Different Manifestations of SDH Loss in Adrenal Medulla vs. Fibroblast Models

Background Introduction

Succinate dehydrogenase (SDH) is a key enzyme in the mitochondrial tricarboxylic acid (TCA) cycle and electron transport chain, responsible for oxidizing succinate to fumarate and participating in electron transfer. SDH consists of four subunits (SDHA, SDHB, SDHC, SDHD), and the loss of function in any subunit can lead to the inactivation of SDH, thereby affecting cellular energy metabolism. SDH deficiency is closely associated with the development of various tumors, particularly pheochromocytoma (PPGL) and paraganglioma (PPGL). These tumors typically originate from neural crest-derived paraganglia cells, where SDH loss leads to succinate accumulation, which in turn inhibits multiple 2-oxoglutarate-dependent dioxygenases, ultimately promoting the expression of genes related to angiogenesis and cell proliferation.

However, why SDH deficiency is tumorigenic only in neuroendocrine cells remains an unresolved mystery. To address this question, researchers compared two SDH-deficient cell models: immortalized mouse chromaffin cells (IMCCs) derived from the adrenal medulla and immortalized mouse embryonic fibroblasts (IMEFs), aiming to reveal the adaptive mechanisms and vulnerabilities of SDH loss in different cell types.

Source of the Paper

This paper was co-authored by Fatimah J. Al Khazal, Sanjana Mahadev Bhat, and others, with the research team coming from multiple institutions, including Mayo Clinic and the Paris Cardiovascular Research Center. The study was published in 2024 in the journal Cancer & Metabolism, titled “Similar deficiencies, different outcomes: Succinate dehydrogenase loss in adrenal medulla vs. fibroblast cell culture models of paraganglioma.”

Research Process and Results

1. Cell Culture and Establishment of SDH-Deficient Models

The study first established two SDH-deficient cell models: SDHB-deficient IMCCs and SDHC-deficient IMEFs. Using gene knockout technology, the researchers successfully constructed SDHB- and SDHC-deficient cell lines and validated the loss of target proteins through Western blot. Additionally, PCR genotyping confirmed the deletion status of SDH subunits.

2. Cellular Phenotype and Metabolic Changes

The study found that SDH deficiency significantly affected cellular phenotypes and metabolism. Both SDH-deficient IMCCs and IMEFs exhibited slowed proliferation, increased cell size, and altered cell cycles. Specifically, SDH-deficient IMCCs showed prolonged G1 phase, while IMEFs exhibited prolonged G2 phase, suggesting potential impacts on DNA repair pathways.

Through metabolomic analysis, the researchers observed a significant increase in succinate levels (approximately 130-fold) in SDH-deficient cells, while levels of other TCA cycle-related metabolites generally decreased. Notably, SDH-deficient IMEFs accumulated lactate, whereas IMCCs did not, indicating that IMEFs rely more on glycolysis to maintain energy supply.

3. Mitochondrial Morphology and Functional Changes

Using electron microscopy, the researchers observed significant changes in mitochondrial morphology in SDH-deficient IMCCs, characterized by mitochondrial swelling, blurred cristae structures, and even the presence of electron-dense bodies. In contrast, SDH-deficient IMEFs showed milder mitochondrial morphological changes. Furthermore, three-dimensional microscopy analysis revealed an overall decrease in mitochondrial volume density in SDH-deficient cells, despite the enlargement of remaining mitochondria.

4. Metabolic Adaptability and Performance Under Hypoxia

Through Seahorse extracellular flux analysis, the researchers found that SDH-deficient IMCCs retained higher basal oxygen consumption rates (OCR) under normoxic conditions, while their metabolic capacity under hypoxic conditions was comparable to that of wild-type cells. In contrast, SDH-deficient IMEFs exhibited significantly reduced metabolic capacity under both normoxic and hypoxic conditions. These results suggest that IMCCs can maintain some oxidative metabolic capacity by preserving Complex I function after SDH loss, whereas IMEFs rely more on glycolysis.

5. Transcriptomic Analysis

RNA sequencing results revealed strikingly different transcriptomic responses to SDH loss in IMCCs and IMEFs. In IMCCs, the expression of multiple Complex I subunit genes was upregulated, while no significant changes were observed in IMEFs. This finding further supports the hypothesis that IMCCs maintain metabolic adaptability by upregulating Complex I-related genes.

Conclusions and Significance

This study, by comparing SDH-deficient IMCCs and IMEFs, revealed the different manifestations of SDH loss in different cell types. IMCCs can maintain some metabolic adaptability by preserving Complex I function after SDH loss, while IMEFs rely more on glycolysis. This finding provides new insights into the tumorigenicity of SDH deficiency in neuroendocrine cells and offers a theoretical basis for developing therapeutic strategies targeting SDH-deficient tumors.

Research Highlights

1. Cell Type Specificity: The study systematically compared the different manifestations of SDH loss in adrenal medulla cells and fibroblasts for the first time, revealing cell type-specific responses to SDH deficiency.
2. Metabolic Adaptability: IMCCs can maintain metabolic adaptability by preserving Complex I function after SDH loss, providing important clues for understanding the mechanisms of SDH-deficient tumorigenesis.
3. Transcriptomic Analysis: Through RNA sequencing, the study uncovered differences in transcriptomic responses to SDH loss in different cell types, laying the groundwork for further research into the molecular mechanisms of SDH deficiency.

Other Valuable Information

The study also found that succinate accumulation in SDH-deficient cells inhibits the activity of histone demethylases (JMJD demethylases), leading to histone hypermethylation. This discovery provides a new perspective on the impact of SDH deficiency on epigenetic regulation.

This research not only deepens our understanding of the tumorigenic mechanisms of SDH deficiency but also provides important theoretical support for developing therapeutic strategies targeting SDH-deficient tumors.