The functional unit of the prostate gland is the acinar which comprises distinct layers of epithelial cells, each characterised by specific gene expression and functions ^[1][30].

The basal layer plays a crucial role in ensuring structural support and tissue integrity. It consists of basal cells that express cytokeratin (CK5, CK14), integrin α2β1, CD44, and TP63 genes, while lacking AR expression ^{[1][2][3][4][5]}[30,32,33,34,35]. These basal epithelial cells also express genes associated with castration resistance, such as BCARC1 (p130cas) and EGFR ^[6][7][36,37], and with angiogenesis, such as NRP1 and VEGFA ^[8][38]. Of note, rare neuroendocrine cells are found within this layer, exhibiting positive staining for Chromogranin A and other neuropeptides ^[9][39]. These cells have a developmental stem cell of origin in common with epithelial cells ^[10][40]. The basal stem cells are responsible for the development and renewal of differentiated and functional luminal cells in adult prostatic glands ^[11][12][41,42]. The luminal cells, in direct contact with the basal cells, secrete substances like prostate-specific antigen (KLK3/PSA) and other kallikreins contributing to the secretion of the seminal fluid. These cells express the AR along with a combination of AR pathway drivers such as FOXA1 and HOXB13, as well as AR-targeted genes such as KLK3, KLK2, TMPRSS2, PSMA, and PSCA ^[13][14][43,44].

Additionally, there is a population of transit amplifying cells that may encompass ‘Club’ (KRT4+, SCGB1A1+) and “hillock” (KRT13+) cells described by G.H. Henry et al. ^[15][16][45,46]. In a normal adult prostate, these cells are rare; they are more frequent in foetal prostates and in pre-tumoral conditions ^[17][47]. These cells are characterised by the co-expression of basal and luminal cytokeratin, high proliferation, and a lack of the cyclin-dependent kinase inhibitor p27 (CDKN1B). This gene is involved in cell cycle arrest, wherein increased level of p27 indicates an exit from the cell cycle ^[18][48]. In adult prostatic glands, p27 is expressed by all cells in the luminal compartment and by a subpopulation of basal cells. Conversely, p27 downregulation occurs not only in most prostate cancers ^[19][49] but also in high grade prostatic intraepithelial neoplasia (PIN) ^[20][50].

Positioned beneath the basal layer and separated by a basement membrane ^[21][51], the non-epithelial prostate microenvironment, collectively termed ‘stroma’, is composed of various cell types. The stroma is a complex cellular network that plays a vital role for normal prostate development and related diseases ^{[22][23][24][25]}[52,53,54,55].

The predominant cell type within the stroma is fibroblasts, which when beside cancer can be known as cancer-associated fibroblasts (CAF). CAFs can be identified by specific markers such as vimentin, ZEB1, MMP2, COL1A1, COL1A2, ASPN, BGN, and SFRP4 ^[22][26][27][52,56,57]. Smooth muscle cells are also present in the stroma and can be distinguished by the expression of ACTA2, MYLK, CALD1, and CNN1 ^[28][29][15,58]. In prostate cancer, the reactive stroma is characterised by a higher proportion of fibroblasts/myofibroblasts, which is offset by a decrease in differentiated smooth muscle cells ^[30][31][32][59,60,61].

Apart from fibroblasts and smooth muscle cells, the stroma contains other components, including blood vessels, lined by endothelial cells expressing endothelial receptors (EDNR and CD31) ^[33][34][62,63]. The stroma also houses nerves and a diverse infiltration of inflammatory immune cells, T cell phenotypes (CD3, CD45), macrophage phenotypes (CD68), granulocytes (CD24, CD15), and B cells (CD19, CD20) ^[35][36][64,65].

Altogether, epithelial layers and stroma form a complex and dynamic network, wherein effective bidirectional communication between prostate epithelial cells and the stroma is crucial for prostate development, renewal, and secretory function ^[37][38][66,67]. The secretion of growth factors, such as transforming growth factor beta (TGFβ) and fibroblast growth factors (FGF), are key players ensuring these functions and efficient communication ^[39][40][68,69]. Indeed, stroma cells secrete FGF7 and FGF10, primarily affecting epithelial cells and lead to the development of prostate cancer by increasing sensitivity to androgens ^{[41][42][43][44][45]}[70,71,72,73,74]. Conversely, epithelial cells secrete FGF2, which regulates fundamental stromal processes such as angiogenesis and cell proliferation ^[46][75]. In addition, TGFβ is an important mediator of bidirectional communication, promoting tumour growth and metastasis by facilitating epithelial-to-mesenchymal transition in epithelial cells ^[47][48][76,77]. TGFβ-secreting epithelial cells influence stromal cell behaviour, including proliferation and extracellular matrix production, thereby supporting tumour invasion and metastasis ^[49][50][78,79].

Stromal and epithelial prostate cells collaborate in the metabolism of sexual steroid hormones and fatty acids. Prostate tissue can locally synthesise dihydrotestosterone (DHT), the most potent androgen, from various androgen precursor molecules ^[51][80]. As a result, despite androgen deprivation, levels of these hormones in the prostate remain high enough to promote cancer progression ^[52][81]. This phenomenon led to the development of new anti-androgen drugs which target the enzymes involved in steroid synthesis or which directly target the AR in prostate cells. Additionally, it is well-established that fatty acid metabolism is a potential target of the epithelial-mesenchymal transition, a key driver of prostate cancer development ^[53][54][82,83]. Prostate cells undergo a shift in lipid beta-oxidation pathways during carcinogenesis, accompanied by increased expression of AMACR, ACLY, ACACA, and FASN enzymes involved in the lipid metabolism ^[55][84].

Polygenic susceptibility to prostate cancer is influenced by functional polymorphisms of a single nucleotide. These polymorphisms play crucial roles during prostate development and homeostasis, influencing the expression of specific prostate transcripts and carrying implications for prostate cancer risk ^[56][85] or fertility. In addition to rare germline mutations, which encompass DNA repair genes associated with prostate cancer susceptibility, these polymorphisms affect key pathways, including those related to the AR (HOXB13, FOXA1) and proliferation (MYC, FGF10) ^[56][85]. These genetic variations have been found to correlate with disparities in prostate cancer risk based on ancestral backgrounds.

Histopathological evaluation of prostate cancer involves the assessment of several key features, including the Gleason score, tumour grade, tumour stage, and the presence of extraprostatic extension ^[57][58][90,91]. Since 1996 ^[59][92], the Gleason score has been a widely used grading system that assesses the architectural patterns of cancer cells. The initial scoring system ranged from 2 to 10, where higher scores indicated a more aggressive disease. However, the scoring system has been revised to now range from 6 to 10 and is transposed into the International Society of Urological Pathology (ISUP) grading system, which ranges from 1 to 5 ^[60][93]. Tumour grade refers to the degree of cellular differentiation and is categorised as low grade (Gleason score 6 or ISUP-1), intermediate grade (Gleason score 7 (3 + 4) or ISUP-2 and Gleason score 7 (4 + 3) or ISUP-3), or high grade (Gleason score 8–10 or ISUP-4-5). Tumour stage provides information about the extent of cancer spread beyond the prostate gland and extraprostatic extension indicates the presence of cancer cells outside the prostatic edge.

Prostate cancer primarily consists of adenocarcinoma, but rare variants, comprising less than 5% of cases, have been identified ^[61][62][63][94,95,96]. These variants include acinar subtypes (such as cribriform, intraductal, mucinous, prostatic intraepithelial neoplasia-like carcinoma, signet ring cell carcinoma, sarcomatoid carcinoma, and pleomorphic giant cell carcinoma) and non-acinar subtypes (such as ductal carcinoma, carcinoma with neuroendocrine differentiation as small cell carcinoma, squamous cell carcinoma, and adenoid cystic carcinoma). Each variant has distinct histological and clinical features, leading to different outcomes. For instance, the mucinous variant tends to have a more favourable prognosis, with an approximately 80% 10-year survival rate, while the neuroendocrine variant has a poorer prognosis, with less than 10% survival at 10 years ^[9][39].

Typical acinar adenocarcinoma is characterised by glandular structures with a luminal phenotype, lacking basal cells and basement membrane layers. The diagnosis of typical acinar adenocarcinoma is confirmed in pathological practice by identifying AMARC+/P63- gland pattern using immunohistochemistry ^[64][97]. New tissue markers associated with prostate cancer, such as PCA3, DLX1, and HOXB6, as well as hypermethylated genes like GSTP1, APC, RASSF1, and copy number variations, have been identified and used in the development of new diagnostic tests based on molecular changes in prostatic secretions collected in the urine ^{[65][66][67][68]}[98,99,100,101]. However, it is important to note that typical prostatic adenocarcinoma exhibits molecular heterogeneity ^[69][70][71][102,103,104]. In addition, gene expression profiling studies have identified distinct molecular subtypes of prostate cancer, including acinar and non-acinar subtypes ^[61][72][73][94,105,106]. The acinar subtype, characterised by glandular structures resembling normal prostate tissue, is the most common. In contrast, the non-acinar subtype lacks typical glandular structures and is often associated with more aggressive disease behaviour, higher Gleason scores, and poorer clinical outcomes compared to the acinar subtype ^[74][107].

Recent advances in genomic profiling have further subdivided primary prostate cancers into subgroups based on their genetic and epigenetic profiles ^{[75][76][77][78][79]}[108,109,110,111,112]. The main subgroup consists of erythroblast transformation-specific (ETS) fusion-positive tumours (59%), resulting from a fusion between androgen-driven genes (TMPRSS2, SLC45A3) and embryogenic/oncogenic genes (ERG, ETV1/4, FLI1) ^[80][81][113,114]. These fusion-positive tumours, in particular TMPRSS2-ERG fusion-positive tumours (46%), often exhibit PTEN deletion and demonstrate upregulation of HDAC1 which is involved in the regulation of the AR. These tumours also show upregulation of NPY and PLA2G7, which are involved in cell growth, migration and invasion, along with downregulation of AZGP1, known to play a role in lipid metabolism ^{[82][83][84][85]}[115,116,117,118]. Although these fusions are predominantly associated with the common acinar adenocarcinoma, they have also been detected in rare variants ^[86][119]. Interestingly, in prostate cancer, harbouring androgen dependent fusion genes, such as TMPRSS2-ERG, the AR switches from an antiproliferative to an oncogenic gene ^[80][113].

Other genetic alterations have been observed in adenocarcinoma, such as PTEN, AR, and SPOP ^[87][88][120,121]. PTEN plays a crucial role in regulating the phosphatidylinositol 3-kinase kinase (PI3K)- protein kinase B (AKT) signalling pathway, which controls cell growth, survival, and metabolism. PTEN is also known as a tumour suppressor gene in prostate cancer, and its loss or inactivation is associated with patterns of increased tumour aggressiveness in localised prostate cancer ^[89][90][122,123]. AR is a critical driver of prostate cancer, playing a key role in the growth and survival of cancer cells. Alterations in AR signalling, such as AR amplification, mutations, or ligand-independent activation, are frequently observed. Constitutively active variant ARs are also found in prostate cancer ^[91][92][93][124,125,126]. SPOP is an E3 ubiquitin ligase with mutations accounting for 11% of cases, which affect protein degradation and influence the development of adenocarcinoma ^[75][108]. Additional genetic alterations include FOXA1 and IDH1 mutations, found in 3% and 1% of cases, respectively ^[75][108].

Similar to acinar adenocarcinoma, rare variants of prostate cancer involve several genes in their development and progression. TP53 mutations are commonly found in small cell carcinoma of the prostate, contributing to its aggressive nature ^[71][94][104,127]. Cribriform patterns can be further classified based on the proportion of cancer cells exhibiting PTEN-loss and PD-L1 overexpression ^[95][128]. Genes such as AR, ERG, FOXA1/2, MUC16, RB1, CDH1, BRCA2, and TP53 have also been involved in different variants of prostate cancer ^[96][97][98][129,130,131].

Nowadays, histopathological patterns are well recognised to play a crucial role in determining the management approach for early-stage prostate cancer ^[99][132]. However, certain patterns such as large cribriform or intraductal patterns are not recommended for active surveillance due to their association with a higher risk of disease progression ^{[100][101][102]}[133,134,135]. Rare events like BRCA2 germline mutations, which are known to have a worse prognosis, also exclude patients from active surveillance management ^[103][136]. The management approach of prostate cancer is complex, as exemplified by TMPRSS2-ERG, a key player in the initial development of prostate cancer ^[104][137], which is not correlated with the progression of the disease to its life-threatening stage. Conversely, molecular events linked to BRCA2, TP53, RB1, and AR become more frequent as prostate cancer progresses from the metastatic stage to the castration-resistant stage, indicating their involvement in the later stages of the disease ^[105][138].

Recently, genomic profiling has emerged as a valuable tool in prostate cancer prognosis and therapeutic decision-making, which may help to decipher the progression of prostate cancer ^[106][107][139,140]. It provides a molecular dimension that complements the traditional histopathological classification. Genomic prognostic signatures based on transcriptomic profiles have been developed to provide clinicians and patients with more confidence in selecting between active surveillance or radical therapies in the early stages of the disease ^[108][141]. These genetic signatures encompass biological processes that may play a central role in tumour initiation and progression. For example, the Prolaris^® test ^[109][110][142,143] focuses on genes associated with proliferation, providing insights into the tumour’s growth rate. On the other hand, tests like OncotypeDx^{® [111][112]}[144,145] or Prostadiag^{® [87]}[120] incorporate multifunctional gene patterns, including proliferation, differentiation, androgen responsiveness, epithelial-mesenchymal transition, and the presence of cancer-associated fibroblasts, to provide a more comprehensive assessment of the tumour’s behaviour. In particular, the Prostadiag^® signature has been extensively studied ^[87][120], wherein three distinct subgroups of tumours based on gene expression patterns have been found. The first subgroup (S1) comprises aggressive tumours (TMPRSS2-ERG+) often characterised by PTEN deletion or TP53 deficiency, indicating a higher risk of disease progression. The second subgroup (S2) consists of tumours (TMPRSS2-ERG+) with a low risk of progression, with a likelihood of less than 10%.

One of the key signalling pathways involved in prostate cancer is the AR pathway ^{[113][114][115]}[146,147,148]. The AR is a transcription factor playing a fundamental role in regulating gene transcription upon binding to androgens. Its activity involves intricate interactions with other transcription factors, nuclear translocation, and binding to response elements, resulting in both genomic and non-genomic activities ^[116][149]. In the context of prostate cancer, the dysregulation of the AR signalling pathway leads to increased AR activity and the expression of genes that promote tumour growth ^{[113][114][115]}[146,147,148]. These modifications arise from various mechanisms, including amplification or mutations in the AR gene, alterations in co-regulatory proteins, and abnormal activation of downstream signalling molecules.

The AR protein is predominantly expressed in the luminal epithelial cells of the prostate ^[117][150]. In the luminal layer, the AR pathway maintains differentiation and secretion functions and blocks the cell cycle ^[118][151]. Ligand binding induces conformational changes that liberate the AR from heat shock proteins and expose its ligand-binding domain, which contains a nuclear localization signal ^[119][120][152,153]. The ligand-bound AR subsequently forms dimers and undergoes phosphorylation, which are translocated to the nucleus. Within the nucleus, the AR binds to specific elements of the androgen response on DNA and recruits coregulators or coactivators such as FOXA1, GATA2, NKX3-1, and HOXB13 ^{[121][122][123][124][125]}[154,155,156,157,158]. In consequence, the transcription of targeted genes (such as KLK2, KLK3, TMPRSS2, CAMKK2, CDH2, SCL43A1, and FKBP5), playing pivotal roles in various biological functions such as tumour progression, cell cycle regulation, glycosylation, calcium metabolism, and lipid metabolism, is enhanced ^{[124][125][126][127]}[157,158,159,160]. AR can also interact with other proteins, such as HES6 and E2F1 ^[128][161], during castration resistant conditions, and can also repress gene expression. In fact, by collaborating with EZH2-mediated repressive chromatin remodelling, the AR facilitates the repression of target genes ^[129][162].

The WNT/β-catenin pathway also plays an essential role in prostate cancer, influencing cell proliferation, invasion, and stem cell-like properties ^[130][131][163,164]. Although WNT-1 is generally found in low levels in primary prostate epithelial cells, its upregulation has been observed in lymph nodes and bone metastases ^[132][165]. Disruption of this pathway can result from genetic mutations or altered expression of key components, including β-catenin or APC. Mutations in APC and CTNNB1 have been identified in up to 22% of castration-resistant prostate cancers ^[133][134][166,167]. Interestingly, stromal cells release WNT proteins that can activate the WNT signalling pathway in tumour cells ^{[130][135][136][137]}[163,168,169,170]. An important downstream target of this pathway is FOXA2, whose induction is essential for bone metastasis development in prostate cancer ^[138][171].

The PI3K/AKT/mammalian target of rapamycin (mTOR) leads to the increased expression of genes involved in cell proliferation, survival, and metabolism. Furthermore, it contributes to the development of resistance to androgen deprivation therapy ^[139][172]. Dysregulation of the PI3K/AKT/mTOR pathway is frequently observed in prostate cancer, with up to 42% of primary tumours and 100% of metastatic samples showing abnormalities in this pathway ^{[105][133][134][140][141]}[138,166,167,173,174].

The RAS/mitogen activated protein kinase (MAPK) cascade transduces extracellular growth signals through transmembrane receptors to regulate gene expression and cellular functions ^[142][143][175,176]. It is frequently deregulated in cancer, including prostate cancer. The cascade involves activation of RAS and the rapidly accelerated fibrosarcoma (RAF)/ mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK) signalling pathway, leading to transcription of target genes such as MYC and cFOS.

Feedback loops and interactions between components enable cross-regulation within the cascades. A complex interplay unfolds between the PI3K/AKT/mTOR pathway and other oncogenic signalling cascades, including AR, MAPK, and WNT pathways, which further promotes the growth of prostate cancer and contribute to drug resistance ^[139][144][172,177]. These pathways interact and regulate each other.