Aberrant expression of the testis kinase TSSK6 activates FAK-STAT3 signaling to promote tumorigenic growth

This study demonstrates that the aberrant expression of the cancer-testis antigen TSSK6 in colorectal cancer promotes tumorigenic growth and invasion by activating a FAK-STAT3 signaling axis that drives STAT3-dependent transcriptional programs for extracellular matrix remodeling and anoikis resistance.

The Gist

The Big Picture: A "Ghost" Protein Waking Up

Imagine your body is a bustling city. Most of the time, certain construction crews (proteins) only work in one specific district: the "Testis District." They build things needed only for making sperm. Once a person is an adult, these crews are supposed to pack up their tools and go home.

However, in some cases of Colorectal Cancer (CRC), these crews don't go home. They sneak into the "Colon District" and start building things they shouldn't. One of these rogue crews is led by a foreman named TSSK6.

This paper asks a simple question: How does this rogue foreman, TSSK6, help cancer cells grow out of control and spread?

The Story: How TSSK6 Takes Over the City

1. The Rogue Foreman (TSSK6)

In healthy cells, TSSK6 is silent. But in about 60% of colorectal cancers, it wakes up. When it wakes up, the cancer cells become super-aggressive. They can grow without being stuck to a surface (like a weed growing in mid-air), they can invade new neighborhoods, and they can form tumors in mice.

The researchers wanted to know: What is TSSK6 actually doing to make the cells so dangerous?

2. The "Construction Site" Connection (FAK)

Cells have little "sticky pads" on their feet called focal adhesions. These are like the suction cups a gecko uses to stick to a wall. They tell the cell, "We are safe here; stay put."

The researchers found that TSSK6 is a master manipulator of these suction cups. It turns on a switch called FAK (Focal Adhesion Kinase).

• The Analogy: Imagine TSSK6 is a hacker who breaks into the security system of the suction cups. Instead of telling the cell to stay put, TSSK6 hacks the system to scream, "EMERGENCY! WE NEED TO MOVE!" This makes the cell loosen its grip and start crawling.

3. The "General" in the Command Center (STAT3)

Once the suction cups (FAK) are activated, they send a message to the cell's command center (the nucleus). The message goes to a general named STAT3.

• The Analogy: Think of STAT3 as a general who usually sits in the office. When TSSK6 hacks the suction cups, it sends a frantic signal that wakes the general up. The general rushes to the front lines (the nucleus) and starts shouting orders to the workers.
• The Result: The general orders the cell to build new roads, tear down old walls, and pack its bags to move. This is the "invasive" behavior of cancer.

4. The Specific Orders (The Gene List)

The researchers looked at the "orders" the general (STAT3) was shouting. They found TSSK6 was specifically ordering the cell to build two very important tools:

1. PLEKHA7 and MEF2C: Think of these as the specific blueprints for "invasion gear." Without these blueprints, the cell can't move effectively.
2. When the researchers removed TSSK6, the general stopped shouting, the blueprints disappeared, and the cancer cells stopped being aggressive.

The Experiment: Proving the Chain of Command

To prove this story was true, the scientists played "Whac-A-Mole" with the different parts of the chain:

• Step 1: They turned off TSSK6. Result: The suction cups stopped moving, the general stayed asleep, and the cancer stopped growing.
• Step 2: They turned off the General (STAT3). Result: Even if TSSK6 was present, the cancer couldn't grow because the orders weren't being shouted.
• Step 3: They used a "hacker blocker" (a drug that stops FAK). Result: TSSK6 was still there, but it couldn't wake up the suction cups, so the general stayed asleep, and the cancer stopped.

The "Spheroid" Test: Watching the Invasion

To see this in action, the scientists grew cancer cells into little 3D balls (spheroids) and put them in a gel that mimics human tissue.

• Normal cells: Stayed in a tight, round ball.
• Cells with TSSK6: The ball started to burst! Little fingers of cells shot out into the gel, trying to invade the surrounding area.
• Cells with TSSK6 + The "Hacker Blocker": The ball stayed round and tight. The invasion stopped.

Why This Matters

This paper tells us that TSSK6 is like a master switch that turns a normal cell into a nomadic, aggressive invader.

1. It's a "Cancer-Testis" Antigen: Because TSSK6 is only supposed to exist in the testis (and not in the rest of the body), it is a perfect target for drugs. If we can make a drug that kills TSSK6, it will likely only hurt the cancer cells and not the healthy cells in the rest of the body.
2. The Pathway: It maps out the exact route the cancer takes: TSSK6 → FAK (Suction Cups) → STAT3 (General) → Invasion.

The Takeaway

Colorectal cancer cells are hijacking a protein meant for making sperm (TSSK6) to help them escape and spread. This protein acts like a remote control that turns on the cell's "move and invade" mode. By understanding this specific remote control, scientists hope to design new drugs that cut the signal, forcing the cancer cells to stop moving and stay put, making them easier to treat.

Technical Summary

1. Problem Statement

Cancer-testis antigens (CTAs) are germ cell-restricted proteins that are frequently aberrantly expressed in various malignancies, including colorectal cancer (CRC). While their expression is well-documented, the specific signaling networks through which these proteins drive tumor progression remain poorly defined.

• Specific Focus: The Testis-Specific Serine Kinase 6 (TSSK6) is expressed in ~60% of CRCs and correlates with reduced overall survival. Previous work established that TSSK6 promotes anchorage-independent growth, invasion, and in vivo tumor formation, and that its kinase activity is essential for these effects.
• Knowledge Gap: The downstream signaling mechanisms by which ectopic TSSK6 drives focal adhesion dynamics, anoikis resistance, and invasive behavior in CRC cells were unknown.

2. Methodology

The authors employed a multi-faceted approach combining loss-of-function (knockdown) and gain-of-function (overexpression) models in colorectal cancer cell lines (HCT116, HT-29, DLD-1, LOVO) and immortalized human colonic epithelial cells (HCECs).

• Phospho-Signaling Screen: A Proteome Profiler Human Phospho-kinase Array was used to identify signaling nodes altered by TSSK6 depletion in HCT116 cells.
• Molecular Biology:
  • siRNA Knockdown: Targeted depletion of TSSK6, STAT3, PLEKHA7, and MEF2C.
  • Overexpression: Lentiviral transduction of TSSK6-V5 in TSSK6-negative cells (DLD-1, HCEC).
  • Inhibition: Pharmacological inhibition of FAK using PF-573228.
• Functional Assays:
  • Soft Agar Assays: To measure anchorage-independent growth.
  • 2.5D Spheroid Invasion Assay: Cells embedded in Matrigel-collagen to assess invasive outgrowth and circularity.
  • Luciferase Reporter Assays: To quantify STAT3 transcriptional activity.
  • Immunofluorescence (IF): To visualize subcellular localization of STAT3 and p-FAK foci.
• Omics Analysis:
  • RNA-Sequencing (RNA-seq): Performed on TSSK6-depleted HCT116 and TSSK6-overexpressing HCECs to identify differential gene expression.
  • Bioinformatics: Enrichr analysis for pathway enrichment (focusing on STAT3, migration, adhesion, and cytoskeleton).
• Validation: Immunoblotting (Western Blot) and RT-qPCR were used to validate protein and mRNA levels of key candidates (PLEKHA7, MEF2C, STAT3, FAK).

3. Key Contributions & Results

A. Identification of the FAK–STAT3 Axis

• Phospho-Screen Results: Depletion of TSSK6 resulted in a significant reduction (≥40%) in the phosphorylation of STAT3 (Ser727), RSK1/2/3, and PYK2.
• Validation: Both loss-of-function (TSSK6 knockdown) and gain-of-function (TSSK6 overexpression) models confirmed that TSSK6 regulates STAT3 Ser727 phosphorylation. While Tyr705 phosphorylation showed variability, Ser727 modulation was consistent.
• Transcriptional Activity: TSSK6 expression increased STAT3-dependent luciferase reporter activity (~4-fold basal) and promoted nuclear accumulation of STAT3.

B. Mechanism: TSSK6 Activates FAK Upstream of STAT3

• FAK Activation: TSSK6 expression led to increased phosphorylation of FAK (Y397) and accelerated cell spreading. Conversely, TSSK6 depletion reduced p-FAK levels.
• Causal Link: Pharmacological inhibition of FAK (PF-573228) abolished TSSK6-induced STAT3 Ser727 phosphorylation. This establishes a linear signaling axis: TSSK6 → FAK activation → STAT3 Ser727 phosphorylation.

C. Identification of Downstream Effectors (PLEKHA7 and MEF2C)

• Transcriptomics: RNA-seq revealed that TSSK6 drives gene programs related to extracellular matrix (ECM) remodeling, cytoskeletal organization, and adhesion.
• Key Targets: Intersection of gain- and loss-of-function datasets identified PLEKHA7 and MEF2C as the most consistently regulated genes.
  • TSSK6 overexpression increased PLEKHA7 and MEF2C mRNA and protein.
  • TSSK6 depletion reduced them.
  • Depletion of STAT3 reduced PLEKHA7 and MEF2C expression, confirming they are STAT3-dependent targets.
• Functional Necessity: Knockdown of either PLEKHA7 or MEF2C significantly reduced TSSK6-driven anchorage-independent growth.

D. Phenotypic Impact on Invasion and Anoikis Resistance

• Spheroid Invasion: TSSK6 expression induced invasive outgrowth of epithelial spheroids into Matrigel-collagen matrices (loss of circularity).
• Dependency: This invasive phenotype was strictly dependent on:
  1. FAK activity (reversed by PF-573228).
  2. STAT3 activity (reversed by STAT3 knockdown).
  3. Downstream effectors (partially reversed by PLEKHA7 or MEF2C knockdown).

4. Significance

• Mechanistic Insight: This study defines a novel oncogenic signaling axis where a germline-restricted kinase (TSSK6) is co-opted by cancer cells to drive the FAK–STAT3 pathway. This explains how TSSK6 promotes the hallmarks of cancer: anchorage-independent growth and invasion.
• Therapeutic Vulnerability: Since TSSK6 is restricted to the testis (and tumors) but absent in most normal somatic tissues, it represents a highly specific therapeutic target. The study suggests that targeting TSSK6 or its downstream effectors (FAK, STAT3, PLEKHA7, MEF2C) could selectively inhibit metastatic progression in CRC without severe off-target toxicity.
• Biological Context: The findings highlight the plasticity of tumor cells, which can repurpose proteins essential for spermatogenesis (cytoskeletal remodeling) to facilitate the cytoskeletal changes required for metastasis and survival in suspension (anoikis resistance).

Conclusion

The authors demonstrate that aberrant TSSK6 expression in colorectal cancer activates focal adhesion kinase (FAK), which in turn phosphorylates STAT3 at Ser727. This activation drives a transcriptional program involving PLEKHA7 and MEF2C, ultimately conferring resistance to anoikis and promoting invasive behavior. These findings position TSSK6 as a critical driver of CRC progression and a potential target for precision oncology.