## 7 Vitamin D in Cosmetics and Cosmetics
### Mechanism of Action  
The mechanism of action of vitamin D when applied topically to the skin consists of several complex biological processes that contribute to its therapeutic effects [74; 81]. Topical vitamin D formulations mostly act locally within the skin in contrast to systemic vitamin D ones, which are converted to their active form in the liver and kidneys. After being applied topically, vitamin D in the form of calcitriol or its analogs interacts with particular skin cellular targets, to produce its advantageous effects. One of the main mechanisms of action involves the \(VDR\), which is expressed in various skin cells including keratinocytes, fibroblasts, and immune cells. The primary mode of action is via its \(VDR\) interaction, where the N-terminal DNA-binding domain of the \(VDR\) transcription factor binds to vitamin D response elements (VDREs) on DNA, the C-terminal ligand-binding domain, and the hinge region that joins these transcription factors' three primary domains. After binding to the ligand 1\(\alpha\),25(OH)2D3, its 12 helices in the ligand-binding domain undergo conformational changes that ultimately enable the domain to interact with coactivators and its heterodimer partner, which is usually the RXR [74]. \(VDR\) attaches itself to VDREs, which are primarily direct repeats of hexanucleotides, spaced three nucleotides apart (DR3 call II gene motifs). This binding enhances the recruitment of several coregulatory complexes necessary for the genomic \(VDR\) action. Due to the gene- and cell-specific nature of these complexes, precise control over gene expression is possible. Coactivators with histone acetyltransferase activity, such as the non-receptor, protein-tyrosine kinase (SRC) family, and corepressors with histone deacetylase activity, like the silencing mediator of retinoic acid and the thyroid hormone receptor (SMRT), as well as the nuclear receptor corepressor (NCoR), are examples of coregulators. Methyltransferases, demethyllases, adenosine triphosphate (ATP)-ase-containing nucleosomal remodeling complexes, and elements of the mediator complex interacting with RNA polymerase II are other interesting factors linked to the \(VDR\) cascade [74]. This multifaceted procedure encourages keratinocytes to differentiate and proliferate, a necessary action for preserving the skin barrier. By upregulating genes involved in keratinocyte differentiation and cornification, vitamin D strengthens the skin's protective layer and promotes the development of a functioning stratum corneum. Furthermore, topically applying vitamin D promotes the synthesis of antimicrobial peptides such as cathelicidins, which are essential for the skin's innate immune response, as they support the skin's defensive processes and aid in the fight against infections. Vitamin D helps the skin keep itself healthy and fight off infections, by improving the synthesis of cathelicidins [37]. Additionally, using vitamin D topically shields the skin from harm caused by UV rays. Antioxidant qualities of vitamin D contribute, in fact, to lessening oxidative stress acceleration by the UV light. The advantages of vitamin D in preserving skin health and averting UV-induced premature aging are consequently more and more clear [37].