Agonists directed against the α and γ isoforms of the peroxisome proliferator-activated receptors (PPARs) have become important for the respective treatment of hypertriglyceridemia and insulin resistance associated with metabolic disease. PPARδ is the least well characterized of the three PPAR isoforms. Skeletal muscle insulin resistance is a primary risk factor for the development of type 2 diabetes. There is increasing evidence that PPARδ is an important regulator of skeletal muscle metabolism, in particular, muscle lipid oxidation, highlighting the potential utility of this isoform as a drug target. In addition, PPARδ seems to be a key regulator of skeletal muscle fiber type and a possible mediator of the adaptations noted in skeletal muscle in response to exercise. In this review we summarize the current status regarding the regulation, and the metabolic effects, of PPARδ in skeletal muscle.

11β-Hydroxysteroid dehydrogenase type 1 (11β-HSD-1) catalyzes the interconversion of inactive cortisone to active cortisol. Overexpression of 11β-HSD-1 in murine adipose tissue results in glucocorticoid receptor (GR)α overexpression, central obesity, and insulin resistance. It is controversial whether 11β-HSD-1 or GRα expression are increased in human adipose tissue in obesity. We studied effects of acquired obesity on 11β-HSD-1 gene (real-time PCR) and protein (Western blotting) expression in sc adipose tissue in 17 monozygotic twin pairs aged 24–27 yr with a mean intrapair difference in body mass index (BMI) of 3.8 kg/m² (range 0.4–10.1 kg/m²). Intrapair correlations were calculated to study effects of acquired obesity on 11β-HSD-1 expression. Western blot analysis of adipose tissue homogenates identified approximately 50- and approximately 68-kDa proteins specific for 11β-HSD-1. Both structural forms correlated positively with 11β-HSD-1 mRNA concentrations. Intrapair differences in 11β-HSD-1 mRNA, and the 50- and 68-kDa proteins in sc adipose tissue correlated positively with those in BMI (kilograms per square meter) (r = 0.78 for 11β-HSD-1 mRNA, P = 0.0002; r = 0.87 for the 11β-HSD-1 50-kDa protein, P = 0.0003; and r = 0.62 for the 11β-HSD-1 68-kDa protein, P = 0.033), total body fat (percent) (r = 0.65, P = 0.005; r = 0.83, P = 0.001; and r = 0.69, P = 0.013, respectively) and sc fat (cubed centimeters) (r = 0.66, P = 0.004; r = 0.94, P = 0.0001; and r = 0.71, P = 0.009, respectively). Furthermore, 11β-HSD-1 mRNA and 50-kDa protein expression, but not 68-kDa protein expression, correlated positively with intrapair differences in intraabdominal fat mass (cubed centimeters) (r = 0.62, P = 0.008; r = 0.69, P = 0.013; r = 0.48, P = 0.112) and serum fasting insulin concentration (milliunits per liter) (r = 0.76, P = 0.0004; r = 0.60, P = 0.037; and r = 0.43, P = 0.160, respectively). Intrapair differences in GRα expression were significantly inversely correlated with those in BMI and total and sc fat mass. In conclusion, expression of 11β-HSD-1 in sc adipose tissue is increased in human acquired obesity and is closely related to accumulation of sc and intraabdominal fat and features of insulin resistance.

The nuclear receptor peroxisome proliferator-activated receptor δ [PPARδ/β (NR1C2)] has been implicated in colorectal carcinogenesis by various molecular genetic observations. These observations have recently been supported by studies of activation of PPARδ by pharmacological agents. Here we present the first report of the stimulation of breast and prostate cancer cell growth using PPARδ selective agonists. Activation of PPARδ with compound F stimulated proliferation in breast (T47D, MCF7) and prostate (LNCaP, PNT1A) cell lines, which are responsive to sex hormones. Conversely, we have found that several steroid-independent cell lines, including colon lines, were unresponsive to compound F. These findings were confirmed with an additional high-affinity PPARδ agonist, GW501516. Conditional expression of PPARδ in MCF7 Tet-On cells resulted in a doxycycline-enhanced response to GW501516, thus providing direct genetic evidence for the role of PPARδ in the proliferative response to this drug. Activation of PPARδ in T47D cells resulted in increased expression of the proliferation marker Cdk2 and also vascular endothelial growth factor α (VEGFα) and its receptor, FLT-1, thus, suggesting that PPARδ may initiate an autocrine loop for cellular proliferation and possibly angiogenesis. Consistent with this hypothesis, we demonstrated a pro-proliferative effect of GW501516 on human umbilical vein endothelial cell cultures and found that GW501516 also regulated the expression of VEGFα and FLT-1 in these cells. Our observations provide the first evidence that activation of PPARδ can result in increased growth in breast and prostate cancer cell lines and primary endothelial cells and supports the possibility that PPARδ antagonists may be of therapeutic value in the treatment of breast and prostate cancer.

We investigated a family from northern Sweden in which three of four siblings have congenital chylomicronemia. LPL activity and mass in pre- and postheparin plasma were low, and LPL release into plasma after heparin injection was delayed. LPL activity and mass in adipose tissue biopsies appeared normal. [(35)S]Methionine incorporation studies on adipose tissue showed that newly synthesized LPL was normal in size and normally glycosylated. Breast milk from the affected female subjects contained normal to elevated LPL mass and activity levels. The milk had a lower than normal milk lipid content, and the fatty acid composition was compatible with the milk lipids being derived from de novo lipogenesis, rather than from the plasma lipoproteins. Given the delayed release of LPL into the plasma after heparin, we suspected that the chylomicronemia might be caused by mutations in GPIHBP1. Indeed, all three affected siblings were compound heterozygotes for missense mutations involving highly conserved cysteines in the Ly6 domain of GPIHBP1 (C65S and C68G). The mutant GPIHBP1 proteins reached the surface of transfected Chinese hamster ovary cells but were defective in their ability to bind LPL (as judged by both cell-based and cell-free LPL binding assays). Thus, the conserved cysteines in the Ly6 domain are crucial for GPIHBP1 function.