Phenotypes and transcriptional regulators of non-luminal epithelial cell identities in castration-resistant prostate cancer.

Abstract

Most prostate cancers have a luminal epithelial cell identity driven by the androgen receptor (AR) transcription factor. As a result, locally advanced or metastatic prostate cancer can be treated with androgen deprivation therapy alone or in combination with androgen synthesis inhibitors like abiraterone or high affinity AR antagonists like enzalutamide. Unfortunately, these therapies are not curative and men eventually develop castration-resistant prostate cancer (CRPC). Targeted therapies can promote lineage plasticity, which is defined as the ability of a cell to alter its identity in response to external pressures. While 70-75% of CRPC restores AR signaling, the remaining 25-30% display lineage plasticity evidenced by AR negative subtypes like neuroendocrine CRPC (NEPC) or “double negative” CRPC (DNPC) lacking both AR and NE markers. These AR-independent manifestations of CRPC represent a clinical challenge because attractive therapeutic targets are largely unknown. The studies performed in this thesis were aimed at characterizing the phenotypes and drivers of non-luminal cell identities in CRPC lineage plasticity and AR independence. We begin by reviewing how basal epithelial cells function not only in benign prostate homeostasis, but also in cancer initiation and castration resistance. We then extend our exploration of non-luminal cell identities in CRPC to include recently-identified prostatic club and hillock epithelial cells. Analysis of RNA-sequencing data from clinical samples revealed that a subset of CRPC tumors historically classified as DNPC co-express genes defining basal, club, and hillock epithelial cells of the benign prostate. A loss-of-function screen revealed KLF5 as a master regulator of this mixed cell identity, which was validated using knockdown and overexpression experiments. We highlight KLF5-mediated upregulation of RARG as a therapeutic vulnerability of this mixed basal, club, and hillock CRPC that can be targeted with differentiation-inducing agents like all-trans retinoic acid. In AR-positive models, we explore how CRPC status affects AR-regulation of KLF5 upon treatment with androgens. However, given KLF5 expression is highest in CRPC subtypes lacking AR, we end by exploring future directions to identify additional regulators of KLF5. Overall, this work links the poorly-characterized DNPC subtype to phenotypes of a mixed basal, club, and hillock epithelial cell identity driven by KLF5.