Peroxisome proliferator activated receptors (PPARs) are a family of 3 nuclear receptors. PPAR is ubiquitously expressed and is thought to be involved in the proliferation of vascular smooth muscle cells, pre-adipocyte cells, and cancer cells (Zhang et al, 2002). Little is known about the endogenous ligands for PPAR , although prostacyclin and some of its mimetics are known activators (Lim and Dey, 2002). Prostacyclin is cytoprotective in some organ systems including the gastrointestinal tract and the pulmonary circulation where its effects are thought to be mediated by cell surface IP receptors. However, we have previously shown that the prostacyclin mimetic, treprostinil sodium (gift from United Therapeutics Corp.) which is currently used in the treatment of pulmonary hypertension, inhibits murine pulmonary cell growth via a PPAR -dependent pathway (Ali et al, 2003). The function of PPAR receptors differ between species, here we have investigated the effects of treprostinil sodium and selective ligands for PPAR , PPAR and PPAR on the proliferation of human pulmonary fibroblasts.

Primary cultures of human lung fibroblasts were purchased from ATCC, seeded at 20,000 cells/ml and kept in DMEM with 10% FCS. At 40% confluence cells were growth arrested (no FCS) for 24 h, after which time drugs were added. 4 h later FCS was returned to cells at final concentration of 3%. Cells were then incubated (5% CO₂ in air, 37  °C) for 5 days and proliferation was quantified using a fluorescence assay (CYQUANT; Cambridge Bioscience). For western blots, cell extracts were probed using primary antibodies specific to PPAR (Mr approximately 50kDa).

A                                           

B

C

Figure 1. Effects of (A) GW0742 or (B) various PPAR ligands (all 10 ^-4 M) on proliferation of human lung fibroblasts. (C) Western blot showing expression of PPAR in human lung cells. Proliferation was calculated as % growth induced by FCS. Data is mean ± s.e.m for n=3-9. *p<0.05; one sample t-test.

Human lung fibroblasts expressed PPAR and the selective PPAR ligand GW0742 (gift from G.S.K.) as well as treprostinil sodium, which inhibits proliferation in murine cells via a PPAR dependent pathway, inhibited proliferation of human lung fibroblasts. PPAR ligands (rosiglitazone and 15d-PGJ₂), but not the PPAR ligand WY-14643 also inhibited proliferation. The mixed PPAR/PPAR ligand bezafibrate had no effect on proliferation. Finally, cicaprost, iloprost and carbaprostacyclin, which are ligands for PPAR in some cells inhibited proliferation of lung fibroblasts. This data suggests that PPAR is a therapeutic target for the treatment of human lung disease, particularly those, such as pulmonary hypertension, where remodelling contributes to symptoms.

Ali, FY et al (2003), Br J Pharmacol 1(4):P004
Lim, H and Dey, SK (2002), Endocrinology 143: 3207-3210
Zhang, J et al (2002), J Biol Chem 277(13): 11505-11512

This work was funded by the Joint Research Board of St Bartholomew’s Hospital, London