Adipose tissue remodeling under ischemia: death of adipocytes and activation of stem/progenitor cells.
BACKGROUND
Following various types of plastic surgery, such as adipose grafting and flap elevation, adipose tissue undergoes ischemia, leading to hypoxia and nutrient depletion. However, few studies have examined ischemic and/or hypoxic changes in adipose tissue.


METHODS
The authors established surgically induced ischemia models by severing blood vessels supplying the inguinal fat pads in mice. The partial pressure of oxygen in adipose tissue was measured with an oxygen monitor, and ischemic changes were analyzed by whole-mount staining, immunohistochemistry, flow cytometry, and Western blotting. The authors also examined cell survival under a hypoxic condition in vitro.


RESULTS
Models for three degrees (mild, intermediate, and severe) of ischemia showed approximately 75, 55, and 20 percent of the partial pressure of oxygen level in normal adipose tissue (50.5±1.3 mm Hg), respectively. Adipose tissue atrophy with substantial fibrosis on day 28 was seen, depending on the severity of ischemia. Intermediate and severe ischemia induced elevated expression of hypoxia-inducible factor 1α and fibroblast growth factor 2 on day 1 and degenerative changes (i.e., apoptosis, necrosis, and macrophage infiltration and phagocytosis) in adipose tissue. Dead cells included adipocytes, vascular endothelial cells, and blood-derived cells, but not adipose-derived stem/progenitor cells. Subsequent to degenerative changes, regenerative changes were seen, including angiogenesis, adipogenesis, and proliferation of cells (adipose-derived stem/progenitor cells, vascular endothelial cells, and blood cells). The authors found that, in vitro, the experimentally differentiated adipocytes underwent apoptosis and/or necrosis under severe hypoxia, but adipose-derived stem/progenitor cells remained viable.


CONCLUSIONS
Severe ischemia/hypoxia induces degenerative changes in adipose tissue and subsequent adaptive tissue remodeling. Adipocytes die easily under ischemic conditions, whereas adipose-derived stem/progenitor cells are activated and contribute to adipose tissue repair.
Tokyo, Japan From the Department of Plastic Surgery, University of Tokyo School of Medicine.
Hirotaka
H
Suga
Hitomi
H
Eto
Noriyuki
N
Aoi
Harunosuke
H
Kato
Jun
J
Araki
Kentaro
K
Doi
Takuya
T
Higashino
Kotaro
K
Yoshimura
D017667:Adipocytes / Q000473:pathology*; D000273:Adipose Tissue / Q000098:blood supply* / Q000473:pathology* / Q000637:transplantation; D053687:Adult Stem Cells / Q000473:pathology*; D000818:Animals; D001284:Atrophy; D015153:Blotting, Western; D016923:Cell Death / Q000502:physiology*; D002470:Cell Survival / Q000502:physiology; D005355:Fibrosis / Q000473:pathology; D007124:Immunoenzyme Techniques; D020287:In Situ Nick-End Labeling; D007511:Ischemia / Q000473:pathology*; D051379:Mice; D008813:Mice, Inbred ICR; D010101:Oxygen Consumption / Q000502:physiology; D012038:Regeneration / Q000502:physiology*; D015427:Reperfusion Injury / Q000473:pathology; D013234:Stem Cells / Q000473:pathology*