Peroxisome proliferator-activated receptor γ-mediated differentiation: A mutation in colon cancer cells reveals divergent and cell type-specific mechanisms

Activation of the nuclear hormone receptor peroxisome proliferator-activated receptor γ (PPARγ) inhibits cell growth and induces differentiation in both adipocyte and epithelial cell lineages, although it is unclear whether this occurs through common or cell-type specific mechanisms. We have identified four human colon cancer cell lines that do no undergo growth inhibition or induce markers of differentiation after exposure to PPARγ agonists. Sequence analysis of the PPARγ gene revealed that all four cell lines contain a previously unidentified point mutation in the ninth α-helix of the ligand binding domain at codon 422 (K422Q). The mutant receptor did not exhibit any defects in DNA binding or retinoid X receptor heterodimerization and was transcriptionally active in an artificial reporter assay. However, only retroviral transduction of the wild-type (WT), but not mutant, receptor could restore PPARγ ligand-induced growth inhibition and differentiation in resistant colon cancer cell lines. In contrast, there was no difference in the ability of fibroblast cells expressing WT or K422Q mutant receptor to undergo growth inhibition, express adipocyte differentiation markers, or uptake lipid after treatment with a PPARγ agonist. Finally, analysis of direct PPARγ target genes in colon cancer cells expressing the WT or K422Q mutant allele suggests that the mutation may disrupt the ability of PPARγ to repress the basal expression of a subset of genes in the absence of exogenous ligand. Collectively, these data argue that codon 422 may be a part of a co-factor(s) interaction domain necessary for PPARγ to induce terminal differentiation in epithelial, but not adipocyte, cell lineages and argues that the receptor induces growth inhibition and differentiation via cell lineage-specific mechanisms.