Identification and functional activation of peroxisome proliferator-activated receptor in human upper aerodigestive cancer

Abstract

Squamous cell carcinoma of the head and neck (SCCHN) is an aggressive malignancy whose carcinogenesis occurs in multiple stages years to decades after carcinogen exposure. In spite of continued advances in the understanding of molecular biology of SCCHN and the introduction of a multitude of multi-modality treatment protocols, the 20-50% survival of stage III and IV disease has not changed appreciably in over twenty years. Efforts to treat or prevent recurrence have predominantly involved the use of cytotoxic chemotherapy, however the use of retinoids as a chemoprevention agent has been clinically assessed. Success has been limited by toxicity of retinoids and reversal of differentiation changes upon cessation of treatment. Peroxisome proliferator-activated receptors (PPARs) are lipid-activated transcription factors belonging to the steroid/thyroid/retinoic acid nuclear receptor superfamily. PPARs primarily target genes involved in lipid homeostasis; one isoform, PPARã, directs the differentiation of precursor cells into adipocytes. PPARã heterodimerizes with RXRá to form a functional transcription factor. The recognition of ETYA as a PPAR activator along with the previously-discovered anti-cancer and differentiation effects of this agent invoked the possibility that a differentiation strategy could be used in the treatment in SCCHN. This study examined PPARã in SCCHN. PPARã protein was expressed in SCCHN cell lines. Treatment of two cell lines with three chemically distinct ligands of PPARã caused dose-dependent inhibition of proliferation. Cells treated with these agents caused cytoplasmic lipid vacuole accumulation consistent with adipogenic phenotype shift. Electromobility supershift analysis demonstrated DR-1 consensus sequence binding activity in SCCHN nuclear extracts. The more-specific cyp4a1 oligonucleotide also demonstrated binding, the amount of which was upregulated with treatment. Supershift analysis demonstrated presence of PPARã in nuclear extracts. Functional activation was assessed using dual transfection reporter gene assays, which demonstrated dose dependent increased PPRE activation with treatment using each agent. We conclude that PPARã ligands may represent a class of drugs which have value in the treatment of SCCHN.