Background: Prostate cancer (PC) growth is driven by androgen receptor (AR) signaling, and AR-targeted therapies extend survival in many patients. However, advanced tumors develop resistance to these therapies via various mechanisms like adopting basal-like or stem cell–like (SCL) transcriptional programs, which are less dependent on AR and linked to aggressive, treatment-resistant disease. As 30-40% of PCs are basal-like, identifying underlying mechanisms in which these PCs become basal or SCL is vital for treating aggressive PCs. CREB5, a transcription factor, has been implicated in the progression of breast, colorectal, and prostate cancers. However, while these studies independently point to CREB5 as a key factor of tumor progression, the underlying transcriptional regulatory role of CREB5 in PC remains elusive.

Methods: We analyzed RNA-sequencing data from 701 patient tumors (n=208, Stand Up To Cancer; n=493, TCGA) to assess correlations between CREB5 expression, AR activity, and basal and SCL gene signatures. In an AR-positive PC cell line (LNCaP), rapid immunoprecipitation mass spectrometry of endogenous protein (RIME) and chromatin immunoprecipitation (ChIP) assays determined CREB5’s protein interactions and its binding to regulatory DNA elements. Functional impact on tumor progression was evaluated by measuring three-dimensional tumorsphere formation in vitro, and assessing tumor growth in mouse xenografts with CREB5 overexpressed in the LNCaP cell line.

Results: High CREB5 expression inversely correlated with AR signaling and positively correlated with basal-like and SCL transcriptional programs in both primary and castration-resistant PCs. We found that CREB5 protein interacted with AP-1 factors, which have previously been implicated in stem cell features, therapy resistance, and metastasis. CREB5 also bound to regulatory elements of several AP-1 factors,  suggesting a complex mechanistic role in regulating activity of numerous AP-1 genes. Functionally, cells overexpressing CREB5 formed significantly more tumorspheres and produced larger tumors in vivo, demonstrating its association with enhanced stemness and aggressive growth in PC.

Conclusion: Altogether, this study indicates that CREB5 enhances PC tumor progression through genes that are associated with SCL transcriptional programs. Therefore, CREB5 may act as a pivotal regulator of differentiation in PC. As CREB5 was associated with AP-1 family members, this may be a mechanism of such tumor progression. Altogether, increased CREB5 may lead to specific mechanisms of therapy response, and future therapeutic strategies may consider antagonizing CREB5 interactions with AP-1 complexes to mitigate tumor progression in deadly PCs.