Background: HNF4␣ is a key factor regulating hepatocyte differentiation and liver-specific functions. Results: Acute disruption of HNF4␣ in the adult liver causes rapid hepatocyte proliferation through direct and indirect control of multiple pathways. Conclusion: HNF4␣ is necessary to maintain the differentiated state of hepatocytes in adult liver. Significance: HNF4␣ expression maintains normal liver function, and its loss stimulates hepatocytes proliferation, possibly leading to cancer.

Peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) inhibits skin tumorigenesis through mechanisms that may be dependent on HRAS signaling. The present study examined the hypothesis that PPARβ/δ promotes HRAS-induced senescence resulting in suppression of tumorigenesis. PPARβ/δ expression increased p-ERK and decreased p-AKT activity. Increased p-ERK activity results from the dampened HRAS-induced negative feedback response mediated in part through transcriptional upregulation of RAS guanyl-releasing protein 1 (RASGRP1) by PPARβ/δ. Decreased p-AKT activity results from repression of integrin-linked kinase (ILK) and phosphoinositide dependent protein kinase-1 (PDPK1) expression. Decreased p-AKT activity in turn promotes cellular senescence through upregulation of p53 and p27 expression. Both over-expression of RASGRP1 and shRNA-mediated knockdown of ILK partially restore cellular senescence in Pparβ/δ-null cells. Higher PPARβ/δ expression is also correlated with increased senescence observed in human benign neurofibromas and colon adenoma lesions in vivo. These results demonstrate that PPARβ/δ promotes senescence to inhibit tumorigenesis and provide new mechanistic insights into HRAS-induced cellular senescence.

The availability of high-affinity agonists for peroxisome proliferator-activated receptor-␤/␦ (PPAR␤/␦) has led to significant advances in our understanding of the functional role of PPAR␤/␦. In this study, a new PPAR␤/␦ antagonist, 4-chloro-N-(2-{[5-trifluoromethyl)-2-pyridyl]sulfonyl}ethyl)benzamide (GSK3787), was characterized using in vivo and in vitro models. Orally administered GSK3787 caused antagonism of 4-[2-(3-fluoro-4-trifluoromethyl-phenyl)-4-methyl-thiazol-5-ylmethylsulfanyl]-2-methylphenoxy}-acetic acid (GW0742)-induced up-regulation of Angptl4 and Adrp mRNA expression in wild-type mouse colon but not in Ppar␤/␦-null mouse colon. Chromatin immunoprecipitation (ChIP) analysis indicates that this correlated with reduced promoter occupancy of PPAR␤/␦ on the Angptl4 and Adrp genes. Reporter assays demonstrated antagonism of PPAR␤/␦ activity and weak antagonism and agonism of PPAR␥ activity but no effect on PPAR␣ activity. Time-resolved fluorescence resonance energy transfer assays confirmed the ability of GSK3787 to modulate the association of both PPAR␤/␦ and PPAR␥ coregulator peptides in response to ligand activation, consistent with reporter assays. In vivo and in vitro analysis indicates that the efficacy of GSK3787 to modulate PPAR␥ activity is markedly lower than the efficacy of GSK3787 to act as a PPAR␤/␦ antagonist. GSK3787 antagonized GW0742-induced expression of Angptl4 in mouse fibroblasts, mouse keratinocytes, and human cancer cell lines. Cell proliferation was unchanged in response to either GW0742 or GSK3787 in human cancer cell lines. Results from these studies demonstrate that GSK3787 can antagonize PPAR␤/␦ in vivo, thus providing a new strategy to delineate the functional role of a receptor with great potential as a therapeutic target for the treatment and prevention of disease.

Background: It is unclear whether ER stress and associated unfolded protein response (UPR) can influence oncogeneinduced senescence. Results: ER stress attenuated senescence by modulating kinases, and a positive feed forward loop was delineated where ER stress caused loss of senescence and promotion of tumorigenesis. Conclusion: A new role for ER stress and UPR that attenuates H-RAS-induced senescence was discovered. Significance: PPAR␤/␦ may suppress RAS-dependent tumorigenesis.