Rb1 and Trp53 cooperate to suppress prostate cancer lineage plasticity, metastasis, and antiandrogen resistance: Beyond the Abstract

Androgen deprivation therapy (ADT) is an effective treatment for most metastatic prostate cancers (mPCa), but eventually the cancer relapses and resists ADT. Newer generation ADTs, like abiraterone acetate (AA) and enzalutamide (Enza), were developed to combat common resistance mechanisms associated with alterations in the androgen receptor (AR) signaling axis.

Unexpectedly, tumors treated with AA or Enza recur within only 1-2 years, often due to a new form of resistance, transformation of prostate adenocarcinoma (PADC) to histologic variants that often exhibit features of neuroendocrine differentiation (NEPC) and are indifferent to AR signaling. Underlying mechanisms of NEPC transformation are largely unknown and experimental models are limited, hindering development of effective remedies.

While genetic alteration of the RB1 tumor suppressor gene is rare in PADC, it is common in NEPC suggesting RB1 loss of function drives NEPC transformation and antiandrogen resistance. To test this hypothesis, we engineered Rb1 loss in a mouse model of PADC initiated by Pten deficiency. While Pten deficient PADC is poorly metastatic, additional loss of Rb1 causes highly metastatic PADC exhibiting neuroendocrine features analogous to those arising in human NEPC. Pten:Rb1 deficient PADC is sensitive to ADT, but relapses with low AR expression and acquired Trp53 mutations. Gene expression profiling confirms neuroendocrine transformation as indicated by elevated expression of markers like Syp and ChgA. Interestingly, both human and mouse NEPC is associated with increased expression of epigenetic reprogramming factors like SOX2 and EZH2. These epigenetic reprogramming factors may cause NEPC transformation and ADT resistance. Consistent with this possibility, treating both human and mouse NEPC cell lines and tumors with clinically relevant Ezh2 inhibitors reversed NEPC transformation and restored sensitivity to antiandrogen therapy.

Lineage plasticity is increasingly appreciated as a general mechanism underlying therapeutic resistance in a variety of cancers as it leads to loss of tumor cell dependence on the signaling pathways targeted by the therapy. For example, non-small-cell lung cancers (NSCLCs) relapsing from EGFR-targeted tyrosine kinase inhibitors exhibit histologic transformation toward small cell lung cancer with neuroendocrine features and loss of EGFR expression. This is analogous to histologic transformation of PADC relapsing from AA or Enza treatment showing a phenotypic switch from AR+ PADC to AR- NEPC.  RB1 loss is a common genetic event in the histologic transformation observed in both lung and prostate cancers. These observations also demonstrate  RB1 tumor suppressor activity is not solely reliant on negative cell cycle regulation, but may also be a critical factor regulating cancer cell reprogramming. RB1 defects have been shown to disrupt the differentiation and increase stem cell properties in multiple organisms and tissues. Therefore, it is expected RB1 deficiency in cancer will create a permissive environment where tumor cells dedifferentiate to stem cell-like states facilitating phenotypic switching and therapeutic resistance. 

We believe that genetic mutations in genes like RB1 and TP53 disrupt the stability of differentiated cell states through epigenetic deregulation, leading to dedifferentiation of tumor cells and enrichment of stem cell properties. Our gene expression profiling analysis demonstrated elevated expression of a wide range of epigenetic regulators in PADC lacking Rb1, including DNA methyltransferase, histone deacetylase, histone methyltransferase, and methyl-lysine readers. This suggests additional possibilities for targeting epigenetic regulators in order to block or reverse lineage plasticity and therapeutic resistance. As lineage plasticity is likely relevant to disease progression and therapeutic resistance in many cancers, our findings could potentially lead to the development of novel drugs and drug combinations for the treatment of a wide variety of cancers where RB1 loss is commonly observed. 

Recently, growing evidence has shifted the cancer therapy spotlight from nonspecific cytotoxic chemotherapy to long-lasting and safer immunotherapy. For example, Sipuleucel-T and PROSTVAC-VF are vaccine-based immunotherapies to target prostate cancer cells that express prostatic acid phosphatase (PAP) or prostate specific antigen (PSA).  Resistance to these therapies, however, may also be impacted by epigenetic reprogramming to stem cell-like states because expression of the relevant antigens relies on a particular differentiated state of PADC cells. Our published study demonstrates inhibiting epigenetic regulators can reverse dedifferentiation, maintaining the differentiated PADC state. We anticipate the use of epigenetic modulating therapy as a means to prolonging the response to not only targeted therapies like ADT, but also to chemo- and immunotherapy in.

Written By: Sheng- Yu Ku1, Spencer Rosario1, Leigh Ellis2, David W. Goodrich1
Department of Pharmacology & Therapeutics, Roswell Park Cancer Institute, Buffalo, NY 14263.
Department of Oncologic Pathology, Harvard Medical School, Brigham and Women's Hospital, Dana-Farber Cancer Institute, Boston, MA 02215

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