IBCN 2018: PPARɣ-Mediated Repression of TFAP2A Expression Identifies an Important Transcriptional Circuit in Basal-Squamous Bladder Cancer
In addition to being transcriptionally regulated, previous studies suggest molecular subtype is “plastic” and can change from a Luminal to Basal-Squamous signature during tumor progression. However, the transcriptional drivers of Basal-Squamous bladder cancer, as well as the degree to which they contribute to the aggressive phenotype typical of this subtype is unknown.
They hypothesized that inactivation of PPARɣ signaling during tumor progression results in increased expression of transcription factors (TFs) specific to the Basal-Squamous subtype. To identify PPARɣ-repressed TFs that promote a Basal-Squamous signature, they initiated a pharmacologic and RNA-seq-based screen following treatment of UMUC1, SW780 and 5637 cell lines with the PPARɣ agonist rosiglitazone (TZD). Hierarchical clustering of RNA-seq data identified a number of TFs regulated by PPARɣ activation. Several of these transcription factors are implicated in urothelial and squamous differentiation. One PPARɣ-repressed TF implicated in skin development and SqD identified by this screen was Transcription Factor Activating Protein 2 alpha (TFAP2A).
Computational analysis of publically available data, as well as immunohistochemistry of our in-house tissue cohort shows TFAP2A and its paralog TFAP2C are significantly overexpressed in Basal-Squamous bladder cancer and in areas of SqD in cystectomy samples. In addition, we show TFAP2A and TFAP2C overexpression are additionally associated with adverse oncologic outcomes. Q-RT-PCR and western blotting analysis also confirmed the ability of PPARɣ activation to repress TFAP2A, and PPARɣ antagonist studies indicate a requirement of a functional receptor for TZD-induced TFAP2A repression.
Additional in vitro experimentation and in vivo tissue recombination experiments show TFAP2A and TFAP2C regulate bladder cancer cell migration, invasion and promote tumor growth, attributes typically associated with the Basal-Squamous subtype. Taken in light of previous studies, these findings definitively identify PPARɣ as a master regulator of bladder cancer cell fate, and further identify TFAP2A as a PPARɣ-repressed transcriptional driver of Basal-Squamous bladder cancer. These results further suggest that PPARɣ inactivation, as well as TFAP2A and TFAP2C overexpression cooperate with other TFs and epigentic changes to promote development of Basal squamous disease during progression. In summary, these findings note tumor heterogeneity and further understanding of bladder carcinogenesis according to molecular subtypes.
Presented by: David J. DeGraff, Pennsylvania State University College of Medicine, Hershey, PA
Written by: Stephen B. Williams, M.D., Associate Professor, Division of Urology, The University of Texas Medical Branch, Galveston, TX. and Ashish M. Kamat, M.D. Professor, Department of Urology, Division of Surgery, The University of Texas MD Anderson Cancer Center, Houston, TX at the 16th Annual Meeting of the International Bladder Cancer Network (IBCN) October 11-13, 2018 - the Inntel Hotels Rotterdam Centre, Rotterdam, The Netherlands