Introduction:

Despite the prevalence of phosphoinositide 3-kinase (PI3K) pathway alterations in prostate cancer (PCa), the clinical efficacy of PI3K-targeted therapies in combination with androgen deprivation therapy (ADT) has been limited. The dichotomy between effectiveness in cancer cell lines and lack of effectiveness in patient tumors led us to hypothesize that the tumor microenvironment (TME) may play an important role in driving drug resistance in PCa. In this study, we designed a comprehensive secreted factor screen to determine novel TME-mediated resistance mechanisms to PI3K pathway inhibitors in PCa.

Methods:

A curated library of 297 secreted factors was screened for their effects on drug response in LNCaP, 22Rv1, VCaP, C42B, and PC3 prostate cancer cell lines. Cells were treated with PI3K pathway inhibitors (GDC-0941, Capivasertib, Sapanisertib) alone and in combination with enzalutamide, an androgen receptor inhibitor. The expression of secreted factors was assessed in human prostate tumors using immunohistochemistry from The Human Protein Atlas. Mechanistic validation was performed using western blotting and viability assays with siRNA and targeted antibodies. In vivo efficacy was tested in cell line-derived xenograft models.

Results:

Primary screening identified Neuregulin 1 (NRG1) as a top secreted factor that conferred resistance to PI3K inhibitors (Figure 1A). NRG1 was found to be expressed in 10/12 (83%) of prostate cancer tumors, in the stromal cell compartments (Figure 1B). Cell viability assays demonstrated that NRG1 treatment leads to resistance to multiple PI3K pathway inhibitors in combination with enzalutamide in a dose-dependent manner (Figure 1C). NRG1-mediated resistance requires ligand activation and phosphorylation of the HER3 receptor, and can be abrogated by treatment with anti-HER3 antibody seribantumab or knockdown with HER3 siRNA. In 22RV1 cell line xenografts, combination therapy with seribantumab, enzalutamide, and pictilisib significantly suppressed tumor growth compared to enzalutamide and pictilisib alone (Figure 1D).

Conclusion:

The TME-derived secretome plays a critical role in modulating resistance to targeted therapies in prostate cancer. NRG1 in the PCa TME, activates HER3 signaling and promotes resistance to both AR and PI3K pathway inhibitors. Targeting HER3 in combination with PI3K inhibitors represents a promising therapeutic strategy to overcome NRG1-mediated resistance mechanisms in advanced prostate cancer.

Funding: AUA/UCF Resident Research Award