protein. Linear representation of the primary sequence of human PrP
showing important protein domains.
The third α helix and the second β sheet are connected by a flexible loop. There are two N-glycosylation sites (residues 183 and 199), which might not be partially or fully glycosylated, resulting in three distinct forms of the PrP
C: the non-glycosylated (~25 kDa), the mono-glycosylated (~25 to 30 kDa) and the bi-glycosylated forms (~35 kDa), respectively. PrP
C is also characterized by the presence of a single disulfide bridge between the two cysteine residues 179 and 214, which allows the link between helix 2 and 3 and serves to stabilize the tertiary structure of the PrP
C (
Figure 2).
1.2. PrPC Expression and Functions
The expression of PrP
C begins at embryogenesis
[1]. The highest level of PrP
C expression was found in the central and peripheral nervous systems
[1]. In adults, strong expressions were detected in the brain, spinal cord, neurons, and glial cells
[7][8]. PrP
C expression was also ubiquitously detected in various cells of the peripheral tissues
[9][10][11][12].
The interest in the study of PrP
C was mainly related to its incrimination in the pathogenesis of the neurodegenerative disorders known as spongiform encephalopathies (SE) or prion diseases
[13][14].
The SE was mainly associated with bovines (BES), and commonly referred to as mad cow disease, which refers to an untreatable and inevitably fatal neurodegenerative illness that affects cattle
[15]. BES is characterized by the aggregation of an abnormal beta-sheet rich isoform of the PrP
C protein called scrapie (PrP
Sc)
[13][14]. In humans, the corresponding form of BES is Creutzfeldt-Jakob’s Disease (CJD), which is also characterized as a brain degenerative disorder
[16]. Fatal familial insomnia (FFI) stands as an exceptionally rare prion disease that induces neurodegeneration and primarily manifests through insomnia, making it incredibly challenging to sleep. The predominant instances of this condition are hereditary in nature, resulting from a mutation in the PRNP gene, while sporadic cases make up the remaining occurrences
[17]. Scrapie gives rise to a lethal degenerative ailment that targets the nervous systems of goats and sheep. Classified as one of several transmissible spongiform encephalopathies (TSEs), it is believed to stem from prions
[18].
In the last decade, PrP
C has also been shown to play a significant role in cancer biology. PrP
C has been found to be upregulated or ectopically expressed in different types of cancer tissues, such as hepatocellular carcinoma, gastric cancer, melanoma, breast cancer, colorectal cancer, pancreatic ductal adenocarcinoma, prostate cancer, osteosarcoma, and glioblastoma
[19][20][21][22][23][24][25][26][27][28]. The increased expression of PrP
C appears to play a crucial role in cancer growth, development, differentiation, invasion, migration, metastasis, chemotherapy resistance, and resistance to apoptosis
[19][20][21][22][23][24][25][26]. The growing body of evidence linking PrP
C to cancer has opened up new avenues for cancer research
[29][30].
The interaction of PrP
C with various proteins and receptors leads to the activation of intracellular signaling pathways that promote tumorigenesis
[31][32]. The differential expression of PrP
C in various types of cancer, its involvement in protein–protein interactions and its activation of downstream pathways confers to this protein a likely role in cancer.
32. PrPC in Human Cancers
32.1. PrPC and Gastric Cancer
The expression of PrP
C has been reported to be highly elevated in gastric cancer tissue, indicating its potential involvement in the pathogenesis of this disease
[20]. Moreover, PrP
C has been shown to promote multidrug resistance in gastric cancer cells by inhibiting apoptosis
[20]. Due to its ability to bind to certain extracellular matrix and adhesive proteins, PrP
C exhibits an adhesive feature, indicating its involvement in cell adhesion
[22][26][33]. A comparison of the PrP
C expression in primary and metastatic sites was conducted in patients with metastatic and non-metastatic gastric cancer
[23]. Although no significant difference in the PrP
C expression was observed between the primary and metastatic sites, a higher staining score for PrP
C was observed in the metastatic compared to the non-metastatic cancers, indicating a potential correlation between the PrP
C expression and gastric cancer aggressiveness.
32.2. PrPC and Melanoma
Previous studies have shown that PrP
C interacts with Filamin A (FLNA) to promote cancer progression
[34]. PrP
C-silenced FLNA-deficient M2 melanoma cells exhibited decreased M2 cell migration in wound healing assays
[35]. This was further reversed by reintroducing PrP
C in PRNP-null M2 cells
[36]. Despite the fact that PrP
C enhances cell migration and alters the cell cytoskeleton organization through FLNA disruption, M2 cells do not express FLNA. Indeed, the effect of PRNP deletion on cell migration was shown to be associated with F-actin protein. The latter, in wild-type M2 cells that are characterized with a higher mobility, shows an expression level which varies according to that of PrP
C [36]. These findings demonstrate that PrP
C negatively regulates F-actin without binding to FLNA. To determine the pathway through which PrP
C affects F-actin, Hsp27 was assessed based on its importance for cell motility and its ability to reduce actin aggregation
[37][38]. The levels and phosphorylation of Hsp27 were evaluated in the presence or absence of PrP
C. There was a significant decrease in phosphorylated Hsp27 at Ser82 when PRNP was deleted, and P-Hsp27 levels were rescued when PRNP was re-expressed in PRNP-null M2 cells
[36]. To identify the kinase responsible for this observation, the inhibition of P38MAPK, Akt, PKD, PKA, and PKC was assessed, as these kinases have been reported to act on Hsp27
[39][40][41][42][43]. Only Akt inhibition decreased the P-Hsp27 levels that were also decreased when PRNP was silenced. The Akt expression was rescued when PrP
C was re-expressed. The binding between Akt and Hsp27 was confirmed by co-immunoprecipitation and co-purification in the presence of PrP
C and was higher in comparison to their binding in PrP
C-null M2 cells
[36]. These findings support the correlation between PrP
C and Akt levels, which will disturb the downstream Akt/Hsp27 interaction, inducing the regulation of actin polymerization and cell migration. PrP
C interaction with FLNA also promotes FLNA interaction with β1 integrin, contributing to melanomagenesis
[44][45]. A7 cells, which express FLNA, exhibited higher spreading and migration ability compared to M2 cells that do not express FLNA
[46]. PrP
C exists as Pro- PrP
C in both A7 and M2 cells, retaining its glycosylphosphatidylinositol anchor peptide signal sequence (GPI-PSS) with an FLNA binding motif.
32.3. PrPC and Breast Cancer
The resistance of cancer cells to apoptosis or drug treatment is one of the main features of tumorigenesis. Epigenetic modifications
[47], ectopic gene expression
[48][49][50], and oncogene overexpression can lead to aberrant expression of anti- or pro-apoptotic proteins. In breast cancer, PrP
C has been reported to contribute to cancer resistance to apoptosis and drug treatment. Chemotherapy of TNF-resistant breast carcinoma cells was effective in patients who were PrP
C-negative. However, PrP
C overexpression in estrogen receptor (ER)-negative breast cancer patients was linked to decreased sensitivity to chemotherapy, indicating that PrP
C could potentially be used as a predictor of adjuvant chemotherapy benefit in ER-negative patients
[48].
Overexpression of PrP
C has also been shown to cause resistance to TRAIL (Tumor necrosis factor-Related Apoptosis Inducing Ligand)-induced apoptosis in Adriamycin (MCF7/ADR)
[51][52][53][54][55][56]. The elevated expression of PrP
C in MCF7/ADR and 2101 cell lines compared to MCF7 cells correlates with the breast carcinoma cells’ resistance to Adriamycin and TRAIL-induced cell death
[57].
Nevertheless, the knockdown of PrP
C using the siRNA-PrP
C strategy in resistant cell lines only restores sensitivity to TRAIL-mediated apoptosis by up to 25% in MCF7/ADR and 60% in 2101 cells. This is achieved through the enhancement of Bid cleavage and caspase-3 processing, concomitantly with Mcl-1 downregulation and activation of pro-apoptotic Bax through the downregulation of Bcl-2
[57]. In addition to its role in acquiring resistance, PrP
C has been shown to be a crucial factor for invasion and migration of MCF7 breast cancer cells. PrP
C overexpression increases matrix metalloprotease-9 (MMP-9) expression by enhancing the association of NF-κB with the promoter of the MMP-9 gene and ERK signaling, similar to that observed in gastric cancer
[58] (
Figure 3).
Figure 3. Cellular regulatory pathways involving PrP
C protein in cancer. The PrP
C protein is involved in different cellular tumorigenesis process, where its activities are regulated directly through protein–protein interactions or/and indirectly using different cellular regulatory pathways. P-gp; P-glycoprotein, GLUT-1; Glucose transporter-1; FLNA; Filamin A.
32.4. PrPC and Colorectal Cancer
Colorectal adenocarcinoma (CRC) cells exhibit high levels of expression of PrP
C compared to normal colorectal cells. PrP
C plays a crucial role in tumor growth and survival by promoting the Warburg effect, which involves increased reliance on glucose metabolism, in the presence of oxygen. This process ensures rapid proliferation and survival of cancer cells
[59][60]. Through the Fyn-HIF-2α pathway, PrP
C increases the expression of GLUT-1, the main glucose transporter, thereby enhancing the dependency of CRC cells on the glycolytic pathway for tumor growth (
Figure 3). In contrast, the depletion of PrP
C suppresses glucose utilization by suppressing GLUT-1 expression, leading to the inhibition of tumor growth both in vitro and in vivo
[61]. Cell surface proteomics studies have identified the differential expression of GLUT-1 and PrP
C as potential biomarkers of colorectal adenoma to carcinoma progression. Hence, these proteins can serve as potential targets for the emerging molecular imaging modalities
[62]. Functional assays have revealed a molecular mechanism that links the levels of PrP
C expression to the regulation of CRC metastasis. Ectopic PrP
C expression was found to promote the in vitro metastatic potential of CRC cells, while inhibition of PrP
C significantly reduced cancer cell motility
[63]. The pathway involving PrP
C-mediated upregulation of SATB1 is a matrix attachment region-binding protein that regulates higher-order chromatin organization and tissue-specific gene expression.
In colorectal and pancreatic ductal adenocarcinoma (PDAC), the overexpression of PrP
C has been shown to confer resistance to anti-cancer drugs, including doxorubicin, etoposide, and vincristine sulfate
[64][65][66][67]. In LS-174T cells overexpressing PrP
C, a higher cell viability and less apoptosis were observed compared to non-transfected cells. The PrP
C anti-apoptotic effect is thought to be mediated through the upregulation of the three proteins that are involved in the inhibition of apoptotic pathway. These include the inhibitor of apoptosis proteins (IAPs)-survivin, the X-linked inhibitor of apoptosis protein (XIAP), and the cellular inhibitor of apoptosis protein-1 (cIAP-1)
[68] (
Figure 3).
At the molecular level, it was proposed that the PrP
C involvement in PDAC is mediated upon its interaction with filamin A (FLNA). This interaction affects the cytoskeleton organization and the expression of different signaling proteins, triggering the cellular proliferation and invasiveness, leading to overall tumor growth
[64][66] (
Figure 3).
43. The Potential Diagnostic and Therapeutic Value of PrPC in Different Types of Cancer
PrP
C expression has been investigated in various types of cancer, including bladder and prostate cancer, osteosarcoma, and glioblastoma
[30][69][70][71][72]. In prostate spheroids, PrP
C expression was inversely correlated with the spheroid diameter and related to the intracellular redox state, potentially by contributing to anti-oxidative defense. Moreover, PrP
C was found to be overexpressed in 90% of prostate cancer biopsies, although its diagnostic or prognostic value remains unknown
[69]. In osteosarcoma, the most common bone malignancy, PrP
C was differentially overexpressed and appeared to be associated with tumor development and aggressiveness, as well as a negative regulator of apoptosis
[70].
PrP
C expression was directly correlated with the proliferation of glioma stem cells (GSC), and its downregulation reduced GSC stemness, cell growth, clonogenicity, and spherogenicity, as well as the ability to develop tumors in animal models. The results imply that PrP
C plays a crucial role in preserving GSC stemness
[73][74].
Hence, blocking its activity could enhance the sensitivity of cancer cells to chemotherapy
[71][75]. Interestingly, PrP
C expression was found to increase the sensitivity to doxorubicin in MDA-MB-435 breast cancer cells, unlike colorectal cancer, suggesting a tumor type-specific mechanism
[76].
Recent studies have also shown that PrP
C is expressed in human lung epithelial cells and is involved in anti-oxidative defense and the maintenance of tight junctions in the epithelial barrier
[77].
54. Targeting PrPC Interactions in Cancer: New Insights and Potential Strategies (Figure 4)
PrP
C plays a central role as a scaffold protein by forming multiprotein complexes with receptors or extracellular molecules. These interactions may contribute to the activation of downstream signal pathways that control numerous biological functions, including cancer stem cell self-renewal, the central entity of tumor maintenance and dissemination
[30][78]. One potential strategy for targeting PrP
C in cancer is to disrupt its interactions with other molecules known to be involved in cancer progression
[75]. For example, PrP
C has been shown to interact with several cell surface receptors, including integrins and laminin receptors. Importantly, these proteins have been reported to play important roles in cancer cell adhesion, migration, and invasion
[69]. Inhibiting these interactions could potentially prevent cancer cells from spreading and invading surrounding tissues. In breast cancer, PrP
C interaction with P-gp was associated with drug resistance, higher aggressiveness, invasion, and migration. PrP
C also interfered in neo-adjuvant chemotherapy response in this cancer
[78].
Figure 4. Potential therapeutic strategies in PrP
C-associated cancer. RISC; RNA-induced silencing complex, SiRNA; small interfering RNA, dsRNA; double-stranded RNA. Several small molecules, including Quinacrine, Chlorpromazine, Amphotericin B, Pentosan polysulfate, and Suramin, have been identified as potential inhibitors of PrP
C.