BMPR1B in TAMs promoted Oral Squamous Cell Carcinoma via increasing SMAD1/EGR1/PD-L1/XBP-1 signaling induced M2 polarization

Technical Summary: BMPR1B in TAMs Promotes Oral Squamous Cell Carcinoma via the SMAD1/EGR1/PD-L1/XBP-1 Signaling Axis

Problem Statement
Oral squamous cell carcinoma (OSCC) remains a malignancy with high incidence and mortality, largely due to its invasive nature, metastatic potential, and resistance to current therapies. A critical factor in this progression is the tumor microenvironment (TME), specifically the role of Tumor-Associated Macrophages (TAMs). While M2-polarized TAMs are known to facilitate tumor immune evasion and progression, the precise molecular mechanisms driving their polarization in OSCC are not fully elucidated. Specifically, the source of activation for Bone Morphogenetic Protein Receptor 1B (BMPR1B) in TAMs and its subsequent role in M2 polarization within the context of OSCC had not been well studied.

Methodology
The study employed a multi-faceted approach combining bioinformatics, single-cell transcriptomics, and extensive in vitro and in vivo experimental validation:

1. Bioinformatics and Single-Cell Analysis:

   • Publicly available OSCC datasets (GSE9844) and The Cancer Genome Atlas (TCGA) head and neck squamous cell carcinoma (HNSC) cohort data were analyzed.
   • Single-cell RNA sequencing (scRNA-seq) data were processed using Seurat, Monocle 3, and CellChat to characterize cellular heterogeneity, macrophage differentiation trajectories, and intercellular communication networks.
   • Virtual gene knockout analyses were performed to simulate the effects of inhibiting specific pathway components (BMPR2, SMAD2, EGR1).
   • Survival analysis (Kaplan-Meier, Cox regression) was conducted to evaluate the prognostic value of BMPR1B expression.

2. Exosome Isolation and Characterization:

   • Exosomes were isolated from OSCC (Ca9-22) cell culture supernatants via differential ultracentrifugation.
   • Exosome markers (CD9, TSG101) and cargo (BMPR2) were validated via Western blotting and Transmission Electron Microscopy (TEM).
   • BMPR2 was knocked down in Ca9-22 cells using siRNA to confirm its role as the exosomal cargo.

3. Cellular Models and Co-culture Systems:

   • THP-1 cells were differentiated into macrophages using PMA to simulate TAMs.
   • Indirect co-culture systems (Transwell) and direct contact co-culture systems were established between OSCC cells and macrophages under hypoxic conditions (1% O₂).
   • Lentiviral vectors were used to generate stable cell lines for overexpression (OE) or knockdown (KD) of BMPR1B, EGR1, PD-L1, XBP-1, and SMAD2/3 in macrophages.
   • Specific inhibitors (Zilurgisertib for SMAD1, EGR-1-IN-1 for EGR1) were utilized to dissect the signaling cascade.

4. Functional Assays:

   • M2 Polarization: Measured via qPCR, ELISA, and Western blotting for markers including Arginase-1, IL-10, CD206, CD163, and TGF-β.
   • OSCC Cell Behavior: Proliferation (colony formation), migration (wound healing), invasion (Transwell), and cell cycle progression (flow cytometry) of Ca9-22 cells were assessed after co-culture with manipulated macrophages.

5. In Vivo Xenograft Model:

   • A subcutaneous xenograft mouse model was established in nude mice using Ca9-22 cells.
   • Tumor volume and weight were monitored over 21 days across eight distinct experimental groups involving combinations of BMPR1B overexpression, EGR1 knockdown, PD-L1 overexpression, and XBP-1 knockdown.

Key Results

• Single-Cell Landscape: Eight major cell populations were identified in the OSCC microenvironment. TAMs in tumor tissues exhibited significantly higher M2 polarization scores and greater developmental heterogeneity compared to normal tissues.
• Prognostic Value: High expression of BMPR1B in HNSC patients was identified as an independent predictor of poor overall survival. Multivariate Cox regression analysis indicated that low BMPR1B expression was associated with a reduced risk of death (adjusted HR = 0.65, 95% CI: 0.49–0.87, P = 0.003), whereas high expression correlated with poorer outcomes.
• Exosomal Mechanism: OSCC cells upregulate exosome biogenesis (CD9, TSG101) and package BMPR2 into exosomes. These exosomes are delivered to TAMs, where exosomal BMPR2 forms a functional complex with BMPR1B on the macrophage surface, initiating downstream signaling. Knockdown of BMPR2 in tumor cells abolished the activation of BMPR1B in macrophages.
• Signaling Cascade: The activation of BMPR1B in macrophages triggers a specific signaling axis:
  1. BMPR1B activation leads to phosphorylation of SMAD1.
  2. SMAD1 phosphorylation regulates the phosphorylation of SMAD2/3.
  3. This cascade upregulates the transcription factor EGR1.
  4. EGR1 induces the expression of PD-L1 and XBP-1 (specifically the spliced sXBP-1 variant).
  5. XBP-1 acts as the terminal executor, driving the transcription of M2 markers (IL-10, Arginase-1, CD206) and proteases (Cathepsin L) associated with tissue remodeling.
• Functional Impact: Macrophages with activated BMPR1B/SMAD1/EGR1/PD-L1/XBP-1 signaling significantly enhanced the proliferation, migration, invasion, and S-phase progression of OSCC cells. Conversely, inhibiting any node in this pathway (e.g., using SMAD1 inhibitors or knocking down EGR1/PD-L1/XBP-1) reversed these pro-tumorigenic effects.
• In Vivo Validation: In mouse xenograft models, overexpression of BMPR1B promoted tumor growth, while the sequential knockdown of EGR1, PD-L1, and XBP-1 progressively reduced tumor volume, confirming the pathway's necessity for tumor progression.

Significance and Claims
The authors claim to have identified a novel trans-cellular mechanism by which OSCC cells reprogram the tumor microenvironment. Key claims include:

1. Trans-Cellular Signaling: The study proposes a "trans-cellular receptor complementation model" where tumor cells provide the Type II receptor (BMPR2) via exosomes to macrophages, which provide the Type I receptor (BMPR1B). This challenges the traditional view of BMP signaling as strictly cell-autonomous (cis-configuration).
2. Mechanistic Insight: The paper elucidates the specific molecular axis (BMPR1B/SMAD1/EGR1/PD-L1/XBP-1) that drives M2 polarization in OSCC, linking endoplasmic reticulum stress (via XBP-1) and immune checkpoint regulation (via PD-L1) to macrophage function.
3. Therapeutic Target: The study identifies this signaling axis as a promising therapeutic target for OSCC. By disrupting the exosomal delivery of BMPR2 or inhibiting downstream nodes (SMAD1, EGR1, PD-L1, XBP-1), it may be possible to reverse M2 polarization and inhibit tumor progression.
4. Novelty: The authors state this is the first study to demonstrate that BMPR1B in the OSCC microenvironment acts through the regulation of TAM polarization rather than directly on tumor cell proliferation, offering new ideas for combination immunotherapy strategies.

The paper concludes that while the proposed mechanisms provide a systematic perspective on immune regulation in OSCC, further large cohort studies and deeper experimental validation of the SMAD phosphorylation cascade details are required.