Peroxisome proliferator-activated receptor β/δ (PPAR-β/δ) is a ligand-activated transcription factor of the PPAR nuclear receptor family that plays important roles in the regulation of cellular metabolism by binding to sequence-specific DNA elements . PPAR-β/δ plays a vital physiological role by regulating inflammation, energy homeostasis, cell proliferation, and differentiation . Besides its expression in peripheral organs, PPAR-β/δ is also highly expressed in the central nervous system . 

The anti-inflammatory effects of PPAR-β/δ occur via a reduction in the binding of nuclear factor (NF)-κB to DNA by interfering with NF-κB subunit p65, thereby inhibiting the transcription of NF-κB target genes, such as inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2), and TNF-α.

Peroxisome proliferator-activated receptor(PPARs) are classified three different isoforms termed PPAR-, PPAR-/ and PPAR- sharing considerable sequence and structural homologies.  

Initial studies demonstrated that PPARs are pivotal participants in the regulation of energy homeostasis by modulating glucose and lipid metabolism and transport,furthermore subsequent studies have shown that PPARs regulate in other cellular functions such as cell proliferation, cell differentiation, apoptosis and inflammation. The expressions of all subtypes of PPAR are identified in normal human epidermal keratinocytes. Of all the subtypes, PPAR-β/δ is expressed dominantly and consistently in keratinocytes, whereas PPAR- and - are expressed lower quantities.PPARs are involved in keratinocyte differentiation, epidermal hyperplasia, inflammation and permeability barrier function.Therefore the activation of PPAR-β/δ by exogenous ligands is expected to maintain the skin homeostasis. 

Takuhiro Yamada, Akihiro Aioi demonstrated that Eucalyptus citriodora extract regulates cutaneous homeostasis including immune dysregulation and skin barrier dysfunction via the modulation of peroxisome proliferator-activated receptor-/ (PPAR-/) pathway.

References:

[1]Gbenou JD, Ahounou JF, Akakpo HB, et al. Phytochemical composition of Cymbopogon citratus and Eucalyptus citriodora essential oils and their anti-inflammatory and analgesic properties on Wistar rats. Mol Biol Rep 2013; 40(2):1127-1134.

[2]Straus DS, Glass CK (2007) Anti-inflammatory actions of ppar ligands: new insights on cellular and molecular mechanisms. Trends Immunol 28(12):551–558

[3]https://link.springer.com/article/10.1007/s11064-020-02956-w

[4]https://systems.enpress-publisher.com/index.php/ti/article/view/1130