The nature of endogenous PPAR ligands are still far from clear, as is whether PPARs act as general lipid sensors or whether high affinity ligands exist in the body. Here we show CYP2J2 can act as an endogenous epoxygenase source of high affinity PPAR ligands. When co-transfected together in vitro
, CYP2J2 induces PPAR, in particular PPARα, activity. In cardiac-specific-CYP2J2 Tr mice, fasting greatly elevates the PPARα target gene PDK4. These results do not exclude a role for CYP2J2 or other CYPs as regulators of PPARβ/δ or –γ. Indeed we found CYP2J2 can activate PPARδ and PPARγ, (albeit it to lower absolute levels then PPARα in our transfection system) and it is known that lipid CYP products (though not the CYP responsible) are endogenous PPARγ activators, induced by laminar shear of human endothelial cells in vitro 
Unlike other proposed PPAR ligand-generating enzymes (e.g. 12/15-lipoxygenase; 
), CYP2J2 did not require additional arachidonic/linoleic acid substrate(s), suggesting a high level of functional coupling between the epoxygenases and PPARs. We also show for the first time a functional in vivo
response for a PPAR ligand generating system. Our results do not rule out the role of other enzymes, such as phospholipases 
or lipoprotein lipases 
implicated in PPARα ligand generation. These enzymes are likely to produce PPAR ligands in parallel to CYPs, and/or supply free fatty acid substrates for CYPs to utilise.
8,9-EET, and 11,12-EET, but not 14,15-EET activated PPARα. 11,12-EET in contrast to 14-15-EET is highly anti-inflammatory and vascular protective 
. Therefore, we propose that PPARα is a likely anti-inflammatory target for 11,12-EET and CYP2J2. Indeed we found the combination of PPARα and CYP2J2 abolished IL-1β induced NFκB activation; a central pro-inflammatory transcription factor and PPARα target 
Many EETs, including 14,15-EET, can also act as cellular hyperpolarising agents 
, however, since 14,15-EET was inactive in our system, hyperpolarization mechanisms are highly unlikely to be involved. Our results are consistent with previous findings that EETs and some of their metabolites can directly bind and activate PPARα 
. Although 14,15-EET did not activate PPARα in our hands, its CYP4A hydroxylase 14,15-EET metabolite 20,14,15-HEET, was the post potent EET product we tested. EETs can be rapidly metabolised by at least 10 different intracellular pathways, and it is estimated that when given exogenously <10% is available free within the cell 
. Our results therefore do indicate that alternative CYP2J2 products exist or further unknown EET metabolites 
are potential endogenous PPARα activators.
There is considerable species difference between CYPs in man and in the mouse. CYP2J2 is the human isoform, in the mouse the situation is far more complex with up to 8 putative homologues (CYP2J5 – CYP2J13; 
). Since epoxygenases are ubiquitous and potentially have many roles, examining the role of endogenous epoxygenases especially in the mouse is extremely difficult. We therefore chose as our main model the established cardiac specific CYP2J2-Tr mouse. We did however test the recently described CYP2J5 knockout mouse 
, the only CYP2J knockout available. However, we did not detect a change in the fasting response or in PDK4 expression, suggesting a lack of involvement of CYP2J5 in PPARα ligand generation or the more likely compensation from other mouse CYP2J or CYP2C EET-producing epoxygenases that are present.
The selective augmentation of PDK4 in cardiac-specific CYP2J2-Tr mice occurred only in female mice. The fasting PPARα response was much stronger in males, and we believe maximally activated. Interestingly, our results are consistent with known gender differences in cardiac PPARα responses in the mouse. Pharmacological stress of the hearts of PPARα knockout mice with Etomoxir to prevent mitochondrial fatty acid import, results in cardiac lipid accumulation and a 100% mortality of male mice but only 25% mortality of female mice 
In conclusion, in vitro CYP2J2 activates PPARα without exogenous stimuli. In vivo CYP2J2 does not appear to be rate-limiting as PPARα target gene (PDK4) expression is only augmented in cardiac-specific CYP2J2 transgenic mouse upon fasting. Therefore, CYP2J2 in vivo is an enzyme apparently quiescent, but capable or responding to changes in lipid availability to generate endogenous PPARα agonists and thereby integrate transcriptional fasting events. CYP2J2 products activate PPARs, in particular PPARα in vitro and in vivo. As lipid-metabolising CYP enzymes have a widespread expression, utilise a variety of lipid substrates and produce a large family of oxidised biologically active lipid mediators, we suggest that lipid metabolising CYPs may represent an important source of PPAR ligands throughout the body.
PPARα is known as a controller of lipid metabolism and inflammation. Linking CYP2J2 and epoxygenases to PPARα has many potential clinical implications. Variants of CYP2J2 with lower activity are known in some populations to be linked to an increased risk of coronary artery disease 
. Epoxygenases such as CYP2J2 in addition to metabolising arachidonic acid may also regulate xenobiotic drug metabolism. Understanding how epoxygenases are regulated, the mediators they produce, and where they work, will give us novel information on biomarkers for dyslipidaemia and inflammation, allow us to understand side-effects of drugs metabolised by epoxygenases, and help us to design novel PPARα ligands based on the structure of high affinity EETs and their metabolites.