Prostate cancer (PCa) is the second leading cause of cancer death in American men after lung cancer, and one in every 41 men will die due to PCa (Chornokur et al., 2011). The first-line treatment for advanced PCa is continuously improving, and drugs with different mechanisms of action are now being used in the treatment of early disease conditions (Kim and Koo, 2020). PCa cells rely on AR signalling for their growth which is commonly called androgen dependent PCa (ADPC), and androgen ablation therapy is the mainstream treatment of ADPC (Sharifi et al., 2005, Wen et al., 2014). Although, ADT is very effective initially, prolonged ADT results in the development of androgen independent PCa (AIPC). AIPC is correlated with an increased mortality rate because of no effective treatment options (Abd Wahab et al., 2020). In AIPC, PCa cells proliferate in the absence of androgens, and it is one of the drawbacks of the treatment of advanced PCa (Feldman and Feldman, 2001). Furthermore, the understudied challenge in PCa treatment is the progression of AIPC to neuroendocrine PCa (Saraon et al., 2014). Nearly 1 % of prostate tumors develop neuroendocrine phenotypes in the absence of androgens (Saraon et al., 2012). Normally the prostate epithelium contains basal cells, luminal cells, and NE (neuron endocrine) cells, which represent only 1 % of all the cells (Cheng et al., 2020). However, the precise role of NE cells is partially known, but recent studies believe that they may have a vital role in prostate cell growth and differentiation by secretion of several factors including peptide hormones (serotonin, histamine, chromogranin A, and calcitonin), thyroid stimulating hormone, parathyroid hormone-related protein, vascular endothelial growth factor, bombesin, vasoactive intestinal peptide, and somatostatin (Markwalder and Reubi, 1999, Petit et al., 2001, Sun et al., 2009) into the tumor microenvironment supporting the growth of surrounding tumor cells in a paracrine manner (Vashchenko and Abrahamsson, 2005). Unlike epithelial cells, NE cells are non-proliferative, terminally differentiative, lack the expression of PSA and AR, and are also resistant to AR blockers (Hu et al., 2015). In PCa tissues, NE differentiated cells are recognized by immunohistochemical staining for markers such as neuron specific enolase (NSE), chromogranin A, synaptophysin, and CD56 (Mjønes et al., 2017).

Recent reports highlight that activation of different signalling pathways such as STAT3 (Chang et al., 2014), AMPK (Shanmugam et al., 2013), and ERK (Bosutti et al., 2016) are involved in acquiring NE characteristic features for PCa cells. In our recent investigations, we reported that IL-6 is involved in the development of AIPC through the activation of STAT3 involving the VCP-Pim-1 axis (Duscharla et al., 2018). In-vitro activation of AMPK and TGF-Beta induces NED of PCa cells through the activation of p38MAPK (Natani et al., 2022). However, no signalling mechanism and/or protein has shown complete involvement in the development of NED of PCa to develop new druggable targets towards novel targeted therapies. Hence, understanding the molecular mechanisms and proteins involved in the progression of AIPC is crucial in developing as a newer therapeutic approach.

Tumor protein D52 (TPD52), a proto-oncogene encoded by tumor susceptible region on chromosome 8, is involved in the progression of PCa. On chromosome 8, the 8q21 region encodes for the TPD52 family of proteins, including TPD52 (184 aa, isoform 3) and PC-1/PrLZ (224 aa, isoform 1), which are known to play an important role in the progression of metastatic PCa (Ummanni et al., 2008). Several studies involving in-vitro and in-vivo models have reported that TPD52 plays an important role in maintaining tumorigenicity and metastasis by increasing the proliferation and invasive potential of tumor cells and also by evading apoptosis (Lewis et al., 2007).

A recent study has shown that PC-1 (isoform-1 of TPD52) is upregulated in NE differentiated LNCaP cells by IL-6 treatment in in-vitro and also activates AR in ligand independent manner for the survival of tumor cells (Moritz et al., 2016). Dasari et al. reported that TPD52 promotes PCa cell survival via activation of different cell survival networks such as AKT, STAT3, and NF-κB (Dasari et al., 2017). In PCa cells, transactivation of NF-κB by TPD52 promotes paracrine activation of STAT3 and also promotes a cross talk between NF-κB and STAT3 signalling axis (Grivennikov and Karin, 2010; Han et al., 2010). Activation of NF-κB can lead to the production of interleukin-6 (IL-6), which in turn activates STAT3 (Hoesel and Schmid, 2013). Earlier studies have reported that activation of STAT3 has been implicated in promoting neuroendocrine differentiation of different cancers, including prostate, lung, and pancreatic cancers (Chang et al., 2005, Liu et al., 2019, Scholz et al., 2003). Recently, Li et al. reported that PrLZ, an isoform of TPD52 family proteins regulates the transactivation of androgen receptor (AR) in castration-resistant PCa and promotes cell growth (Li et al., 2013). Therefore, we hypothesized that TPD52 might be driving the castration resistant PCa cells emerging from NEPC cells causing resistance to PCa therapies. In this study, we investigated the role of TPD52 and its molecular mechanism in promoting NED of LNCaP cells.