Abstract
Background/Aim: Diagnostic and prognostic biomarkers in localized prostate cancer (PC) are insufficient. Treatment stratification relies on prostate-specific antigen, clinical tumor staging and International Society of Urological Pathology (ISUP) grading, whereas molecular profiling remains unused. Integrins (ITG) have an important function in bidirectional signaling and are associated with progression, proliferation, perineural invasion, angiogenesis, metastasis, neuroendocrine differentiation, and a more aggressive disease phenotype in PC. However, ITG subunit expression in localized PC and their utility as prognostic biomarkers has not yet been analyzed. This study aimed to fill this gap and provide a comprehensive overview of ITG expression as well as ITG utility as biomarkers. Patients and Methods: The Cancer Genome Atlas (TCGA) and the Memorial Sloan Kettering Cancer Center (MSKCC) prostate adenocarcinoma cohorts were analyzed regarding ITG expression in correlation to ISUP, N- and American Joint Committee on Cancer (AJCC) stage and were correlated with disease-free survival (DFS). Statistical tests used included the Mann-Whitney U-test, logrank test and uni- and multivariable cox regression analyses. Results: After grouping for ISUP (1 and 2 vs. 3-5), N0 vs. N1 and AJCC stage (≤2 vs. ≥3), multiple ITGs showed significant expression differences. The most consistent results were observed for ITGα4, ITGαX, ITGα11, ITGβ2 and ITGα2. In multivariable cox regression, ITGα2, ITGα10, ITGαD, ITGαB2 (TCGA), ITGα11 and ITGβ4 (MSKCC) were independent predictors of DFS. Conclusion: The utility of ITGs as PC biomarkers was herein shown.
Prostate cancer (PC) is the second most common cancer in men and ranks fifth in cancer-related mortality worldwide (1). The introduction of prostate-specific antigen (PSA) screening led to a significant reduction of mortality over the last decades. Next to PSA, current PC screening and treatment decision/stratification is based on digital rectal examination (DRE) and prostate biopsy with or without magnetic resonance imaging (MRI) guidance (2). However, there are several limitations in detection and risk stratification. PSA is organ-, but not tumor-specific. This explains why PC detection biopsy is only positive in around 25% for patients with PSA 2-10 ng/ml (3). False-positive results above thresholds (e.g., in patients with benign prostatic hyperplasia or prostatitis) limit the accuracy. Another critique on the use of PSA screening in PC detection is overdiagnosis and subsequently overtreatment of patients, in which an insignificant PC would never surface during the patient lifespan (3). Additionally, biopsy undersampling may lead to underestimation of the true tumor grade and upgrading after radical prostatectomy (RP) (4-6). Once diagnosed, PC shows an enormous biological heterogeneity: Even though some patients die of metastatic disease within a few years, others can live for 10-20 years with an organ-confined disease (7). This might be due to genomic diversity-like differences in the transcriptome or copy-number alterations, which may lead to an increase in oncogenic pathways (7). Due to this heterogeneity and the above-mentioned sampling errors, PC prognosis is variable even among patients with the same Gleason score (GS) (6, 8). For improving PC screening as well as determining treatment response and a change in systemic therapy, better prognostic biomarkers are urgently needed. Integrins (ITGs) act as receptors, play important roles in cell-extracellular matrix (ECM) adhesion as well as in cell-cell and cell-ECM interaction. For interaction, they provide signal transduction in either direction (inside-out and outside-in) (9, 10). Integrin activated pathways have an impact on cell-cycle regulation (11) and, thereby, on proliferation, differentiation and cell death (10), as well as migration, gene expression and activation of growth factor receptors (12). These characteristics show the important role of ITGs in cancer in general (13). Additionally, in PC the tumor microenvironment is different compared to normal tissue microenvironment (12) indicating a stronger cell/ECM interaction (14). Without proper ECM anchoring adherent non-cancer cells cannot survive. In neoplastic cells this anchorage requirement is lost (10). Pickup et al. argue that biophysical and biochemical cues from the ECM, associated to the tumor, are critical for the malignancy since they influence each of the formerly established cancer hallmarks (e.g., induced angiogenesis, initiation of invasion, resistance to stressors inducing cell death) (15). Moreover, in PC different integrins were found to be associated with tumor progression, development of chemotherapy resistance (16), proliferation, perineural invasion, angiogenesis (17), metastases formation (18), neuroendocrine differentiation (19) and a more aggressive disease phenotype (11, 14, 20-24). Furthermore, it is supposed, that deregulation of integrin, and thereby cellular adhesion, facilitates the invasion of tumors and occurrence of metastases (14). ITGs are αβ heterodimers. In total, there are 24 known ITG heterodimers and 18 different α- and 8 different β-subunits (25). This raises not only the question on consistent differences in specific ITG subunits, but also which ITGs might be most promising for further research.
This analysis aimed to answer these questions in localized PC and provide a better understanding of the role of ITGs in the context of tumor biology.
Materials and Methods
mRNA expression data. The cBioPortal (https://www.cbioportal.org) database was screened for available PC cohorts with mRNA expression data. After screening all available cohorts for mRNA expression data for localized prostate cancer, the ITG data sets of The Cancer Genome Atlas (TCGA) Prostate Adenocarcinoma cohort (Firehose Legacy) consisting of data of 501 patients and the Memorial Sloan Kettering Cancer Center (MSKCC) Prostate Adenocarcinoma cohort (7) consisting of 240 samples were downloaded (download date: December 31, 2019) (26, 27). At the time the study was conducted, these cohorts were the only publicly available cohorts of patients with localized PC and mRNA expression data. Furthermore, the MSKCC cohort was the first public resource of integrated genomic profiling in PC. The well-known TCGA program has been collecting clinicopathologic data and molecular profiles in more than 30 different cancer types (28).
The mRNA expression data are available as normalized mRNA expression z-scores. The respective normalization of mRNA expression data was performed before uploading data to cBioPortal. Of note, the method of normalization differs between the two cohorts: In the MSKCC cohort the mRNA expression is normalized to transcript expression in 29 normal prostate tissues (7). In the TCGA cohort the mRNA expression data is normalized relative to diploid samples or relative to all samples. To gain the most representative result, we used mRNA expression z-scores relative to diploid samples.
Clinical data. Clinical data of the respective cohorts (e.g., age, International Society of Urological Pathology (IUSP) grading, disease-free survival (DFS), etc.) were also downloaded from cBioPortal and merged with RNA expression data.
Exclusion criteria. RNA expression data of cell lines, samples obtained from metastases, patients with metastatic disease and patients without follow-up regarding DFS were excluded. Hereafter, the MSKCC cohort consisted of 104 patients. In the TCGA cohort, 489 patients were left for analysis after exclusion of patients without follow-up data for DFS. The methodology is presented as a flowchart in Figure 1.
Statistical analysis. For comparison of the ITG expression the Mann-Whitney U-test was used. Furthermore, for each ITG a partition test was performed to determine the best cut-off regarding DFS in both cohorts. All patients with DFS <3 months were excluded from analyses and partition tests were performed with a minimum number of 20% of the respective cohort for partition test. Analyses to correlate each ITG subunit with DFS included Kaplan-Meier analysis and logrank test. Hereafter, univariable and multivariable cox regression analyses were performed including further possible predictors of DFS [age at diagnosis, ISUP grading, American Joint Committee on Cancer (AJCC) stage and N stage]. Frequencies and proportions were assessed for categorical variables, while means, medians, and interquartile ranges (IQR) were computed for continuous variables. All tests were two-sided with a statistical significance set at p<0.05. Analyses were conducted using the JMP®15 software (SAS Institute Inc., Cary, NC, USA) and GraphPad Prism9 (GraphPad Software, Inc., San Diego, CA, USA).
Results
A descriptive analysis of the cohort characteristics is shown in Table I. In a first step the expression of all ITGs in both cohorts as “ITG fingerprints” was analyzed. Results are shown in Figure 2.
ITG expression was analyzed in ISUP groups ISUP 1&2 and ISUP 3-5 (Figure 3). The comparison of ITG expression in these ISUP groups revealed significant differences in ITGα11 and ITGαX expression, which was significantly higher in the ISUP 3-5 group in both cohorts (p<0.01 and p<0.05 respectively), ITGβ6 expression was elevated in the MSKCC cohort in ISUP 1 and 2 (p<0.01). In the TCGA cohort, further significant differences in expression between ISUP 1 and 2 and 3-5 were found for ITGα2, ITGα4, ITGα5, ITGα6, ITGα7, ITGα9, ITGα10, ITGαD, ITGαE, ITGα2b, ITGβ2, ITGβ3 and ITGβ8 (all p-values <0.05). Results of group comparisons are shown in Figure 3C and Figure 3F.
Additionally, the expression of all ITGs was analyzed in relation to N status. In the TCGA cohort significant differences were found for ITGα2 (p<0.01) with a significantly lower expression in the lymph node-positive group. All other significant expression results showed a higher expression in ITGα4, ITGαL, ITGβ2 (p<0.05), ITGαD, ITGβ7 (p<0.01), ITGα11 and ITGαX (p<0.01). Results are shown in Figure 4A-C.
As a third variable ITG expression was analyzed in relation to AJCC stage. Patients were grouped in AJCC stage ≤2 and ≥3. Comparison of these two groups revealed no significant results in the MSKCC cohort (Figure 5A and B). In the TCGA cohort, significant differences in expression could be shown for ITGα2b, ITGβ2 (p<0.05), ITGα4, ITGα10 (p<0.01), ITGαX (p<0.01), ITGα2 and ITGα11 (p<0.01) (Figure 5C-E).
In a last step, DFS was analyzed dependent upon the ITG expression. Kaplan-Meier curves and logrank test revealed a significantly longer survival for several ITGs. Results are shown in Figure 6A-M (TCGA cohort) and Figure 6N-P (MSKCC cohort).
Furthermore, significant ITGs, as depicted in Kaplan-Meier analyses and logrank test and further possible clinical predictors of DFS, were subsequently analyzed in uni- and multivariable cox regression analyses. Results are shown in Table II.
Discussion
In the ERSPC trial, PSA-screening was associated with a significantly lower risk of metastatic disease and reduced mortality (29). However, other studies were not able to detect this effect (30, 31), thus contributing to the ongoing controversial discussion on the benefits and possible pitfalls of PSA screening. Both the PSA and other assessment tools (prostate biopsy, biopsy GS, DRE and imaging) are prone to errors. After diagnosis of localized PC, treatment options include active surveillance, local therapy, radical prostatectomy and radiation. Usually, decisions are based on GS. Epstein et al. showed that approximately 25% of GS 5-6 tumors on biopsy will be GS 7 or higher at RP (6). Furthermore, in patients with the same GS prognosis is variable (6). This indicates the need for additional and more accurate biomarkers to improve PC prediction as well as its management and therapy stratification following diagnosis. In this study, we aimed to provide further evidence for the utility of ITGs as predictive and prognostic biomarkers in localized PC.
The most consistent results were seen for ITGα11, ITGαX and ITGβ2 with higher expression in ISUP 3-5 (ITGα11 and ITGαX both cohorts, ITGβ2 in TCGA) and AJCC ≥3 as well as N1 in the TCGA cohort. Furthermore, a high ITGα11 expression could be associated with a shorter DFS in both cohorts. ITGαX and ITGβ2 were associated with a shorter DFS in the TCGA cohort. However, these integrins do not appear to have been thoroughly studied in prostate cancer to date.
In the TCGA cohort, ITGα2 showed very consistent results with lower expression in ISUP 3-5, AJCC ≥3, N1 and a significantly shorter DFS in the group with low expression. However, other studies showed exosome-mediate transfer of ITGα2 to promote migration and invasion in PC cells (32). Salemi et al., showed that an ITGα2β1 inhibition attenuates prostate cancer cell proliferation by cell-cycle arrest, promoting apoptosis and reducing epithelial-mesenchymal transition (11). Although these results seem contradictory, it is important to note that the studies examined different aspects and no absolute expression values are available. Therefore, the results are not directly comparable.
Further consistent results were seen for ITGα9, ITGβ3 and ITβ8 (all with lower expression in ISUP 3-5 and shorter DFS for low expression), ITGα10 and ITGα2b (both with higher expression in ISUP 3-5 and AJCC ≥3 and shorter DFS for high expression), ITGαD (higher expression in ISUP 3-5, N1 and shorter DFS for high expression) and ITGαE (higher expression in ISUP 3-5 and shorter DFS for high expression) in the TCGA cohort.
In a recent systematic review and meta-analysis including 14 studies Drivalos et al. showed a correlation of a low expression of ITGα6 and ITGβ1 with high GS and a “borderline trend” between advanced clinical stage and low ITGα6 expression (33). Whereas our analysis could not confirm the ITGβ1 results, we also found lower ITGα6 expression associated to higher ISUP grades. Another study found ITGα6β1 to promote cell survival and resistance to PI3K inhibitors in castration-resistant PC as ITGα6 is a direct transcriptional target of the androgen receptor (34). Taken together, this could generate the hypothesis, that higher ISUP grades might be less androgen dependent. Matching this, it is known that a higher ISUP grade is associated with faster castration resistance (35).
In localized PC ITGα3 and ITGα3β1 have been associated with worse disease outcomes by Pontes-Júnior et al. (36). The same working group revealed a correlation between negative ITGβ1 expression and biochemical recurrence and time to recurrence after RP (37). These results could not be confirmed in this analysis.
Further evidence exists for ITGαVβ6 and ITGαVβ3: ITGαVβ3 may promote a shift towards a neuroendocrine phenotype, but not αVβ6, since the latter is confined to adenocarcinomas of the prostate (38). Even though in our analyses no neuroendocrine phenotypes were included, high ITGβ3 expression was associated with shorter DFS. This might be associated with a molecular shift towards a neuroendocrine phenotype, which is associated with worse prognosis (39).
Lately ITGαV was suggested as a novel non-invasive marker for PC detection, since significantly lower levels were found in patients with PC compared to healthy controls (40). However, this analysis could not find significant ITGαV expression differences in patients with localized PC.
ITGs have also been associated with other cancer entities: For example, increased ITGβ1 expression could be associated with decreased survival in invasive breast cancer (41) and increased ITGα6β4, ITGαVβ5, ITGα3, ITGα5, ITGβ1 and ITGβ3 expression with decreased OS in non-small cell lung cancer (NSCLC) (42-45). In esophageal adenocarcinoma ITGαV expression was correlated with shortened OS (46) and in colon carcinoma a correlation between ITGαVβ3 expression and a shorter relapse-free and OS as well as presence of liver metastases could be shown (47). As mentioned above, our analysis also revealed a high ITGβ3 with a shorter DFS. This matches the mentioned findings in NSCLC and colon carcinoma. ITGβ3 might be strongly associated to worse prognosis across multiple cancer entities and should be further characterized in future studies.
These findings and the fact that commercially available ITG antibodies already exist in non-cancer treatment [Natalizumab, ITGα4β1/α4β7 antibody, used for multiple sclerosis and Crohn’s disease (48); Vedolizumab, ITGα4β7 antibody, used in adult inflammatory bowel disease (49)], the interest for integrins as drug targets has risen (45). Recent endeavors have led to the development of integrin antagonists (14) and anti-angiogenic cancer therapy based on ITGαVβ3 antibodies (50). These showed a decrease in metastasis and angiogenesis as well as improved survival in mice (51). However, these drugs have not yet been tested in clinical trials.
Despite all the evidence on expression of single ITGs and their correlation with progression and survival, molecular pathways by which integrins contribute to cancer progression and metastasis still need to be investigated since much more mechanistic insight is required to determine which integrin blocking strategies may be applied to which types and stages of cancer. However, not only expression in tumor tissue or metastases but also the mechanisms, by which PC may communicate, must be considered. Recent studies isolated extracellular vesicles (EV) from blood of patients with PC and showed that ITGαVβ3 is transferred to recipient cells using these EV (52). Additionally, it must be considered that not only gene expression, but also epigenetic and epitranscriptomic modification of ITGs might be a promising mechanism to develop diagnostic or therapeutic strategies.
Limitations. The explorative character of this study must be emphasized. This approach led to a large amount of data, that need to be further characterized in future studies. For this reason, an additional analysis of known and possible ITG heterodimers was not performed, as this would lead to a huge and confusing amount of data. At the same time their explorative character is an unbiased approach: All ITG subunits were analyzed and interpreted with the same thoroughness. Another possible limitation is the relatively small number of patients in the MSKCC cohort. Additionally, information on N status was only available in the TCGA cohort and existed for only 417 of the 489 patients included.
Conclusion
In conclusion this study – for the first time – provided a comprehensive overview and presents evidence for the utility of ITG subunit expression as biomarkers in localized PC. ITGα2, ITGα4, ITGα11, ITGαX, ITGβ2 showed consistent results in both cohorts. After further validation, they might be used for treatment stratification and prognosis – either alone or in combination with other biomarkers. Additionally, the identified ITGs might be interesting for the development of liquid biopsy-based analyses, and functional experiments.
Acknowledgements
The results shown here are, in part, based upon data generated by the TCGA Research Network: https://www.cancer.gov/tcga and in part upon data generated by the MSKCC cohort from Taylor et al. (7). Data was obtained using the cBioPortal (26, 27). K.N., P.N. and T.S.W received research funding from the Hector Stiftung II, Weinheim, Germany. J.H. and T.S.W. received research funding from the Southwest German Society of Urology (Südwestdeutsche Gesellschaft für Urologie), Stuttgart, Germany.
Footnotes
Authors’ Contributions
Conceptualization: T.S.W. and M.Ne.; Data curation: L.F. and M.Ne.; Formal analysis: M.Ne. and L.F.; Funding acquisition: K.N., P.N., J.H. and T.S.W.; Investigation: M.Ne, T.S.W. and F.We.; Methodology: M.Ne. and T.S.W.; Project administration: M.Ne. and T.S.W.; Supervision: T.S.W. and P.N.; Validation: T.S.W., K.N. and F.We.; Visualization: M.Ne., F.We., F.Wa.; Writing – original draft: M.Ne. and T.S.W.; Writing – review and editing: M.Ne., K.N., F.We., N.W., F.H., F.Wa., M.Ni., M.S.M., P.E., J.H. and T.S.W.
Data Availability Statement
The data supporting the reported results can be found at cBioPortal under <https://www.cbioportal.org/study/summary?id=prad_tcga (TCGA cohort)> and <https://www.cbioportal.org/study/summary?id=prad_mskcc> (MSKCC cohort).
Conflicts of Interest
The Authors declare no conflicts of interest. Funding by the Hector Stiftung II and the Southwest German Society of Urology had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.
- Received October 20, 2022.
- Revision received November 5, 2022.
- Accepted November 14, 2022.
- Copyright © 2023 International Institute of Anticancer Research (Dr. George J. Delinasios), All rights reserved.
This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY-NC-ND) 4.0 international license (https://creativecommons.org/licenses/by-nc-nd/4.0).