Exosomal PSM-E Inhibits Macrophage M2 Polarization to Suppress Prostate Cancer Metastasis via the RACK1 Signaling Axis

Academic Background

Prostate cancer (PCA) is a prevalent cancer among men and the second leading cause of cancer-related deaths. While early-stage prostate cancer can be treated with surgery or radiotherapy, approximately one-third of cases progress to a more aggressive form, leading to poor prognosis. Despite various treatment strategies, such as androgen deprivation therapy (ADT), managing metastatic prostate cancer remains challenging. Therefore, a deeper understanding of the mechanisms underlying prostate cancer metastasis is crucial for developing effective treatments.

Prostate-specific antigen (PSA) is currently the most commonly used biomarker for prostate cancer diagnosis and prognosis. However, its limited sensitivity and specificity make it difficult to distinguish between prostate cancer, prostatitis, and benign prostatic hyperplasia (BPH), leading to overdiagnosis and overtreatment. Thus, there is an urgent need to develop more efficient biomarkers and sensitive methods for early diagnosis.

In recent years, tumor-associated macrophages (TAMs) in the tumor microenvironment have been recognized as significant contributors to prostate cancer progression. Macrophages exist in two polarized forms: M1 macrophages enhance immune responses and exhibit anti-tumor effects, while M2 macrophages suppress immune activity and promote tumorigenesis. Studies have shown that TAMs in the tumor microenvironment typically exhibit an M2-like phenotype, promoting the growth, invasion, and metastasis of prostate cancer cells. Additionally, TAM infiltration is an independent risk factor for prostate cancer recurrence, with patients having fewer TAMs generally showing better prognosis.

Exosomes are extracellular vesicles with diameters ranging from 30 to 150 nanometers and play a crucial role in mediating intercellular signaling within the tumor microenvironment. Exosomes carry mRNA, proteins, and non-coding RNAs, influencing recipient cells and their microenvironment. Tumor cells can manipulate immune cells through exosome-delivered proteins, creating a microenvironment conducive to tumor growth and metastasis. However, the specific role of exosomal proteins in prostate cancer metastasis remains incompletely understood.

Prostate-specific membrane antigen (PSMA) is a type II transmembrane glycoprotein produced by prostate epithelial cells. Its expression is significantly elevated in prostate cancer, castration-resistant prostate cancer, and metastatic prostate cancer. A novel splice variant of PSMA, PSM-E, is specifically overexpressed in prostate cancer and strongly correlates with tumor stage and grade. Studies have shown that PSM-E can inhibit the proliferation, migration, and invasiveness of prostate cancer cells, but its specific mechanisms within the prostate cancer microenvironment remain unclear.

Research Background and Objectives

This study aims to investigate the role of prostate cancer-derived exosomal PSM-E in regulating macrophage M2 polarization and its mechanisms in suppressing tumor invasion and metastasis. By examining the expression of PSM-E in serum and urine exosomes and its correlation with clinical features of prostate cancer, the study further elucidates the molecular mechanisms by which PSM-E inhibits macrophage M2 polarization through the RACK1 signaling axis, providing new insights for prostate cancer diagnosis and treatment.

Research Methods

Study Subjects and Sample Collection

The study included 93 participants, comprising 45 controls and 48 prostate cancer patients. Serum and urine samples were collected from all participants. The clinical pathological diagnosis of prostate cancer patients was confirmed by at least two pathologists according to the American Joint Committee on Cancer (AJCC) guidelines.

Exosome Isolation and Characterization

Exosomes were isolated from serum and urine samples using centrifugation and characterized using transmission electron microscopy (TEM) and Western blotting to analyze the morphology and expression of PSM-E protein.

Cell Culture and Transfection

Human prostate cancer cell lines (PC3 and LNCaP) and the human monocyte cell line THP-1 were used in the experiments. The interaction between PSM-E and RACK1 was studied by transfecting cells with PSM-E-Flag and RACK1-HA plasmids.

In Vitro Experiments

Wound healing assays and Transwell invasion assays were conducted to study the effects of exosomal PSM-E on the migration and invasion of prostate cancer cells. Real-time quantitative PCR and Western blotting were used to analyze the regulatory effects of exosomal PSM-E on macrophage M2 polarization.

In Vivo Experiments

A prostate cancer model was established in C57BL/6J mice, and the effects of exosomal PSM-E on tumor growth and macrophage M2 polarization were observed through intraperitoneal injection of exosomal PSM-E.

Research Results

Expression of Exosomal PSM-E in Serum and Urine of Prostate Cancer Patients

The study found that the expression of PSM-E in serum and urine exosomes was significantly higher in prostate cancer patients compared to the control group. The expression level of urinary exosomal PSM-E was positively correlated with the Gleason score and pathological stage of prostate cancer.

Diagnostic Value of Exosomal PSM-E

LC-MS/MS analysis revealed that the concentration of urinary exosomal PSM-E was significantly higher in prostate cancer patients compared to the control group. ROC curve analysis showed that the AUC value of urinary exosomal PSM-E for prostate cancer diagnosis was 0.8904, significantly outperforming traditional PSA biomarkers.

Exosomal PSM-E Inhibits Prostate Cancer Cell Migration and Invasion

In vitro experiments demonstrated that exosomal PSM-E significantly inhibited the migration and invasion of prostate cancer cells.

Exosomal PSM-E Inhibits Macrophage M2 Polarization

The study found that exosomal PSM-E inhibited macrophage M2 polarization by suppressing the RACK1-FAK-ERK signaling pathway, thereby inhibiting prostate cancer invasion and metastasis.

In Vivo Validation

In a mouse prostate cancer model, exosomal PSM-E significantly suppressed tumor growth and reduced the infiltration of M2 macrophages in tumor tissues.

Conclusion

This study is the first to reveal the molecular mechanism by which prostate cancer-derived exosomal PSM-E inhibits macrophage M2 polarization through the RACK1-FAK-ERK signaling axis, thereby suppressing tumor metastasis. Urinary exosomal PSM-E, as a novel non-invasive biomarker, holds significant diagnostic and prognostic value. Targeting exosomal PSM-E may represent an innovative therapeutic strategy for preventing prostate cancer progression.

Research Highlights

1. Exosomal PSM-E as a Novel Biomarker: Urinary exosomal PSM-E demonstrated high sensitivity and specificity in prostate cancer diagnosis, outperforming traditional PSA biomarkers.
2. PSM-E Inhibits Macrophage M2 Polarization: Exosomal PSM-E suppressed macrophage M2 polarization through the RACK1-FAK-ERK signaling pathway, highlighting its critical role in the tumor microenvironment.
3. In Vitro and In Vivo Validation: Both in vitro and in vivo experiments validated the inhibitory effects of exosomal PSM-E on prostate cancer cell migration, invasion, and tumor growth.

Research Significance

This study provides new insights into the diagnosis and treatment of prostate cancer. Exosomal PSM-E, as a novel non-invasive biomarker, holds promise for early diagnosis and prognosis prediction in prostate cancer. Additionally, targeting exosomal PSM-E may represent an innovative therapeutic strategy for preventing prostate cancer progression, offering significant clinical potential.