*2.3.1. 25-Hydroxylation*

Various enzymes may be associated with the first hydroxylation of 25(OH)D, but CYP2R1 seems to be the key to this hydroxylation [50]. In humans, other cytochromes P450 such as CYP3A4, CYP27A1, and CYP2J2 show activity of 25-hydroxylase to vitamin D molecules but less efficient. CYP2J3, CYP2D25, and CYP2C11 also show activity of 25-hydroxylase but are only expressed in male pigs and rats, respectively [51]. 25-Hydroxylation appears to be functional in AT. Interestingly, in human AT, biopsies have confirmed the expression of CYP27A1, CYP2R1, and CYP2J2, suggesting that human AT and adipocytes are able to convert vitamin D3 into 25(OH)D.

#### *2.3.2. 1α-Hydroxylation*

to VD response elements in the promoter region of target genes [36]. VDR expression has been identified in most human tissues, including in osteoblasts, skin keratinocytes, macrophages, smooth muscle, pancreatic β-cells and epithelial cells [37, 38], and it is also highly expressed

The action of 1,25(OH)2D is mediated through the VDR, which regulates the transcription of many target genes [13]. There are more than 1000 genes that are directly or indirectly regulated by 1,25(OH)2D and involved in various physiological processes such as cell proliferation,

VDR expression is increased in obese, which has more VAT than lean subjects, but the physiological relevance of this upregulation has not yet been elucidated. VAT *VDR* gene expression correlated positively with body mass index (BMI) [39]. The ubiquitous expression of VDR may underlie the diverse effects of VD and provide a mechanistic basis for the link between VDD and a number of disorders that are linked with obesity like certain types of cancer, inflammatory bowel disease, cardiovascular diseases (CVD), diabetes (type 1 and type

Expanding to another approach, there are associations of VDR variants with the more metabolically active fat, VAT, which is more closely tied to the metabolic consequences of adiposity. Association of VDR SNP rs4,328,262 with VAT supports the notion that the VDR gene is likely to be related to the development of obesity and obesity-related outcomes [43]. Polymorphisms in the VDR gene might play a role in regulating AT activity body fatness and susceptibility to adiposity among African Americans, albeit genetic factors that contribute to adiposity are certainly more complex than to be explained totally by variations in

The formation, activation, and catabolism of 25(OH)D are complex processes, which involve mitochondrial and microsomal cytochrome P450 enzymes. In humans, four cytochrome P450 enzymes, CYP2R1, CYP3A4, CYP27A1, and CYP2J2, [44–47] possess 25-hydroxylase activity, with CYP2R1 being the most specific. Hydroxylation in the 1α-position is effected by the mitochondrial CYP27B1. This process was classically located to the kidney, but recently, extrarenal 1α-hydroxylase activity has been described in several other tissues [48]. 1,25(OH)2D stimulates its own degradation by induction of the 24-hydroxylase (CYP24A1), which catabolizes 25(OH)D and 1,25(OH)2D to calcitroic acid and other inactive metabolites [49].

Various enzymes may be associated with the first hydroxylation of 25(OH)D, but CYP2R1 seems to be the key to this hydroxylation [50]. In humans, other cytochromes P450 such as CYP3A4, CYP27A1, and CYP2J2 show activity of 25-hydroxylase to vitamin D molecules but less efficient. CYP2J3, CYP2D25, and CYP2C11 also show activity of 25-hydroxylase but are only expressed in male pigs and rats, respectively [51]. 25-Hydroxylation appears to be functional in AT. Interestingly, in human AT, biopsies have confirmed the expression of

in adipocytes.

146 Adiposity - Omics and Molecular Understanding

a single gene.

**2.3. Hydroxylase enzymes**

*2.3.1. 25-Hydroxylation*

differentiation, apoptosis, and angiogenesis [38].

2), and the metabolic syndrome [40–42].

25(OH)D is then secreted into the circulation or directed to 1α-hydroxylase CYP27B1 mitochondria to be metabolized to 1,25(OH)2D. CYP27B1 is the key enzyme 1α-hydroxylation, and its activity is regulated by the parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), calcium, and phosphorus and self-regulated by 1,25(OH)2D via negative feedback mechanism [18]. CYP27B1 mRNA, which encodes the 1α-hydroxylase that converts 25(OH)D to the biologically active 1,25(OH)2D, was present at significant levels in SAT and VAT. This gene was mainly expressed in the stromal vascular fraction of human AT that contains preadipocytes, macrophages, and endothelial cells. The expression of CYP27B1 has also been detected in adipocytes of murine [17] and human AT biopsies [52].
