L-PGDS Compensates for a Lack of PPARc2 Figure 6. Gene expression and lipid levels in liver. A) DKO mice have increased expression of genes involved in hepatic lipogenesis B) DKO mice have accelerated hepatomegaly C) DKO mice do not have overt hepatosteatosis. D) DKO mice have elevated hepatic TG levels. All mice n = 8 per group C57Bl/6 males 5 months old. a, P,0.05 WT vs PPARc2, b, P,0.05 WT vs DKO c, P,0.05 PPARc2 vs DKO, d P,0.05 L-PGDS vs DKO. doi:10.1371/journal.pone.0039512.g006 While the alterations in carbohydrate metabolism and circulat- ing TG levels in the DKO mice were consistent with alterations in subcutaneous lipid handling genes, the lack of a reduction in metabolic rate and increase in circulating FFA levels suggested that the overall fuel availability was not compromised. Indeed, the loss of appropriate lipid release and synthesis of fatty acids in subcutaneous WAT appeared to be over-compensated for by increased markers of lipogenesis in liver and by increased markers of lipid release from epididymal WAT, on mRNA, protein and phosphoprotein levels. On a protein levels the same pattern was not as clear, as both PPARc2 KO and DKO mice had reduced levels of ATGL and HSL, as well as reduced levels of HSL phosphorylation, however the DKO mice had significantly elevated levels of ATGL protein and an increased pHSLactive/ pHSLinhib ratio compared to PPARc2 KO mice alone. An important caveat is that while these markers support the concept of depot-specific alterations in lipolysis, direct measurements of lipolysis would be required to further confirm this finding. The markers of increased lipid release in epididymal WAT were associated with multiple markers of increased inflammation in this tissue. In humans, mesenteric adipose tissue has both higher rates of lipolysis [9] and is more inflamed than subcutaneous adipose depots in obesity [28,29]. Consistent with these observations, a recent study has demonstrated elevated lipolysis from adipose tissue can directly promote macrophage infiltration [30]. Further- more, at least one report has demonstrated that L-PGDS expression is lower in mesenteric adipose tissue than subcutaneous adipose tissue depots [19]. We hypothesise that in the context of an underlying impairment in adipose tissue function caused by reduced PPARc function, loss of L-PGDS reduced subcutaneous adipose tissue function further in terms of both uptake and release of lipid and resulted in an over-reliance on intra-abdominal adipose tissue, associated with elevated inflammation. The changes in gene expression and lipid levels in liver, despite the lack of a primary change in L-PGDS expression, may be secondary to alterations in adipose tissue lipid handling observed in the DKO mice. The adipose tissue expandability hypothesis has principally been focused on the concept that a reduction in adipose tissue mass leads to ectopic deposition of lipid in non adipose organs, [31]. More recently, evidence from mice and humans suggests that adipose tissue dysfunction in terms of lipid uptake and release, may be the most important factor that leads to ectopic lipid deposition [32] [27]. The accumulation of triglyceride in liver and the elevated levels of the potentially lipotoxic species lysophosphatidylcholine [33–34] in the DKO mice when com- pared to PPARc2 KO mice occur despite no differences in fat pad masses between these animals. Despite similar fat pad masses between PPARc2 and DKO mice, DKO mice had increased ATGL levels and a pro-activation HSL-phosphorylation pattern in epididymal white adipose tissue. The changes in ATGL and HSL were reversed in subcutaneous adipose tissue. Overall, these data suggest that functional changes primarily occurring in adipose tissue depots could be mediating secondary changes in liver via lipotoxic process. An important teleological question is why L-PGDS, an apparently positive factor for metabolic health in this setting, is elevated in response to ablation of PPARc2 another apparently positive factor. We and others have put forward the concept of allostasis, which suggests that complex systems, such as mice, have multiple parallel systems which perform similar though non- identical roles. When one system is ablated, other systems are able to partially compensate for the loss of the primary system, albeit usually at some cost to fitness [13]. Mechanistically, how L-PGDS acts to compensate for a loss of PPARc2 remains unresolved. Measurements of eicosanoids isolated from BAT from WT, L- PGDS KO, PPARc2 KO and DKO mice revealed some subtle alterations in lipoxygenase products. However, cyclooxygenase PLoS ONE | www.plosone.org 10 July 2012 | Volume 7 | Issue 7 | e39512