Time course of histomorphological changes in adipose tissue upon acute lipoatrophy

Abstract

SUMMARY Background and Aims Crown-like structures (CLS) are characteristic histopathology features of inflamed adipose tissues in obese mice and humans. In previous work, we suggested that these cells derived from macrophages primarily involved in the reabsorption of dead adipocytes. Here, we used a well-characterized transgenic mouse model in which the death of adipocytes in adult mice is inducible and highly synchronized. In this “FAT-ATTAC” model, apoptosis is induced through forced dimerization of a caspase-8 fusion protein. Methods and Results 0, 0.5, 1, 2, 3 and 10 days post induction of adipocyte cell death, we analyzed mesenteric and epididymal adipose depots by histology, immunohistochemistry and electron microscopy. Upon induction of caspase-8 dimerization, numerous adipocytes lost immunoreactivity for perilipin, a marker for live adipocytes. In the same areas, we found adipocytes with hypertrophic mitochondria and signs of organelle degeneration. Neutrophils and lymphocytes were the main inflammatory cells present in the tissue, and the macrophages were predominantly Mac-2 negative. Over the course of ablation, Mac-2 positive macrophages substituted for Mac-2 negative macrophages, followed by CLS formation. All perilipin-negative, dead adipocytes were surrounded by CLS structures. The time course of histopathology was similar in both fat pads studied, but occurred at earlier stages and was more gradual in mesenteric fat. Conclusion Our data demonstrate that CLS formation results as a direct consequence of adipocyte death, and that infiltrating macrophages actively uptake remnant lipids of dead adipocytes. Upon induction of adipocyte apoptosis, inflammatory cells infiltrate adipose tissue initially consisting of neutrophils followed by macrophages that are involved in CLS formation.

MECHANISMS IN ENDOCRINOLOGY: White, brown and pink adipocytes: the extraordinary plasticity of the adipose organ

“…All dead adipocytes gave rise to CLS, in line with our hypothesis (76). The time course of fat histopathology in this model disclosed that CLS form after adipocyte death, demonstrating that this event may be sufficient to recruit macrophages and induce CLS formation (76). Altogether, we think that the death and degeneration of hypertrophic adipocytes, with the consequent exposure to the extracellular milieu of nuclear and cytoplasmic (mainly, lipid droplets) determinants that are normally segregated into the cell, represent the primary events triggering the inflammatory and immune reactions in the obese adipose tissue.…”

“…To assess whether CLS originate from adipocyte debris, we used Philip Scherer's transgenic model, where white adipocyte apoptotic death is specifically induced by administration of a dimeriser that activates caspase 8 (75). All dead adipocytes gave rise to CLS, in line with our hypothesis (76). The time course of fat histopathology in this model disclosed that CLS form after adipocyte death, demonstrating that this event may be sufficient to recruit macrophages and induce CLS formation (76).…”

Obese adipocytes show ultrastructural features of stressed cells and die of pyroptosis

“…It may be speculated that NLRP3 infl ammasome activation in hypertrophic adipocytes is triggered by the cholesterol crystals, which ultimately drive cells to pyroptosis. Importantly, in FAT-ATTAC mice, the apoptotic mode of cell death of hypertrophic adipocytes is not characterized by formation of cholesterol crystals ( 36 ); accordingly, the hypertrophic adipose tissue shows no signs of NLRP3 infl ammasome or caspase-1 activation (present results). Data from different experimental settings suggest that, in addition to cholesterol crystals, other factors may activate the NLRP3 infl ammasome in hypertrophic adipocytes, including extracellular ATP and high glucose concentrations, which may follow rupture of adipocyte cell membranes ( 29,47 ), and increased ROS and free radical production, as also suggested by the increased expression of anti-ROS enzymes detected in our tissue samples.…”

“…That NLRP3-dependent caspase-1 activation and production of infl ammatory cytokines play a prominent role in metabolic syndrome has recently been confi rmed in mice lacking NLRP3, ASC, or caspase-1, which are resistant to the development of high-fat diet-induced obesity and insulin resistance ( 42 ). adipocyte apoptosis is induced through forced dimerization of a caspase-8 fusion protein ( 19,33 ), disclosed an analogous sequence of morphological events: apoptotic adipocytes fi rst exhibit organelle alterations; then they lose perilipin immunoreactivity, degenerate, and recruit neutrophils and lymphocytes; and fi nally, they are resorbed by macrophages in CLSs ( 36 ). Thus, the death of hypertrophic adipocytes appears to be a major event triggering macrophage recruitment and CLS formation in both genetic obesity and in the adipose tissue-specifi c apoptosis model.…”

Regulation of adipocyte lipolysis

“…Taken together, ghrelin constitutes a negative regulator of basal and TNF-a-induced autophagy in human visceral adipocytes (200) . Novel fascinating findings in the field of adipocyte apoptosis have been recently reported (398,399) . White adipose tissue inflammation, a characteristic feature of obesity, results from the death of hypertrophic adipocytes that are subsequently cleared by macrophages, giving rise to crown-like structures (CLS).…”