The NFκB transcription factor controls many processes that influence carcinogenesis and cancer progression. Over-expression of NFκB and its transcribed genes are involved in tumor growth, angiogenesis, metastasis, and appear to be correlated with resistance to chemotherapy, advanced tumor stage, PSA recurrence and pre-surgical PSA levels in PC [1–3]. Indeed, there is recent evidence that IL-6 exposure (an NFκB target gene product) induces neuroendocrine differentiation of PC tumour sub-clones, conveying anti-apoptotic phenotype and resistance to chemotherapy [4, 5]. Furthermore, activation of NFκB was proved to be sufficient to maintain androgen-independent growth of prostate and PC by up-regulating androgen receptor action .
The ubiquitin-proteasome system (UPS) is an indispensable cellular regulatory machine with proteolytic and non-proteolytic functions affecting many cancer-related processes including cell cycle regulation, oxidation damage control, apoptosis, cell trafficking, DNA repair, transcription and chromatic re-modelling [7–10]. Ubiquitination regulates at least three steps in the NFκB pathway: degradation of IκB, processing of NFκB precursors, and activation of the IKK, the latter both degradation-dependent and -independent [1, 7].
Neuropeptides (NPs) are naturally occurring peptides that include endogenous opioids Met- and Leu-enkephalin, substance P, bradykinin, angiotensin 1 and 2, ET-1 and bombesin (BBS)-like peptides. Acting as paracrine hormones, they induce responses in many organ systems [11–13]. In relevance to cancer, NPs act as potent mitogens for many cancer types, including small-cell lung cancer and PC [14, 15]. NPs like ET-1 and BBS have been shown to stimulate PC growth and new bone formation in vitro[15–20], promote cell migration [21–24] and show potent synergy with other growth factors implicated in PC progression [22, 25, 26]. Advanced and metastatic PC shows upregulated endothelin type A receptor (ETAR) and reduced endothelin type B receptor (ETBR) expression, therefore sustaining unattenuated ETAR-mediated ET-1 action [17, 18], and higher plasma ET-1 levels compared to hormone-naïve cancer . This pattern of receptor expression also predicts recurrence of PC following radical prostatectomy . Similarly, advanced PC overexpresses gastrin-releasing peptide (GRP) family receptors, via which BBS-like peptides exert their action [27–30].
NEP is a cell membrane enzyme that hydrolyzes and inactivates NPs [11–13]. During transition to androgen-independence, NEP was shown to be downregulated or silenced , frequently via promoter hypermethylation . Furthermore, it appears that androgen deprivation therapy (ADT) leads to emergence of clones that have downregulated NEP expression, as the latter is transcriptionally activated by androgens [31, 33, 34]. This decrease in NEP expression leaves unopposed the autocrine and paracrine mitogenic action of NPs to act as an alternative growth pathway for PC cells in a low androgen environment [3, 21, 22, 35].
The NFκB/UPS pathway and the NEP/NPs axis are therefore two systems that have been previously shown to be greatly involved in PC progression [1, 3, 16, 31, 36–39], but not investigated for their precise interrelation and dynamics. We have observed that with regard to the steady state pattern between these two pathways in vitro, AI cells have increased UPS/NFκB activation and a rich NP milieu due to low NEP activity, while AD cells exhibit an exact mirror image . In this work we have investigated the hypothesis that these pathways are directly linked, and that this link has specific dynamics in PC progression. Elucidation of such biological influences could identify potential benefit from combined clinical targeting of these pathways in castration-resistant, advanced stage PC patients.