Cardiac REDD1 alters glucose and fatty acid metabolic gene expression via an mTORC1-independent, PPAR alpha-dependent mechanism and drives hypertrophic growth

This study demonstrates that cardiac REDD1 promotes glucose oxidation and hypertrophic growth by suppressing PPARα activity and its downstream targets (such as PDK4) through an mTORC1-independent mechanism.

The Gist

The Heart's Fuel Switch: Meet REDD1

Imagine your heart is a high-performance hybrid car. It has two main fuel tanks: one for Fatty Acids (like diesel) and one for Glucose (like gasoline). Under normal, calm driving conditions, the heart prefers the "diesel" (fatty acids) because it's efficient for long, steady cruising.

However, when the car hits a steep hill or needs to speed up (like during exercise or heart stress), it needs to switch to "gasoline" (glucose) for a quick, powerful burst of energy.

This paper discovers a new "mechanic" in the heart called REDD1. Its job is to manage the fuel switch.

The Big Discovery: REDD1 is the "Glucose Manager"

For a long time, scientists knew REDD1 was important, but they mostly thought of it as a "brake" on a different system called mTORC1 (which controls cell growth). This paper reveals a brand-new, super important job for REDD1: It tells the heart to stop burning fat and start burning sugar.

Here is how the story unfolds, step-by-step:

1. The Fuel Dilemma (The Randle Cycle)

Think of the heart's metabolism like a busy kitchen. You can't cook a steak (fat) and a salad (sugar) on the same burner at the same time without a fight.

• PPARα is the "Fat Chef." When he is active, he orders the kitchen to burn fat and shuts down the sugar station.
• PDK4 is the "Sugar Lock." When PDK4 is active, it locks the door on the sugar burner, preventing glucose from being used.

2. The New Mechanic: REDD1

The researchers found that when glucose levels rise (or when the heart is stressed), REDD1 shows up.

• What REDD1 does: It grabs the "Fat Chef" (PPARα) and tells him, "Take a break!"
• The Result: When the Fat Chef takes a break, he stops ordering fat-burning recipes. He also stops making the "Sugar Lock" (PDK4).
• The Outcome: The sugar burner unlocks! The heart can now burn glucose efficiently.

The Analogy: Imagine REDD1 is a security guard who kicks the "Fat Chef" out of the kitchen. Once the Chef is gone, the "Sugar Lock" is removed, and the kitchen switches to cooking with sugar.

3. The Surprise: It's Not About the "Growth Brake"

Scientists previously thought REDD1 only worked by hitting the "brake" on the mTORC1 system (the engine's growth regulator).

• The Experiment: The researchers used a drug (Everolimus) to artificially hit that "brake" hard.
• The Result: Even with the brake slammed down, the fuel switch didn't happen.
• The Conclusion: REDD1 has a secret, independent superpower. It controls the fuel switch without needing to touch the growth brake. It works on its own track.

4. The Stress Test (Heart Disease)

The researchers then tested what happens when the heart is under pressure (simulated by tightening a band around the aorta, like a traffic jam for blood flow).

• Normal Heart: When stressed, the heart naturally increases REDD1. This helps it switch to glucose, which helps the heart muscle grow bigger and stronger to handle the stress (a good, adaptive response).
• REDD1-Deleted Heart: When they removed REDD1 from the mice, the heart couldn't switch to glucose. It kept trying to burn fat even though it was stressed.
• The Consequence: The heart couldn't grow properly to handle the stress. It stayed smaller and weaker than it should have.

Why Does This Matter?

Think of the heart as a building that needs to be renovated when a storm hits (heart disease).

• With REDD1: The renovation crew knows to switch from "wood" (fat) to "steel" (glucose) to reinforce the building quickly.
• Without REDD1: The crew keeps trying to use wood, but the storm is too strong. The building doesn't get reinforced, and the renovation fails.

The Takeaway for Everyday Life

1. REDD1 is a Metabolic Switch: It's a crucial molecule that helps the heart decide whether to burn fat or sugar.
2. It's a "Sugar Promoter": It shuts down fat burning to make room for sugar burning, which is vital when the heart is working hard or under stress.
3. It's Independent: It does this job all by itself, not just by controlling the famous "growth brake" (mTORC1).
4. Future Medicine: Understanding this switch could help doctors treat heart failure. If we can control REDD1, we might be able to help a failing heart switch its fuel source to keep it running strong, or stop it from growing too big and failing.

In short: REDD1 is the heart's smart manager that says, "Stop burning fat, we need sugar right now to get through this stress!" And it does this without asking permission from the usual growth controllers.

Technical Summary

1. Problem Statement

The heart relies primarily on fatty acid oxidation for energy under physiological conditions but shifts toward glucose oxidation during pathological stress (e.g., pressure overload-induced hypertrophy) and specific physiological states (e.g., insulin stimulation). While the metabolic shift is well-documented, the molecular mechanisms controlling this fuel preference remain unclear.

Regulated in development and DNA damage 1 (REDD1) is a known negative regulator of mTORC1 signaling and is induced by glucose, insulin, and stress. Previous studies suggested REDD1 deletion leads to systemic glucose intolerance, yet its specific role in cardiac metabolic substrate preference and the mechanism governing the glucose-to-fatty acid switch were unexplored. The authors hypothesized that REDD1 is a critical regulator of cardiac fuel preference, potentially acting independently of its canonical mTORC1-inhibitory function.

2. Methodology

The study utilized a multi-faceted approach combining in vitro cell culture models and in vivo mouse genetics:

• Cell Models: Human AC16 cardiomyocytes with REDD1 deletion (AC16ΔREDD1) and wild-type (WT) controls.
• Animal Models:
  • Global REDD1 knockout mice (Redd1-/-).
  • Cardiomyocyte-specific REDD1 knockout mice (Redd1^Fl/Fl, αMHC-Cre).
  • Wild-type and floxed controls.
• Interventions:
  • Metabolic Stress: Culturing cells in varying glucose concentrations (0, 2.5, 5.5, 25 mM).
  • Pharmacological Inhibition: Treatment with Everolimus (mTORC1 inhibitor) and GW6471 (PPARα inhibitor).
  • Pathological Model: Transverse Aortic Constriction (TAC) surgery to induce pressure overload and cardiac hypertrophy for 2 weeks.
• Assays:
  • Molecular: qPCR, Western blotting (phosphorylation status of PDH, PDK4, mTORC1 targets), PPARα activity assays, and nuclear extraction.
  • Metabolic: PDH activity assays, mitochondrial stress tests (Seahorse XF Analyzer measuring OCR/ECAR), and untargeted metabolomics.
  • Omics: RNA-sequencing (RNA-Seq) and Gene Ontology/KEGG pathway analysis.
  • Phenotypic: Echocardiography, gravimetric analysis (Heart Weight/Body Weight, Heart Weight/Tibia Length), histology (H&E, WGA staining for cardiomyocyte cross-sectional area), and pathological marker analysis (Nppb, CARP).

3. Key Contributions

This study establishes three major novel findings:

1. REDD1 as a Metabolic Switch: REDD1 is required to enhance glucose oxidation and suppress fatty acid oxidation in cardiomyocytes.
2. mTORC1-Independence: The metabolic effects of REDD1 are independent of its well-established role as an mTORC1 inhibitor.
3. PPARα-Dependent Mechanism: REDD1 exerts its metabolic control by directly inhibiting the transcriptional activity of PPARα, thereby downregulating PDK4 and fatty acid catabolic genes.

4. Key Results

A. REDD1 Regulates Glucose Oxidation via PDH

• Induction: Physiological glucose levels induce REDD1 expression in cardiomyocytes.
• PDH Regulation: In REDD1-deleted cells/mice, PDK4 (Pyruvate Dehydrogenase Kinase 4) expression is significantly elevated. This leads to increased phosphorylation (inhibition) of PDH at Serine 300 (and Serine 293 in some contexts), reducing PDH activity.
• Consequence: Reduced PDH activity prevents the conversion of pyruvate to acetyl-CoA, shifting metabolism away from glucose oxidation. This is evidenced by reduced Maximal Respiratory Capacity (MRC) and increased glycolytic capacity (ECAR) in REDD1-deficient cells.

B. REDD1 Suppresses Fatty Acid Metabolism

• Transcriptomics: RNA-Seq of Redd1^Fl/Fl, αMHC-Cre hearts revealed upregulation of genes involved in fatty acid uptake (Cd36), transport (Fabp4), activation (Acsl1), and β-oxidation (Acadm, Hadhb, etc.).
• Mechanism: REDD1 normally suppresses these genes. Its absence leads to a hyper-active fatty acid catabolic state.

C. The Mechanism is mTORC1-Independent but PPARα-Dependent

• mTORC1 Independence: Treatment with Everolimus (mTORC1 inhibitor) successfully blocked mTORC1 signaling (reduced pP70S6K) but failed to normalize the elevated PDK4, pPDH, or the altered metabolic profile in REDD1-deficient cells. This proves the metabolic switch is not mediated via mTORC1.
• PPARα Dependence:
  • REDD1 deletion increased PPARα activity (nuclear binding) without changing PPARα protein levels.
  • Treatment with the PPARα inhibitor GW6471 in REDD1-deficient cells normalized the expression of PDK4 and ACSL1 and reduced pPDH levels.
  • This confirms REDD1 acts by inhibiting PPARα activity, which in turn suppresses PDK4 and fatty acid genes.

D. Role in Pathological Hypertrophy

• Induction: Pressure overload (TAC) induces REDD1 expression in WT hearts.
• Hypertrophic Growth: WT mice subjected to TAC developed significant hypertrophy (increased HW/BW, HW/TL, cardiomyocyte CSA, and markers Nppb/CARP).
• REDD1 Requirement: Cardiomyocyte-specific REDD1 deletion (Redd1*^Fl/Fl, *αMHC-Cre + TAC) significantly attenuated hypertrophic growth compared to TAC-treated controls.
• Metabolic Link: In TAC-treated WT mice, PDK4 and pPDH levels dropped (facilitating glucose oxidation for growth). In REDD1-deficient mice subjected to TAC, these levels remained high (similar to sham), preventing the necessary metabolic shift and blunting the hypertrophic response.

5. Significance

• Novel Pathway: The study uncovers a critical, mTORC1-independent pathway where REDD1 acts as a "metabolic gatekeeper" by inhibiting PPARα. This redefines REDD1's function beyond cell growth regulation to direct metabolic control.
• Randle Cycle Integration: The authors propose REDD1 as a novel component of the Randle Cycle (glucose-fatty acid cycle), where glucose-induced REDD1 inhibits PPARα to favor glucose utilization over fatty acids.
• Therapeutic Implications:
  • Heart Failure: Since the early adaptive phase of hypertrophy requires a shift to glucose oxidation, REDD1 is essential for this compensatory mechanism. However, in later stages of heart failure, PPARα activation has been shown to be beneficial. The study suggests that while REDD1 inhibition might be detrimental in early hypertrophy, it could be a therapeutic target in established heart failure to restore PPARα activity and improve cardiac energetics.
  • Metabolic Disease: Understanding this pathway offers new insights into glucose intolerance and metabolic flexibility in cardiac and potentially other tissues (liver, skeletal muscle).

In conclusion, the paper demonstrates that REDD1 is a glucose-induced, PPARα-dependent inhibitor of fatty acid oxidation that is essential for the metabolic remodeling required for cardiac hypertrophic growth, operating via a mechanism distinct from its mTORC1 regulation.