How the loss of PDHK2 and PDHK4 enzymes leads to ketoacidosis, hypothermia, and death during fasting
Imagine your body as a hybrid vehicle, capable of seamlessly switching between different fuel sources—glucose from carbohydrates and fatty acids from fats. This remarkable capability, known as metabolic flexibility, allows us to maintain energy through feast and famine. Recent research on genetically modified mice has revealed what happens when this switching mechanism fails—with dramatic consequences that include ketoacidosis, hypothermia, and even death during fasting.
At the heart of our metabolic system lies a crucial molecular complex that decides whether we burn sugar or fat. The pyruvate dehydrogenase complex (PDC) serves as the body's ultimate fuel selector, controlling the conversion of pyruvate (derived from glucose) into acetyl-CoA, which can then enter the citric acid cycle to produce energy 4 .
This decision represents a critical metabolic branch point—pyruvate can either be converted to acetyl-CoA for energy production, or conserved for glucose synthesis through gluconeogenesis. The PDC doesn't operate autonomously; its activity is regulated by two opposing enzymes: pyruvate dehydrogenase kinases (PDKs) and pyruvate dehydrogenase phosphatases (PDPs) 6 .
The PDC acts as a gatekeeper between glycolysis and the citric acid cycle, determining whether glucose is fully oxidized or conserved.
The phosphorylation-dephosphorylation mechanism of PDC regulation allows for precise control over fuel selection based on nutritional status and energy demands. This system ensures that glucose is conserved during fasting for organs that critically depend on it, like the brain.
The four PDK isoenzymes (PDK1-4) phosphorylate the PDC, rendering it inactive, while PDPs activate it through dephosphorylation. In the well-fed state, when glucose is abundant, the PDC remains active, promoting glucose oxidation. However, during fasting or starvation, our bodies need to conserve glucose for organs that depend on it. This is where PDKs play their crucial role—they act as metabolic brakes that shut down glucose oxidation and promote fatty acid utilization instead 6 .
| Isoenzyme | Primary Tissues | Key Regulatory Factors | Functional Importance |
|---|---|---|---|
| PDK1 | Heart, pancreatic islets | HIF1α | Hypoxia response |
| PDK2 | Heart, liver, kidney | Nutritional factors, hormones | Fed state regulation |
| PDK3 | Testis, kidney, brain | HIF1α, ChREBP | Tissue-specific regulation |
| PDK4 | Skeletal muscle, heart, liver | Fasting, diabetes, fatty acids | Fasting adaptation |
Table 1: The Four PDK Isoenzymes and Their Expression Patterns
The importance of different PDK isoenzymes varies by nutritional state. PDK2 is particularly important in the fed state, while PDK4 becomes critical during fasting 1 . During extended periods without food, increased PDK4 expression inactivates the PDC, conserving three-carbon compounds like pyruvate, lactate, and alanine for glucose production in the liver 6 . This adaptation simultaneously promotes fatty acid oxidation and ketone body production as alternative energy sources.
In type 2 diabetes, this system goes awry. PDK4 expression becomes upregulated even in the fed state, contributing to the hyperglycemia characteristic of diabetes by shunting glucose toward gluconeogenesis 6 . This inappropriate suppression of PDC activity creates a metabolic inflexibility that worsens diabetic symptoms, making PDKs attractive potential therapeutic targets.
To understand the individual and combined roles of PDK2 and PDK4, researchers created genetically modified mice lacking one or both of these enzymes 1 2 . The results revealed both compensatory mechanisms and vulnerability points in metabolic regulation.
Mice lacking only PDK2 showed higher PDC activity and lower blood glucose in the fed state, but relatively normal glucose levels during fasting 1 . This suggested that PDK4 could compensate for the lack of PDK2 during fasting conditions. Similarly, PDK4-deficient mice displayed relatively mild metabolic alterations, with noticeable effects primarily appearing after fasting 1 .
The real surprise came when researchers created double-knockout mice (DKO) lacking both PDK2 and PDK4. These mice showed intensified effects in both fed and fasted states, with markedly improved glucose tolerance, lower insulin levels, and increased insulin sensitivity 1 2 . These sound like beneficial effects—and they were, under normal feeding conditions. However, when subjected to long-term fasting (48 hours), the DKO mice developed a metabolic crisis, succumbing to hypoglycemia, ketoacidosis, and hypothermia 1 .
| Mouse Model | Fed State Effects | Fasted State Effects | Response to Long-term Fasting |
|---|---|---|---|
| PDHK2-KO | Higher PDC activity, Lower blood glucose | Minimal changes | Normal survival |
| PDHK4-KO | Minimal changes | Higher PDC activity, Lower blood glucose | Normal survival |
| Double-KO | Markedly improved glucose tolerance, Increased insulin sensitivity | Severe hypoglycemia, Ketoacidosis | Lethal outcome |
Table 2: Metabolic Characteristics of PDK Knockout Mice
Survival rates of different PDK knockout mice during 48-hour fasting
To understand why the double-knockout mice failed to survive prolonged fasting, researchers designed a comprehensive study comparing wild-type, single-knockout, and double-knockout mice 2 . The experimental approach included:
PDHK2-KO and PDHK4-KO mice were crossbred to generate double-knockout mice
Mice were subjected to 48-hour fasting periods with careful monitoring
Blood was collected to measure glucose, ketone bodies, pH, and other metabolites
[U-13C6]glucose infusion to track metabolic fluxes
The results revealed a perfect storm of metabolic disturbances in the fasted double-knockout mice:
| Parameter | Wild-Type Mice | Double-Knockout Mice | Physiological Significance |
|---|---|---|---|
| Blood Glucose | Maintained | Severe reduction | Brain and RBCs lack fuel |
| Blood β-hydroxybutyrate | Moderate elevation | Extreme elevation | Pathological ketone production |
| Blood pH | Normal | Acidic | Metabolic acidosis |
| Bicarbonate | Normal | Low | Compromised acid-base balance |
| Body Temperature | Maintained | Significant drop | Impaired thermoregulation |
Table 3: Metabolic Parameters in 48-Hour Fasted Mice
The stable isotope studies revealed that slightly more glucose was being converted to ketone bodies in the DKO mice, creating a vicious cycle where precious glucose reserves were being inappropriately diverted 2 . This inappropriate metabolic flux contributed to the severe hypoglycemia observed in the double-knockout mice during fasting.
These findings extend far beyond theoretical interest, with significant implications for understanding and treating human metabolic diseases:
The research reveals a potential paradox in targeting PDKs for diabetes treatment. While partial inhibition of PDKs might benefit type 2 diabetes patients by increasing glucose oxidation and lowering blood glucose, complete inhibition could be dangerous 4 .
Researchers are now exploring whether a therapeutic window exists where partial PDK inhibition could provide glucose-lowering benefits without creating vulnerability to fasting-induced complications.
The study highlights the concept of metabolic inflexibility—the inability to adapt fuel oxidation to fuel availability—which characterizes conditions like type 2 diabetes, obesity, and metabolic syndrome .
The dramatic metabolic crisis observed in PDHK2/PDHK4-double-knockout mice during fasting underscores a fundamental biological principle: our metabolic systems depend on delicate balances and backup mechanisms. While pharmaceutical interventions often aim for maximum effect, this research reminds us that subtle modulation may often be safer and more effective than complete inhibition.
The survival of organisms during periods of food scarcity depends on sophisticated metabolic adaptations that have evolved over millennia. The PDK system represents a crucial component of this adaptation, allowing us to conserve precious glucose when it becomes scarce while switching to alternative fuel sources. When this system fails, as in the double-knockout mice, the consequences are severe and potentially fatal.
As research continues, scientists are working to translate these findings into safer, more effective treatments for metabolic diseases that affect millions worldwide. The goal is not to completely override our evolved metabolic regulations, but to gently correct them when they go awry—finding that perfect balance that maintains metabolic flexibility without creating vulnerability.