Mitochondrial Sirt3 loss disrupts fat-burning brake in female fat cells

Scientists at the Ruđer Bošković Institute in Zagreb have uncovered a striking sex-specific weakness in how fat cells regulate their own fat storage. In a study published in Molecular Biology Reports, a team led by Sandra Sobočanec and with Ena Šimunić and Kate Šešelja as joint first authors reports that losing Sirtuin 3, a mitochondrial enzyme long tied to oxidative metabolism and stress adaptation, selectively dismantles an antilipogenic signaling circuit in female-derived fat cells. The finding offers a molecular explanation for why adipose tissue behaves so differently between the sexes and why female fat cells may be particularly vulnerable when this mitochondrial guardian is absent.

Sirtuin 3, or Sirt3, is one of a family of NAD+-dependent deacetylases that reside inside mitochondria and remove chemical acetyl tags from metabolic enzymes. This deacetylation keeps enzymes involved in fatty acid oxidation, the respiratory chain, and antioxidant defense running at peak efficiency. Decades of work have linked Sirt3 loss to metabolic syndrome, fatty liver, cardiac lipid buildup, and pancreatic beta cell failure. Yet its role during adipogenesis, the process by which precursor cells mature into fat-storing adipocytes, remained poorly defined, particularly in light of well-documented biological sex differences in adipose tissue function and distribution.

To probe this question, the researchers turned to mouse embryonic fibroblasts, or MEFs, derived from both male and female Sirt3 wild-type and knockout mice. These cells were coaxed through a standard adipogenic differentiation protocol into lipid-rich, adipocyte-like cells. The team then deployed an unusually broad analytical arsenal: Oil Red O staining to visualize accumulated lipids, confocal microscopy and lipid droplet morphometry to quantify storage architecture, immunoblotting to track signaling proteins, fatty acid profiling by gas chromatography, LC–MS metabolomics to map the cellular lipidome, and transmission electron microscopy to inspect organelle ultrastructure. Each experimental group contained three to five biological replicates.

The first major observation was that all four groups, male and female, wild-type and knockout, successfully induced adipogenic markers and formed lipid-laden adipocyte-like cells. Sirt3 was not required for the basic program of fat cell differentiation. However, the female knockout adipocytes stood out dramatically, displaying the strongest lipid storage phenotype of any group. This female-biased accumulation suggested that Sirt3 exerts a brake on lipid storage specifically in female cells, a brake that disappears when the enzyme is genetically removed.

The mechanistic heart of the study lies in the AMPK–Acc1 signaling axis. AMP-activated protein kinase, or AMPK, is a cellular energy sensor that switches on when energy supplies run low. One of its key downstream targets is acetyl-CoA carboxylase 1, or Acc1, the rate-limiting enzyme of de novo lipogenesis. When AMPK phosphorylates Acc1, Acc1 activity drops, malonyl-CoA synthesis falls, and fat synthesis is throttled back. In wild-type female adipocytes, the researchers found that bortezomib, a clinically used proteasome inhibitor, increased phosphorylation of both AMPK and Acc1, engaging this antilipogenic circuit. But in Sirt3 knockout female cells, basal phosphorylated AMPK relative to total AMPK was already reduced, and the cells failed to mount a comparable AMPK–Acc1 response to bortezomib.

Even more telling was the result with AICAR, a synthetic AMPK activator that bypasses upstream signaling. When the researchers applied AICAR to the knockout female adipocytes, it failed to restore the AMPK–Acc1 signaling axis. This suggests that the defect in Sirt3-deficient cells is not merely a failure of upstream AMPK activation but reflects a deeper breakdown in the coupling between AMPK and its lipogenic target, a breakdown that cannot be rescued simply by pushing AMPK harder pharmacologically.

Fatty acid profiling added a second layer to the story. Sirt3 deficiency increased the abundance of stearoyl-CoA desaturase 1, or Scd1, the enzyme that introduces a double bond into saturated fatty acids, and raised the C16:1 to C16:0 desaturation index, a biochemical readout of Scd1 activity. Elevated desaturation is associated with monounsaturated fatty acid production and has been linked to triglyceride synthesis and metabolic disease in both human and mouse studies. Intriguingly, bortezomib reduced Scd1 abundance in the knockout cells but did not normalize the elevated desaturation ratio, indicating that the desaturation phenotype persists even when Scd1 protein levels are partially brought down.

Metabolomics revealed a predominantly bortezomib-associated remodeling of glycerophospholipids and lysophospholipids, the structural lipid families that form cellular membranes and lipid signaling intermediates. This lipidome shift points to membrane composition changes accompanying the proteasome inhibitor treatment. Qualitative transmission electron microscopy then uncovered bortezomib-associated organelle stress, most pronounced in the Sirt3 knockout adipocytes, consistent with the idea that cells lacking this mitochondrial quality-control enzyme are less equipped to buffer proteotoxic and lipotoxic challenges during adipocyte maturation.

Taken together, the data support a model in which female adipocytes rely on Sirt3-dependent metabolic control during adipogenesis. When Sirt3 is present, it helps maintain AMPK–Acc1 antilipogenic signaling, keeps Scd1-associated desaturation in check, and supports lipid turnover. When Sirt3 is absent, the antilipogenic brake weakens, lipid storage intensifies, desaturation rises, and organelle stress accumulates under proteasome challenge. Male adipocytes, by contrast, did not show the same degree of dependence on Sirt3 for these phenotypes, underscoring the sexual dimorphism that has long been recognized in adipose biology but rarely traced to a single mitochondrial enzyme.

The findings carry implications that extend beyond basic cell biology. Because impaired adipogenesis contributes to adipose tissue dysfunction in obesity and metabolic disease, identifying Sirt3 as a sex-specific regulator of lipid storage signaling opens potential avenues for therapeutic exploration, particularly in conditions where female fat tissue metabolism goes awry. The study also highlights proteasome inhibitors like bortezomib not just as cancer drugs but as experimental tools that expose hidden vulnerabilities in metabolic signaling. As researchers continue to map how mitochondrial sirtuins shape tissue-specific and sex-specific metabolism, this work adds a compelling piece to the puzzle of why fat biology is never one-size-fits-all.

Subject of Research: Sirt3 deficiency and sex-specific adipocyte lipid metabolism

Article Title: Sirt3 deficiency impairs AMPK–Acc1 antilipogenic signaling in female adipocytes

Article References: Šimunić, E., Šešelja, K., Podgorski, I. I., Belužić, R., Hadžija, M. P., Mavar, M. P., Rešetar, M., Halasz, M., Jaganjac, M., Bujak, I. T., Protrka, M., Balog, T., Stojanović, S., Korać, A., & Sobočanec, S. (2026). Sirt3 deficiency impairs AMPK–Acc1 antilipogenic signaling in female adipocytes. Molecular Biology Reports, 53(1), Article 1726. https://doi.org/10.1007/s11033-026-12892-7

Image Credits: AI Generated

DOI: 10.1007/s11033-026-12892-7

Keywords: Sirt3, deficiency, impairs, AMPK, Acc1, antilipogenic, signaling, female, adipocytes, scientific research