Sublethal stress from polypharmacy modulates scavenging function and fenestrations in mouse liver sinusoidal endothelial cells

This study demonstrates that a polypharmacy cocktail of four common drugs induces sublethal, concentration-dependent metabolic changes in mouse liver sinusoidal endothelial cells, specifically causing synergistic defenestration and altered endocytosis at first-pass concentrations, which underscores the need to account for combinatorial drug effects on liver endothelial function in safety assessments.

The Gist

The Liver's "Doorman" and the Drug Overload

Imagine your liver is a bustling, high-security factory that processes everything you eat and drink. To keep this factory running smoothly, it has a special team of gatekeepers called Liver Sinusoidal Endothelial Cells (LSECs).

These gatekeepers have a very specific job: they stand guard at the factory's front door (the blood vessels) and manage a series of tiny, dynamic windows called fenestrations. Think of these windows as security gates that are usually wide open, allowing essential supplies (nutrients, oxygen) to pass through to the workers inside (liver cells) and letting waste products out.

This study asks a simple but critical question: What happens to these security gates when an older person takes multiple medications at once?

In the medical world, taking many drugs at once is called polypharmacy. It's common in older adults, but it can lead to falls, confusion, and other health issues. While we know drugs can hurt the liver, we didn't know exactly how they affect these specific "gatekeeper" cells, especially at the low, non-lethal doses people actually take.

The Experiment: A Stress Test for the Gates

The researchers set up a simulation using mouse cells to test four very common medications:

1. Metoprolol (for heart/blood pressure)
2. Citalopram (for depression/anxiety)
3. Oxybutynin (for bladder control)
4. Oxycodone (for pain)

They tested these drugs in two scenarios:

• The "Steady State" (The Daily Routine): This is the low level of drug circulating in your blood after you've been taking it for a while.
• The "First Pass" (The Rush Hour): When you swallow a pill, it goes straight to the liver first. The concentration of the drug hitting the liver is much higher here—like a sudden rush of traffic—before it spreads to the rest of your body.

They also tested a "Cocktail" where all four drugs were mixed together, mimicking a patient taking a complex regimen.

What They Discovered

Here is the breakdown of what happened to the liver's "security gates":

1. The Cells Didn't Die, But They Got Stressed

First, the good news: The drugs didn't kill the cells. The gatekeepers were still alive. However, they were under sublethal stress. It's like a worker who isn't fired but is so overwhelmed they start making mistakes or changing their behavior.

2. The Gates Started Closing (Defenestration)

This was the big finding. When the drugs hit the liver at high concentrations (the "First Pass" or rush hour), the tiny windows started to close up.

• Citalopram and Oxybutynin were the worst offenders. They acted like a heavy hand slamming the security gates shut.
• Metoprolol and Oxycodone were gentler, only slightly narrowing the gates.
• The Cocktail Effect: When all four drugs were mixed together at high concentrations, the gates closed even more dramatically than with any single drug. It was a synergistic effect—the drugs worked together to clog the system more than the sum of their parts.

3. The "Vacuum Cleaner" Went into Overdrive

The liver cells also have a "vacuum cleaner" function (endocytosis) to suck up waste and old proteins.

• Surprisingly, when the gates were under stress from the high drug concentrations, the vacuum cleaners turned up to max speed.
• The cells tried to compensate for the stress by working harder to clean up.
• The Exception: Citalopram closed the gates but didn't make the vacuum cleaner work faster. This suggests that different drugs mess with the cell's structure and its cleaning function in completely different ways.

Why This Matters: The "Traffic Jam" Analogy

Imagine your liver is a highway interchange.

• The Fenestrations (Windows) are the on-ramps and off-ramps.
• The Drugs are like construction vehicles blocking the road.

When you take one drug, it's like one construction vehicle. The traffic (blood flow) slows down a bit, but the highway keeps moving.
When you take a polypharmacy cocktail (multiple drugs), it's like a massive construction zone with heavy trucks blocking the on-ramps. The gates close, and the traffic jams.

Even though the liver cells (the workers) are still alive, they can't get the supplies they need, and they can't get the waste out efficiently. This "traffic jam" at the cellular level might explain why older people on multiple medications feel frail, get sick more easily, or why their bodies can't process new drugs correctly.

The Bottom Line

This study reveals that taking multiple medications can silently damage the liver's entry system without killing the cells outright.

• High concentrations (like when a pill is first swallowed) cause the most damage.
• Mixing drugs can make the problem much worse than taking them one by one.
• The damage is structural: The physical "windows" of the liver close up, potentially leading to long-term liver dysfunction and making it harder for the body to handle medications.

The Takeaway for Patients:
This research highlights why doctors need to be very careful when prescribing multiple drugs to older adults. It's not just about whether the drugs interact chemically; it's about how the liver itself physically reacts to the stress of processing them all at once. It suggests that "less is more" when it comes to medication, and that the health of the liver's "gatekeepers" is a crucial piece of the puzzle in keeping us healthy.

Technical Summary

1. Problem Statement

Polypharmacy (the concurrent use of multiple medications) is increasingly prevalent in aging populations and is linked to adverse outcomes such as frailty, cognitive decline, and increased mortality. While the liver is the primary site of drug metabolism, the specific sublethal effects of common medications on Liver Sinusoidal Endothelial Cells (LSECs) remain poorly understood.

LSECs are specialized endothelial cells characterized by fenestrations (transcellular pores) and high endocytic activity, which are critical for the bidirectional exchange of substances between blood and hepatocytes. Disruption of these structures (defenestration) can impair liver function and drug clearance. The study addresses a critical gap: understanding how common drugs, both individually and in combination (polypharmacy), affect LSEC viability, endocytic function, and ultrastructure at physiologically relevant concentrations, specifically distinguishing between systemic steady-state levels and higher first-pass concentrations encountered during oral drug absorption.

2. Methodology

The study utilized an in vitro model using primary LSECs isolated from male C57BL/6JRj mice.

• Drug Selection: Four commonly prescribed, high Drug Burden Index (DBI) drugs were selected: citalopram (antidepressant), oxybutynin (anticholinergic), metoprolol (beta-blocker), and oxycodone (opioid).
• Concentration Modeling:
  • Steady-State Concentrations: Derived from previous in vivo mouse studies (Mach et al., 2021) measuring systemic levels after chronic therapeutic dosing.
  • First-Pass Concentrations: Calculated by dividing steady-state concentrations by the oral bioavailability of each drug, simulating the higher concentrations LSECs encounter immediately after oral ingestion.
  • Polypharmacy Cocktail: A combination of all four drugs at the respective steady-state and first-pass concentrations.
• Assays:
  • Viability: Resazurin assay to measure mitochondrial metabolic activity.
  • Scavenging Function: Radiolabeled formaldehyde-treated serum albumin ($^{125}$I-FSA) assay to quantify endocytic uptake and degradation via scavenger receptors (Stabilin-1/2).
  • Morphology: Scanning Electron Microscopy (SEM) followed by quantitative image analysis using a custom StarDist deep learning model. Metrics included fenestration frequency (pores/$\mu m^2$), porosity (% area covered), and pore diameter distribution.
• Statistical Analysis: Data were normalized to controls and analyzed using One-way ANOVA or Kruskal-Wallis tests with multiple comparisons.

3. Key Contributions

• Differentiation of Exposure Levels: The study explicitly models the difference between systemic steady-state exposure and the higher first-pass hepatic exposure, revealing that first-pass concentrations induce significantly more severe morphological and functional changes.
• Sublethal Stress Characterization: The research demonstrates that common drugs can induce significant functional and structural alterations in LSECs without causing cell death (cytotoxicity), highlighting a "sublethal" mechanism of drug-induced liver stress.
• Synergistic vs. Antagonistic Effects: The study investigates how drug combinations interact, finding that while some drugs act additively, others (like citalopram) may modulate the effects of the cocktail differently depending on the exposure level.
• Uncoupling of Structure and Function: A novel finding is the dissociation between structural changes (defenestration) and functional changes (endocytosis), suggesting distinct molecular pathways for these responses.

4. Key Results

A. Cell Viability

• No Cytotoxicity: None of the treatments (monotherapies or cocktail) at steady-state or first-pass concentrations significantly reduced cell viability.
• Metabolic Shift: Interestingly, the first-pass polypharmacy cocktail caused a significant increase in resazurin reduction (128% of control). The authors interpret this not as improved health, but as a stress-induced metabolic shift (potentially driven by ROS and calcium signaling) that accelerates mitochondrial respiration.

B. Endocytic (Scavenging) Function

• First-Pass Enhancement: Exposure to first-pass concentrations significantly increased endocytic activity for oxybutynin, metoprolol, oxycodone, and the polypharmacy cocktail (up to 185% of control).
• Citalopram Exception: Citalopram was the only drug that did not increase endocytosis at either concentration, despite causing morphological changes.
• Steady-State: No significant changes in endocytosis were observed at steady-state concentrations.

C. Morphology (Fenestrations)

• Steady-State Effects: All monotherapies significantly reduced porosity (surface area covered by pores), but fenestration frequency remained largely stable. This suggests a redistribution of pore sizes rather than total loss.
• First-Pass Effects: First-pass concentrations caused profound defenestration (loss of pores) across all groups.
  • Citalopram and Oxybutynin: Caused the most severe, dose-dependent reduction in both porosity and frequency.
  • Polypharmacy Cocktail: Triggered synergistic defenestration at first-pass concentrations, reducing porosity and frequency significantly more than steady-state levels.
  • Metoprolol and Oxycodone: Produced mild, non-dose-dependent effects on morphology.
• Pore Size Distribution: First-pass treatments increased the proportion of large fenestrations (>200 nm), while steady-state treatments reduced the proportion of small fenestrations (<100 nm).

5. Significance and Implications

• Mechanism of Polypharmacy Toxicity: The study provides a cellular mechanism for the systemic frailty and functional decline observed in polypharmacy patients. Sublethal stress on LSECs impairs the liver's filtration and clearance capabilities.
• Pharmacokinetic Impact: Defenestration reduces the permeability of the sinusoidal barrier, potentially impairing the transfer of drugs and substrates to hepatocytes. This could alter drug clearance rates (e.g., reduced first-pass metabolism), leading to unpredictable systemic drug levels and toxicity.
• Drug Safety Assessment: Current safety assessments often focus on hepatocyte toxicity. This study argues for the inclusion of endothelial responses (fenestration integrity and endocytic function) in drug safety and pharmacokinetic modeling, particularly for elderly patients on multiple medications.
• Aging and Polypharmacy: The findings suggest that polypharmacy may mimic or accelerate "aging-like" stress responses in the liver endothelium, even in young models, potentially explaining why polypharmacy exacerbates frailty in the elderly.

In conclusion, the paper establishes that common medications, particularly in combination and at first-pass concentrations, induce sublethal but functionally significant remodeling of liver sinusoidal endothelial cells, compromising liver filtration and potentially contributing to the adverse outcomes associated with polypharmacy.