Natural helix 9 mutants of PPARγ differently affect its transcriptional activity

Marjoleine F. Broekema, Maarten P.G. Massink, Cinzia Donato, Joep de Ligt, Joerg Schaarschmidt, Anouska Borgman, Marieke G. Schooneman, Diana Melchers, Martin N. Gerding, René Houtman, Alexandre M.J.J. Bonvin, Amit R. Majithia, Houshang Monajemi, Gijs W. van Haaften, Maarten R. Soeters, Eric Kalkhoven

The nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ) is the master regulator of adipocyte differentiation, maintenance, and function. Loss-of-function mutations in the PPARG gene cause familial partial lipodystrophy subtype 3 (FPLD3), characterized by progressive and gradual change of subcutaneous adipose tissue distribution during the peripubertal phase. Broekema et al. have identified and characterized a novel PPARγ L451P mutant in a family affected by FPLD3. They show that this mutant significantly impairs the transcriptional activity of PPARγ due to a range of molecular defects.

Objective: The nuclear receptor PPARγ is the master regulator of adipocyte differentiation, distribution, and function. In addition, PPARγ induces terminal differentiation of several epithelial cell lineages, including colon epithelia. Loss-of-function mutations in PPARG result in familial partial lipodystrophy subtype 3 (FPDL3), a rare condition characterized by aberrant adipose tissue distribution and severe metabolic complications, including diabetes. Mutations in PPARG have also been reported in sporadic colorectal cancers, but the significance of these mutations is unclear. Studying these natural PPARG mutations provides valuable insights into structure-function relationships in the PPARγ protein. We functionally characterized a novel FPLD3-associated PPARγ L451P mutation in helix 9 of the ligand binding domain (LBD). Interestingly, substitution of the adjacent amino acid K450 was previously reported in a human colon carcinoma cell line.

Methods: We performed a detailed side-by-side functional comparison of these two PPARγ mutants.

Results: PPARγ L451P shows multiple intermolecular defects, including impaired cofactor binding and reduced RXRα heterodimerisation and subsequent DNA binding, but not in DBD-LBD interdomain communication. The K450Q mutant displays none of these functional defects. Other colon cancer-associated PPARγ mutants displayed diverse phenotypes, ranging from complete loss of activity to wildtype activity.

Conclusions: Amino acid changes in helix 9 can differently affect LBD integrity and function. In addition, FPLD3-associated PPARγ mutations consistently cause intra- and/or intermolecular defects; colon cancer-associated PPARγ mutations on the other hand may play a role in colon cancer onset and progression, but this is not due to their effects on the most well-studied functional characteristics of PPARγ.