Molecular Cell Biology Lecture #18: Metabolic Health

Molecular Cell Biology Lecture #18: Metabolic Health

Presented by: Mitra Esfandiarei, Ph.D.
Email: mesfan@midwestern.edu
Date: October 6, 2025

## Learning Objectives

  • Understand the concept and definition of metabolic health

  • Explain the role of AMP-activated protein kinase (AMPK) in metabolism.

  • Understand how AMPK is activated by nutrition deprivation (Low ATP, High AMP).

  • Understand the two-step pathway of full AMPK activation and the role that AMP and Liver Kinase B1 (LKB1) play in the process.

  • Explain the downstream events of AMPK activation:

    • Impact on glucose uptake

    • Glycolysis

    • Lipolysis

    • Lipogenesis

    • Fatty acid oxidation

    • Autophagy

  • Explain how AMPK's effects on acetyl-CoA carboxylases (ACC) can impact fatty acid oxidation in mitochondria and understand the role of malonyl-CoA and CPT-1 in the process.

  • Understand how a high sugar diet could impact the insulin signaling pathway.

  • Understand how a high sugar diet could impact the AMPK signaling pathway.

  • Explain the mechanism of metformin action on AMPK.

  • Understand how metformin affects downstream signaling pathways of AMPK.

  • Explain the interplay between AMPK & mTORC1 and their opposing functions in metabolic pathways.

  • Understand how activation of AMPK leads to inhibition of:

    • Protein synthesis

    • Glycolysis

    • Lipogenesis

Metabolic Health

  • Definition: Metabolic health refers to the body’s ability to efficiently manage:

    • Blood sugar

    • Lipids

    • Energy balance

    • Hormone regulation

  • Characteristics of Optimal Metabolic Health:

    • Normal blood glucose levels

    • Healthy blood lipid levels

    • Controlled blood pressure

    • Balanced energy storage and expenditure

  • Dietary Impact:

    • Poor dietary choices, especially those high in sugars, refined carbohydrates, unhealthy fats, and ultra-processed foods can disrupt metabolic processes.

    • Associated conditions include:

    • Obesity

    • Insulin resistance

    • Type 2 diabetes

    • Cardiovascular diseases

    • Non-alcoholic fatty liver disease (NAFLD)

AMP-Activated Protein Kinase (AMPK)

  • Definition: Adenosine monophosphate-activated protein kinase (AMPK) is a protein kinase that functions as a crucial energy sensor maintaining cellular energy homeostasis.

  • Functionality:

    • AMPK promotes catabolic processes (e.g., glucose uptake, fatty acid oxidation) and inhibits anabolic processes (e.g., lipid and protein synthesis) to restore energy balance.

  • Activation Mechanism:

    • AMPK is activated in low energy states (high AMP/ATP ratio) such as during:

    • Exercise

    • Fasting

    • Caloric restriction

    • High AMP binds to the γ-subunit of AMPK, causing conformational changes that promote partial activation.

    • Full activation requires phosphorylation by Liver Kinase B1 (LKB1) on the catalytic subunit.

    • Note: LKB1 is constitutively active in many tissues and targets partially active AMPK under energy-depleted conditions.

Downstream Effects of AMPK

  • When energy level is low:

    • AMPK promotes catabolic processes (glucose uptake, fatty acid oxidation) and blocks anabolic processes (lipid synthesis).

  • Consequences of Activation:

    • Increased glucose uptake:

    • AMPK increases the translocation of GLUT4 to the plasma membrane, enhancing glucose uptake, especially in muscle cells.

    • Inhibition of lipid synthesis:

    • AMPK phosphorylates and deactivates acetyl-CoA carboxylase (ACC), effectively inhibiting lipid synthesis.

    • Promotion of fatty acid oxidation:

    • Inhibiting ACC lowers malonyl-CoA concentrations, thus enhancing fatty acid beta-oxidation in mitochondria.

    • Inhibition of protein synthesis:

    • AMPK suppresses the mTORC1 pathway, halting protein synthesis and growth during energy-deficient states.

AMPK Activation and Fatty-Acid Oxidation

  • Activation Trigger: AMPK activates in low cellular energy states (increased AMP/ATP ratio) and promotes:

    • Phosphorylation of ACC:

    • Impairs the production of malonyl-CoA from acetyl-CoA.

    • Malonyl-CoA Effects:

    • Normally inhibits Carnitine Palmitoyl Transferase I (CPT1), a critical enzyme for mitochondrial fatty-acyl/carnitine shuttle.

    • Reduced malonyl-CoA increases CPT1 activity, allowing long-chain fatty acids to enter mitochondria for energy production through beta-oxidation.

Dietary Influence on Signaling Pathways

  1. Insulin Signaling Pathway

    • Influence of Diet: High consumption of refined sugars leads to spikes in blood glucose, overstimulating the insulin signaling pathway.

    • Pathway Disruption: Cells gradually become resistant to insulin, impeding glucose uptake and causing insulin resistance and Type 2 diabetes.

    • Key Molecules: Insulin receptor, IRS-1/2, PI3K, AKT, GLUT4.

      • PI3K/Akt/mTORC1 overactivation leads to decreased GLUT4 and hyperglycemia.

  2. AMPK Signaling Pathway

    • Influence of Diet: Diets high in refined carbohydrates and caloric excess suppress AMPK activity.

    • Pathway Disruption: Reduced AMPK activation hampers fatty acid oxidation and causes lipid accumulation, contributing to obesity and metabolic syndrome.

    • Key Molecules: AMPK, ACC, malonyl-CoA, CPT1.

Targeting AMPK in Type 2 Diabetes: Metformin

  • Metformin: An AMPK activator recommended as a first-line treatment for Type 2 diabetes.

  • Mechanism of Action:

    • Primarily taken up by liver mitochondria, it inhibits mitochondrial complex I activity, reducing ATP production and activating AMPK.

    • Inhibits gluconeogenesis, resulting in decreased fasting glucose levels.

The Interplay Between AMPK & mTORC1

  • AMPK and mTORC1 exhibit opposing effects on regulating SREBP-1, a transcription factor essential in lipid biosynthesis:

    • AMPK Activation: When energy is scarce, AMPK inhibits mTORC1 to prevent unnecessary energy-consuming processes

    • mTORC1 Activation: Under nutrient-rich conditions, it promotes anabolic processes, enhancing lipid biosynthesis.

  • Pathological Implications: Dysregulation between AMPK and mTORC1 can lead to:

    • Chronic activation of mTORC1 seen in conditions like obesity, contributing to lipid accumulation and cardiovascular risk.

    • Therapeutic benefits of drugs like metformin, which activate AMPK and inhibit mTORC1 to improve metabolic health.

Fructose Impact on Metabolic Health

  • Definition: A simple monosaccharide present in fruits, honey, and root vegetables.

  • Insulin Secretion: Fructose does not directly trigger insulin secretion since it is not metabolized by pancreatic beta cells.

  • Absorption in Intestines:

    • Absorbed via the GLUT5 transporter in enterocytes.

    • High doses (>1g/kg) can lead to fructose spillage into the liver through the portal vein.

  • Metabolism in the Liver:

    • Rapidly phosphorylated to fructose-1-phosphate by fructokinase, leading to increased de novo lipogenesis.

    • Consumption of non-carbohydrate food prior to fructose aids its intestinal absorption and metabolism.

Low vs. High Fructose Diet

  • Dietary Effects on Health:

    • Meals with high fructose intake can significantly disrupt metabolic health and emphasize the consequences of excessive fructose consumption on hepatic pathways.

The Lipogenic & Glycolytic Functions of Fructose in the Liver

  • Fructose metabolism produces intermediates for glycosylation and fatty acid synthesis, leading to potential metabolic disturbances:

    • Glycolysis Pathway: Breakdown of fructose into glycolytic intermediates (DHAP, GAP).

    • Lipogenic Pathway: Excess conversion of glycolytic intermediates to fatty acids, leading to fatty acid accumulation in the liver.

  • Regulatory Role of PFK-1: Fructose metabolism bypasses PFK-1, leading to uncontrolled glycolysis and lipogenesis.

Comparison of Fructose Lipogenic vs. Glycolytic Pathways

  • Glycolytic Features: Energy production (ATP generation) and metabolic intermediates leading to TCA cycle activity.

  • Lipogenic Features: Focus on fat synthesis (production of triglycerides) leading to liver complications such as hepatic steatosis.

Meal Fructose Analysis

  • Example Meal Ingredients:

    • Big Mac Sandwich: 4g fructose, 8.1g total sugar

    • Medium Fries: 0.3g fructose, 0.6g total sugar

    • Medium Coca-Cola: 36g fructose, 68g total sugar

    • Total Fructose for Meal: 40.3g

    • Recommendation: Limit daily fructose intake to no more than 1 mg/kg of body weight (approx. 60-70 mg for an average adult).