Limited cooperation between peroxisome proliferator-activated receptors and retinoid X receptor agonists in sebocyte growth and development.

We have found that sebaceous epithelial cell (sebocyte) differentiation is induced by cognate ligand-agonists of either peroxisome proliferator-activated receptors (PPARs) or retinoid X receptors (RXRs). In this study, we tested the hypothesis that PPAR-RXR cooperation is used in sebocytes as was reported to occur in gene transfection systems and liposarcoma cells through PPAR-RXR heterodimerization. PPAR agonists at maximally effective concentrations were tested in combination with a specific RXR agonist (the rexinoid CD2809) at doses ranging from submaximal to maximal in a primary rat preputial cell monolayer culture system. We evaluated ligand-agonists of PPARalpha (WY-14643 = WY), PPARgamma (troglitazone = TRO), and PPARdelta,alpha (carbaprostacyclin = cPGI2). Cell differentiation was determined by analysis of lipid staining and proliferation by cell counting. The RXR agonist induced a more diffuse and granular pattern of lipid staining throughout colonies than did PPAR agonists. The PPAR ligands WY, TRO, and cPGI2 induced 37, 35, and 59% lipid-forming colonies (LFCs), respectively (P < 0.05 vs controls, which averaged 19%). Low-dose rexinoid (10(-8) M) alone exerted no significant effect but amplified the effect of cPGI2 (P < 0.05). Middose rexinoid (10(-7) M), which alone induced about 40% LFCs, had an additive effect on differentiation with WY, TRO, and cPGI2 (71, 48, and 83% LFCs respectively, P < 0.05 vs each agonist alone). Proliferation was enhanced significantly by either rexinoid or cPGI2, but there was no change in growth when the two were added together. The greater effectiveness of cPGI2 than the other PPAR agonists may be explained by the predominance of PPARdelta gene expression in cultured sebocytes, as demonstrated by RNase protection assay. These studies demonstrate that a submaximal dose of RXR agonist augmented the stimulation of sebocyte differentiation by PPAR agonists, as expected from PPAR-RXR heterodimerization. However, the evidence for PPAR-RXR cooperativity is limited. The pattern of lipid staining is compatible with an independent effect of rexinoid on sebocyte differentiation. Furthermore, since there is no enhancement of the growth-promoting effects of RXR agonist and cPGI2 when they are combined, this effect also does not appear to be mediated by PPAR-RXR interaction.