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Development and Validation of a New BAG-1L–Specific Antibody to Quantify BAG-1L Protein Expression in Advanced Prostate Cancer – Beyond the Abstract

The androgen receptor (AR) remains the main therapeutic target in prostate cancer.1 Therapies targeting the AR signaling axis have improved the outcome for men with advanced castration-sensitive prostate cancer (CSPC) and castration-resistant prostate cancer (CRPC).1,2 Despite this, the development of treatment resistance remains inevitable, and is, in part, driven by persistent AR signaling for which all currently available AR-targeting therapies are ineffective.1 One potential therapeutic strategy is to target key co-regulators that enhance AR transactivation.

The molecular co-chaperone BAG-1 exists as three major isoforms (BAG-1L, BAG-1M, and BAG-1S) in humans that are generated by alternative initiation of translation from a single mRNA.3 Consistent with this, BAG-1L has a unique N-terminus, which contains a nuclear localization sequence, and unlike BAG-1M and BAG-1S, is found in the nucleus where it binds to and enhances transactivation of AR. Pre-clinical studies have identified BAG-1L as a promising target to inhibit AR function in androgen receptor-dependent prostate cancer.4,5

Despite these promising data, the study of the biological and clinical relevance of BAG-1L has been limited by the lack of availability of a validated BAG-1L–specific antibody, with the reliance upon antibodies that recognize all three BAG-1 isoforms.6 To this end, we developed (in collaboration with RevMAb Biosciences) and validated a BAG-1L–specific antibody to allow the research community to interrogate the biological relevance and clinical significance of BAG-1L in malignant and non-malignant disease.6

Firstly, we validated that the newly developed BAG-1L–specific antibody (RM310) recognized BAG-1L (and not other BAG-1 isoforms) across several cell lines with and without genomic manipulation.6 Following this, we performed exploratory immunohistochemistry (IHC) analyses and determined the expression of BAG-1L protein in normal human, unmatched primary and metastatic CPRC, and matched CSPC and CRPC tissues, demonstrating predominantly nuclear staining.6 Next, we identified that BAG-1L protein expression was higher in CRPC metastases than in unmatched, untreated castration-sensitive primary prostate cancer samples.6 In contrast, BAG-1L protein expression did not change between matched CSPC and CRPC biopsies, where all patients had gone on to develop CRPC, suggesting that BAG-1L protein expression may be associated with more aggressive biology.6 Finally, in BAG-1L protein expression at the time of CRPC biopsy did not associated with clinical benefit from AR-targeting therapy or docetaxel chemotherapy.6

It is important to acknowledge the limitations of this study including pre-analytical processing, heterogenous patient cohorts, different quantification strategies, and small heterogenous (treatments and time of biopsies) patient cohorts.6 However, the development and validation of a BAG-1L–specific antibody is an important step for a research community that has relied on antibodies recognizing all BAG-1 isoforms to study this functionally distinct family member. Critically, this antibody provides a research reagent that can be further developed to interrogate the biological and clinical significance of BAG-1L in malignant and non-malignant diseases.

Written by: Antje Neeb,1 Ines Figueiredo,1 Johann de Bono,1,2 & Adam Sharp1,2

  1. Institute of Cancer Research, London, United Kingdom.
  2. Royal Marsden NHS Foundation Trust, London, United Kingdom.

References:

  1. Westaby D, Fenor de La Maza MLD, Paschalis A, Jimenez-Vacas JM, Welti J, de Bono J, et al. A New Old Target: Androgen Receptor Signaling and Advanced Prostate Cancer. Annu Rev Pharmacol Toxicol. 2022;62:131-53.
  2. Sartor O, de Bono JS. Metastatic Prostate Cancer. N Engl J Med. 2018;378(17):1653-4.
  3. Packham G, Brimmell M, Cleveland JL. Mammalian cells express two differently localized Bag-1 isoforms generated by alternative translation initiation. Biochem J. 1997;328 ( Pt 3)(Pt 3):807-13.
  4. Cato L, Neeb A, Sharp A, Buzon V, Ficarro SB, Yang L, et al. Development of Bag-1L as a therapeutic target in androgen receptor-dependent prostate cancer. Elife. 2017;6.
  5. Lee, II, Kuznik NC, Rottenberg JT, Brown M, Cato ACB. BAG1L: a promising therapeutic target for androgen receptor-dependent prostate cancer. J Mol Endocrinol. 2019;62(4):R289-R99.
  6. Neeb A, Figueiredo I, Gurel B, Rodrigues DN, Rekowski J, Riisnaes R, et al. Development and Validation of a New BAG-1L Specific Antibody to Quantify BAG-1L Protein Expression in Advanced Prostate Cancer. Lab Invest. 2023:100245.

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