44]. PTH, secreted by the parathyroid gland in response to calcium levels, stimulates the expression of CYP27B1, leading to an increase in calcitriol production [45]. Although calcitriol signals its degradation via CYP24A1, PTH sustains calcitriol levels by activating the renal cAMP–PKA pathway and invoking the CYP24A1 mRNA degradation [46]. FGF-23, secreted by osteoblasts and osteocytes in response to both phosphate and calcitriol levels [42], reduces serum calcitriol levels by inhibiting the expression of CYP27B1 and simultaneously enhancing the expression of CYP24A1 in the kidney [47].