The HPV E7 oncoprotein promotes LIM and SH3 Domain Protein 1 (LASP1) transcription via the Rb/E2F1 signalling pathway in HPV-positive cervical cancer cells

This study reveals that the HPV E7 oncoprotein drives LASP1 transcription in HPV-positive cervical cancer cells by degrading Rb to activate E2F1, which directly binds to and promotes the LASP1 promoter, thereby establishing an additional mechanism alongside miR-203 downregulation that underscores LASP1's critical role in cancer progression.

The Gist

Imagine your body is a bustling city, and your cells are the individual buildings. Usually, there's a strict Mayor (a protein called Rb) who keeps the construction crews (cell division) on a tight schedule, ensuring buildings are only expanded when it's safe and necessary.

Then, there's a Villain called HPV (Human Papillomavirus). When this virus infects a cell, it sends out a henchman named E7. E7's job is to hijack the city's power grid. He does this by kidnapping the Mayor (Rb) and throwing him in jail (degrading him). With the Mayor gone, the construction crews go wild, building non-stop. This uncontrolled growth is what leads to cancer.

In this specific study, scientists discovered a new way E7 causes chaos. They found that E7 doesn't just let the construction crews run wild; he specifically turns up the volume on a very loud, very aggressive Foreman named LASP1.

Here is the story of how they figured it out, broken down simply:

1. The Missing Foreman (LASP1)

LASP1 is like a foreman who tells the cell to move faster, build more, and even invade neighboring properties (metastasis). The scientists already knew that in HPV-positive cervical cancer, this Foreman is working overtime. They previously found that E7 silences a "stop sign" (a tiny molecule called miR-203) that usually tells LASP1 to calm down.

But they wondered: Is there another way E7 is shouting "Go, go, go!" to LASP1?

2. The Chain of Command (Rb → E2F1 → LASP1)

The scientists realized there was a second, direct line of communication. Here is the new chain of command they discovered:

• The Villain (E7): As usual, E7 kidnaps the Mayor (Rb).
• The Freed Worker (E2F1): Normally, the Mayor (Rb) holds a worker named E2F1 in a cage, preventing him from giving orders. When E7 kicks the Mayor out, E2F1 is freed!
• The New Boss: Once free, E2F1 rushes to the construction site of the LASP1 gene. He doesn't just knock on the door; he grabs the keys, unlocks the front door, and flips the switch to "Maximum Power."
• The Result: The LASP1 gene starts churning out massive amounts of the aggressive Foreman, making the cancer cells grow and spread even faster.

3. The Experiments (How they proved it)

To prove this theory, the scientists played a few games of "What if?" in the lab:

• The "Bad Cop" Test: They took E7 and broke its handcuffs (the part that grabs the Mayor). When E7 couldn't grab the Mayor, he couldn't free E2F1, and the LASP1 lights stayed dim. This proved E7 needs to defeat the Mayor to turn on LASP1.
• The "Overachiever" Test: They forced the cell to have too much E2F1. Suddenly, LASP1 went crazy, even without E7. This proved E2F1 is the direct boss of LASP1.
• The "Locksmith" Test: They looked at the LASP1 gene's blueprint and found a specific "lock" (a DNA sequence) that only E2F1 has the key for. When they broke that lock using gene-editing tools (CRISPR), E2F1 couldn't turn the switch on, and LASP1 levels dropped.
• The "Rescue" Test: They stopped E2F1 from working, which slowed down the cancer cells. But when they added extra LASP1 back in, the cells started growing again! This showed that LASP1 is a crucial part of the reason why the cancer grows so fast.

The Big Picture

Think of HPV E7 as a master criminal who uses two different tricks to rob the city:

1. Trick 1: He silences the "Stop" signs (miR-203) so the Foreman (LASP1) isn't told to stop.
2. Trick 2 (The New Discovery): He kidnaps the Mayor (Rb) to unleash a new boss (E2F1) who directly orders the Foreman (LASP1) to work overtime.

Why does this matter?
This discovery is like finding a second security camera in a bank vault. We already knew one way the virus stole the money (via miR-203), but now we know it has a second, very direct way (via Rb/E2F1). Understanding both paths gives doctors more targets to aim at. If we can block the "lock" on the LASP1 gene or stop E2F1 from turning it on, we might be able to stop the cancer from growing, even if the virus is still there.

In short: HPV E7 breaks the rules, frees a boss (E2F1), and that boss forces the cancer engine (LASP1) to run at full speed.

Technical Summary

1. Problem Statement

• Context: High-risk Human Papillomavirus (HPV) infection is the primary cause of cervical cancer. The viral oncoproteins E6 and E7 drive oncogenesis by degrading p53 and the Retinoblastoma protein (Rb), respectively.
• The Gap: The protein LASP1 (LIM and SH3 Domain Protein 1) is known to be overexpressed in HPV-positive cervical cancers and promotes tumor progression. Previous work by the authors established that HPV E7 upregulates LASP1 by suppressing the microRNA miR-203 (a post-transcriptional mechanism).
• The Question: Whether HPV E7 regulates LASP1 through additional transcriptional mechanisms remained unclear. Specifically, the role of the Rb/E2F1 signaling pathway in driving LASP1 expression needed investigation.

2. Methodology

The study employed a combination of molecular biology, cell biology, and bioinformatics approaches using HPV-positive (HeLa, SiHa) and HPV-negative (C33A) cervical cancer cell lines, as well as HEK293T cells.

• Reporter Assays: The LASP1 promoter was cloned into a pGL3.0 luciferase vector. Cells were transfected with wild-type (WT) and mutant HPV E7 proteins (HPV16 and 18), Rb, and E2F1 to measure promoter activity.
• Mutagenesis: Site-directed mutagenesis was used to generate specific HPV E7 mutants to dissect functional domains:
  • Rb-binding deficient mutants (HPV16 E7 Δ22-26, HPV18 E7 Δ24-27, HPV18 E7 C27S).
  • HDAC-binding mutant (L67R).
  • Phosphorylation mutant (SS32/34AA).
• Gene Manipulation:
  • Overexpression: Transient transfection of E2F1, Rb, and LASP1.
  • Knockdown/Knockout: Use of siRNA and shRNA to deplete E7 and E2F1. CRISPR/Cas9 was used to delete specific E2F1 binding sites in the endogenous LASP1 promoter.
• Binding Analysis:
  • ChIP-qPCR: Chromatin Immunoprecipitation to verify direct binding of E2F1 to the LASP1 promoter.
  • Bioinformatics: Analysis of public ChIP-seq datasets (GSE20303, GSM935484, etc.) from HeLa S3 cells to map E2F1, RNA Pol II, and H3K4me3 enrichment.
  • Motif Search: LASAGNA-Search 2.0 was used to identify putative E2F1 binding motifs.
• Functional Assays: Cell proliferation (growth curves) and colony formation assays were performed to assess the rescue of growth defects in E2F1-depleted cells upon LASP1 restoration.
• Validation: Western blotting and RT-qPCR were used to quantify protein and mRNA levels.

3. Key Contributions

• Novel Transcriptional Mechanism: The study identifies a second, distinct mechanism by which HPV E7 upregulates LASP1: direct transcriptional activation via the Rb/E2F1 pathway, complementing the previously known miR-203 suppression.
• E2F1 as a Direct Regulator: This is the first demonstration that E2F1 acts as a direct transcriptional regulator of LASP1.
• Specific Binding Site Identification: The authors identified a specific E2F1 binding motif (CCTGAGAGCGCTGAG) within the LASP1 promoter and validated its necessity for expression.
• Functional Hierarchy: The study establishes that LASP1 is a downstream effector of E2F1 that can partially compensate for the loss of E2F1 in driving cell proliferation.

4. Key Results

• HPV E7 Drives LASP1 Promoter Activity: High-risk (HR) HPV E7 (types 16 and 18) significantly increased LASP1 promoter activity in a dose-dependent manner. Low-risk (LR) HPV E7 types did not show this effect (and in some cases suppressed it).
• Rb Binding is Essential: Mutants of HPV E7 that cannot bind Rb (Δ22-26, Δ24-27, C27S) failed to activate the LASP1 promoter. Mutants affecting other interactions (HDAC binding, phosphorylation) retained the ability to activate the promoter. This confirms Rb degradation is the critical step.
• Rb/E2F1 Axis Regulation:
  • Overexpression of Rb suppressed LASP1 promoter activity and protein levels.
  • Overexpression of E2F1 enhanced LASP1 expression.
  • Depletion of E2F1 in HPV+ cells significantly reduced LASP1 mRNA and protein levels.
• Direct Binding Confirmation:
  • ChIP-seq data showed E2F1 enrichment at the LASP1 promoter.
  • ChIP-qPCR confirmed E2F1 physically binds to the LASP1 promoter in HeLa and SiHa cells.
  • Mutating the specific E2F1 binding site in the promoter abolished E2F1-driven activation.
  • CRISPR/Cas9 deletion of the E2F1 binding site in the endogenous genome significantly reduced LASP1 expression.
• Functional Rescue: Depletion of E2F1 caused significant growth defects and reduced colony formation in cervical cancer cells. Overexpression of LASP1 in these E2F1-depleted cells partially rescued the proliferation defects, indicating LASP1 is a key downstream mediator of E2F1-driven oncogenesis.

5. Significance

• Mechanistic Insight: The paper elucidates a dual-layer regulation of LASP1 by HPV E7:
  1. Post-transcriptional: Suppression of miR-203 (previously known).
  2. Transcriptional: Degradation of Rb $\rightarrow$ Release of E2F1 $\rightarrow$ Direct binding to LASP1 promoter.
• Oncogenic Driver Validation: It reinforces LASP1 as a critical driver of HPV-associated cervical cancer, linking viral oncoproteins directly to cytoskeletal and motility regulators via the cell cycle machinery.
• Therapeutic Implications: Understanding that LASP1 is a convergence point for Rb/E2F1 signaling and miR-203 regulation suggests that targeting LASP1 or the specific E2F1-LASP1 interaction could be a viable therapeutic strategy for HPV-positive cancers, potentially overcoming resistance mechanisms that might target only one regulatory arm.
• Differentiation of HPV Types: The study highlights a functional divergence between High-Risk and Low-Risk HPV E7 proteins regarding LASP1 regulation, correlating with their oncogenic potential.