L-Lactate reprograms tumor-associated macrophages to drive pancreatic cancer progression via BCL3 lactylation

This study reveals that in pancreatic ductal adenocarcinoma, tumor-secreted L-lactate is consumed by macrophages to induce site-specific lactylation of the transcriptional co-regulator BCL3 at lysine 21, which drives nuclear translocation and NF-κB p50-mediated transcriptional reprogramming that suppresses inflammation, promotes tumor progression, and correlates with poor patient survival.

The Gist

The Big Picture: A Toxic Neighborhood and a Helpful Neighbor

Imagine the pancreas in a person with pancreatic cancer (PDAC) as a toxic, high-pressure construction site. The cancer cells are the workers who are running a marathon, burning through energy at a crazy speed. Because they are working so hard, they produce a massive amount of waste: lactic acid (lactate).

Usually, we think of this waste as just garbage that makes the area acidic and painful. But this paper discovered something surprising: The cancer cells aren't just dumping this waste; they are feeding it to their neighbors.

The neighbors in this story are Macrophages. Think of macrophages as the "security guards" or "janitors" of the immune system. Their normal job is to patrol the neighborhood, eat bad bacteria, and call for help if there's an intruder (the cancer).

The Plot Twist: The Janitors Get Brainwashed

In a healthy body, these security guards (macrophages) would see the cancer, get angry, and try to destroy it. But in this cancerous neighborhood, the cancer cells are constantly shouting, "Hey! Here's a bucket of fresh lactate!"

The paper found that the macrophages drink this lactate. But it's not just a drink; it's a mind-control drug.

1. The Delivery: The macrophages have special "straws" (transporters called MCT1) that suck up the lactate from the cancer cells.
2. The Switch: Once inside the macrophage, the lactate acts like a key. It attaches a tiny chemical tag (called L-lactylation) to a specific protein inside the cell called BCL3.
   • Analogy: Imagine BCL3 is a light switch on the wall. The lactate tag is a sticky note that says "Turn this switch ON."
3. The Hijack: This sticky note forces the BCL3 switch to move into the "control room" (the cell nucleus). Once there, it grabs onto a specific team of workers (the p50 subunit of NF-κB) and pushes the "attack team" (the p65 subunit) out the door.

The Result: From Guardian to Accomplice

Because the "attack team" is kicked out, the macrophage stops trying to fight the cancer. Instead, it flips a switch to become a helper.

• Before: The macrophage was a "Good Cop" (M1 type), trying to kill the tumor.
• After: The macrophage becomes a "Bad Cop" (M2 type). It stops fighting, starts building a protective shield around the tumor, and even sends signals that help the cancer cells grow faster.

The paper shows that if you stop the macrophages from drinking the lactate, or if you break the "sticky note" (the lactylation) on the BCL3 switch, the macrophages stay as "Good Cops" and the tumor stops growing.

The Real-World Proof

The researchers tested this in two ways:

1. In the Lab: They fed lactate to immune cells, and the cells immediately started helping cancer cells grow. When they blocked the lactate, the cells went back to fighting the cancer.
2. In Mice: They gave mice with pancreatic cancer extra lactate to drink. The tumors grew huge. But, if they removed the macrophages from the mice, the extra lactate did nothing. This proved that the lactate needs the macrophages to do its dirty work.

Why This Matters for Patients

The researchers looked at real human tissue samples and found a scary pattern:

• Patients whose tumors had a lot of these "brainwashed" macrophages (with the lactate tag on BCL3) had much shorter survival times.
• These tumors were also "immune-excluded," meaning the good immune cells (like CD8+ T-cells, the "special forces") were locked out of the tumor area, unable to fight.

The Takeaway

This study reveals a secret handshake between cancer cells and the immune system. The cancer cells use their own waste (lactate) to reprogram the body's security guards into bodyguards for the tumor.

The Good News: This gives doctors a new target. Instead of just trying to kill the cancer cells, we might be able to develop drugs that:

1. Block the "straws" so macrophages can't drink the lactate.
2. Erase the "sticky note" (lactylation) so the BCL3 switch stays off.
3. Wake up the "Good Cops" to start fighting the tumor again.

In short: Stop the lactate, and you might just turn the tumor's best friends back into its worst enemies.

Technical Summary

1. Problem Statement

Pancreatic ductal adenocarcinoma (PDAC) is characterized by a dense, immunosuppressive tumor microenvironment (TME) dominated by tumor-associated macrophages (TAMs) that exhibit a pro-tumorigenic M2-like phenotype. While PDAC is known for its hyper-glycolytic nature (Warburg effect) and high lactate production, the mechanism by which tumor-derived lactate reprograms TAMs to support tumor progression remains unclear. Specifically, it is unknown how extracellular lactate is sensed by immune cells to induce specific transcriptional programs, and whether this process involves post-translational modifications (PTMs) beyond simple metabolic byproduct accumulation.

2. Methodology

The study employed a multi-omics and multi-modal approach combining clinical samples, in vitro models, and in vivo animal models:

• Metabolomics & Transcriptomics: Untargeted metabolomics (GC-MS) on paired human PDAC and adjacent normal tissues, integrated with public single-cell RNA sequencing (scRNA-seq) datasets (CRA001160, GSE197177) to identify lactate consumers and transporter signatures.
• Cell Models: Co-culture systems using human (THP-1) and murine (RAW264.7) macrophages with PDAC cell lines (PANC-1, KPC1199, etc.). Conditioned media (CM) experiments were used to isolate soluble factors.
• Genetic & Pharmacological Manipulation:
  • Knockdown/Overexpression: shRNA-mediated knockdown of LDHA (tumor cells) and Bcl3 (macrophages); rescue experiments with Wild-Type (WT) and lactylation-deficient (K21R) Bcl3 mutants.
  • Inhibitors: Pharmacological blockade of lactate transport (AR-C155858 for MCT1/2), lactylation "writer" p300 (C646), and BCL3-NF-κB interaction (JS6).
  • Stereoisomer Controls: Use of D-lactate and Sodium Acetate to distinguish specific L-lactate signaling from non-specific acidification or carbon source effects.
• In Vivo Models: Orthotopic PDAC models in C57BL/6J mice using KPC1199 cells. Strategies included Ldha-knockdown tumors, dietary lactate supplementation (L-NaLac vs. D-NaLac/NaAce), and a "macrophage replacement" model where endogenous macrophages were depleted (clodronate liposomes) and replaced with genetically modified RAW264.7 cells.
• Molecular & Structural Biology:
  • Proteomics: Global proteomics and L-lactylome profiling (LC-MS/MS) to identify lactylated proteins.
  • Site Identification: Mass spectrometry to pinpoint lactylation sites.
  • Structural Modeling: AlphaFold-Multimer to predict the interaction between the P300 catalytic domain and BCL3.
  • Validation: Co-immunoprecipitation (Co-IP), Western blotting, subcellular fractionation, immunofluorescence (IF), and multiplex immunofluorescence (mIF) on human tissue microarrays (TMA).

3. Key Contributions & Results

A. TAMs are the Primary Consumers of Tumor-Derived Lactate

• Metabolomic profiling revealed that despite robust glycolysis in PDAC, extracellular lactate does not accumulate proportionally in tumor tissue, suggesting rapid consumption.
• scRNA-seq analysis identified macrophages as the dominant cell population expressing high-affinity lactate transporters (specifically SLC16A1/MCT1), significantly higher than epithelial or stromal cells.
• Inhibition of MCT1/2 blocked lactate uptake and prevented the acquisition of the M2-like phenotype.

B. L-Lactate Drives M2 Polarization via Protein Lactylation

• Tumor-derived L-lactate (but not D-lactate or acetate) induced a pro-tumorigenic M2 phenotype (upregulation of ARG1, MRC2, IL10; downregulation of CD86) in macrophages.
• This reprogramming was dependent on protein L-lactylation (Kla). Inhibiting the lactylation writer p300 (via C646) or blocking lactate uptake abolished the M2 shift.
• Crucially, the effect was stereoselective (L-lactate only) and independent of acidification, confirming a specific signaling role for lactate.

C. BCL3 is the Central Effector of Lactylation

• Integrated proteomics and lactylome profiling identified BCL3 (B-cell lymphoma 3) as a top nuclear target of lactylation.
• Site-Specific Modification: Mass spectrometry identified Lysine 21 (K21) as the primary lactylation site on BCL3.
• Mechanism of Action:
  1. Lactylation at K21 promotes nuclear translocation of BCL3.
  2. Lactylated BCL3 binds with high affinity to the NF-κB p50 subunit.
  3. This interaction competitively displaces the pro-inflammatory p65 (RelA) subunit from p50.
  4. The resulting BCL3-p50 complex represses pro-inflammatory genes (p65-dependent) and enforces a tumor-supporting transcriptional program.
• Structural modeling suggested K21 interacts with the P300 catalytic pocket, facilitating the modification.

D. In Vivo Validation of the BCL3-K21 Axis

• Macrophage Replacement Model: In mice with depleted endogenous macrophages, reconstitution with WT-BCL3 macrophages restored lactate-driven tumor growth. However, reconstitution with K21R-mutant (lactylation-deficient) macrophages failed to support tumor progression, maintaining a pro-inflammatory (CD86+) phenotype despite high systemic lactate.
• Dietary Lactate: Oral administration of L-NaLac accelerated tumor growth, an effect that was abolished by macrophage depletion or by using K21R-mutant macrophages.

E. Clinical Relevance

• A "Lactylation-Driven Macrophage Signature" (LDMS) derived from the study correlated with poor overall survival and advanced TNM stages in the TCGA-PAAD cohort.
• Multiplex IF on human PDAC tissue microarrays showed that high BCL3-Lactylated Macrophage Scores (BKMS) correlated with:
  • Larger tumor size.
  • Exclusion of CD8+ T cells (immune-excluded phenotype).
  • Independent poor prognosis in multivariate Cox regression analysis.

4. Significance

• Mechanistic Insight: The study establishes a direct molecular link between tumor metabolism (glycolysis/lactate) and immune cell reprogramming via a specific PTM (lactylation) on a non-histone protein (BCL3).
• Paradigm Shift: It challenges the view of lactate as merely a waste product or acidification agent, redefining it as a critical signaling molecule that drives immune evasion through stereoselective sensing.
• Therapeutic Target: The identification of the BCL3-K21 lactylation axis offers a novel therapeutic target. Disrupting this axis (e.g., via K21 mutation or specific inhibitors) could reverse the immunosuppressive TME, restore anti-tumor immunity, and inhibit PDAC progression.
• Biomarker Potential: The BCL3-lactylated macrophage signature serves as a robust prognostic biomarker for PDAC patients, potentially guiding stratification for metabolic-immunotherapies.

In summary, this paper elucidates a "metabolic-signaling axis" where PDAC cells export L-lactate, which is imported by TAMs to lactylate BCL3 at K21. This modification rewires NF-κB signaling to suppress inflammation and promote tumor growth, providing a compelling rationale for targeting lactylation in pancreatic cancer therapy.