active endocrine organ that targets various tissues in the body to maintain homeostasis. A lack or complete absence of adipose tissue (lipodystrophy) or overproduction of adipose tissue (obesity) can result in metabolic complications such as type 2 diabetes, insulin resistance, hepatic steatosis, and hypertriglyceridemia. The plastic nature of adipose tissue, with its regenerative properties and ability to expand and contract in response to shifts in energy balance, is most evident in the obesity epidemic. For many years, scientists and clinicians have investigated the development of adipose tissue at the cellular level; however, with modern biotechnological advances in stem cell research, the study of adipogenesis has been focused on the discovery and function of adipose-derived stem cells (ASCs). Capable of differentiating into cells of nonmesodermal and mesodermal origin, ASCs serve as an important target for adipose tissue engineering and regenerative medicine.
Adipose Tissue: A Source of Multipotent Stem Cells
Stems cells are cell populations that possess multilineage potential, self-renewing capacity, and long-term viability. Originating from the stroma of bone marrow, mesenchymal stem cells (MSCs) have been widely studied as an example of adult stem cells that are capable of differentiating into chondrocytes, osteoblasts, adipocytes, and myoblasts in vivo and in vitro. While MSCs are promising candidates for disease management and mesenchymal defect repair, the use of MSCs in the clinic has been limited due to complications associated with morbidity, pain, and low cell count/tissue volume upon harvest.
In recent years, researchers have focused attention to adipose tissue as an alternative source of adult stem cells. Much like bone marrow, adipose tissue is derived from mesenchymal origin and contains an easily isolated stroma. ASCs exhibit stable proliferation and growth kinetics in culture and in the presence of specific inducing factors can differentiate into chondrogenic, osteogenic, adipogenic, and myogenic lineages. Due to the ubiquitous nature of human adipose tissue, large quantities of ASCs can be easily obtained with little patient discomfort or donor site morbidity.
The multipotent nature of ASCs is evident in various human pathologies. In children with progressive osseous heteroplasia (POH), an autosomal dominant genetic defect that causes ectopic bone formation within subcutaneous adipose depots, chondrocytes and osteoblasts can be found within colonies of adipocytes. Histologic analysis implies a tripotent capacity of ASCs to differentiate into cells of chondrogenic, osteogenic, and adipogenic origin. Obesity also presents additional evidence supporting the presence of stem cells in adipose tissue. Adipocytes have a turnover rate ranging between 6 and 15 months. While various behavioral, genetic, and epigenetic factors can contribute to obesity, in vivo studies have demonstrated the existence of stem cell populations that replace mature adipocytes throughout the lifetime of humans.
Adipose-Derived Stem Cell Isolation
Humans contain five major types of adipose tissue: bone marrow, mammary, mechanical, brown, and white. Each type of adipose depot serves a unique biological function and contains a distinct stem cell profile. White adipose tissue contains higher amounts of multipotent stem cells compared to brown adipose tissue, with subcutaneous depots providing higher yields of ASCs compared to visceral fat. Within subcutaneous white adipose tissue, greater numbers of stem cells have been harvested from arm regions compared to the abdomen, thigh, and breast. ASCs recovered from superficial abdominal regions were found to be the most resistant to apoptosis. Furthermore, ASCs from younger donors have demonstrated greater cell adhesion and proliferation compared to older donors. It has yet to be determined which adipose tissue depot serves as the optimal location for stem cell recovery.
Samples of subcutaneous adipose tissue are often obtained from subjects under local anesthesia. Current methods for isolating ASCs depend on collagenase digestion of tissue followed by centrifugation to isolate primary adipocytes from the stromal vascular fraction (SVF). ASCs display morphology similar to fibroblasts, which makes phenotypic identification difficult, and do not exhibit the intercellular lipid droplets that are found in adipocytes. Isolated ASCs are grown in monolayer culture utilizing specific cell culture techniques.
Regeneration, Repair, and Tissue Engineering
Traditionally, rehabilitation of injured or diseased organs and tissues has required tissue replacement through the use of autologous tissues since the body rejects tissue transplants with foreign antigen. With advances in modern biotechnology, researchers have placed an emphasis on developing tissue-engineered substitutes that are better suited in restoring, maintaining, and improving tissue function. The technology of tissue engineering involves an interdisciplinary field of physicians, engineers, and scientists who utilize adult stem cells to be directly implanted into the host or expanded in culture. In the latter technique, stem cells are differentiated and combined with tissue-engineered scaffolds and growth factors to develop tissue and organ systems. Tissue engineering can be used as a tool for transplantation, rehabilitation, and reconstructive surgery.
ASCs have the potential to regenerate and repair different types of tissues through a variety of mechanisms. ASCs can provide a beneficial impact on diseased or injured tissues/organs by producing and secreting soluble factors. Some of the growth factors and cytokines secreted by ASCs include hepatocyte growth factor (HGF), insulin-like growth factor (IGF-1), vascular endothelial growth factor (VEGF), tumor necrosis factor (TNF-α), fibroblast growth factor (FGF), adiponectin, transforming growth factor-β (TGF-β) and other angiogenic, anti-apoptotic, and anti-inflammatory factors. Certain soluble factors can also promote tissue repair and wound healing by recruiting endogenous stem cells. This newly formed stem cell population acts in a paracrine manner that can be stimulated to differentiate along the lineage pathway required for tissue repair.
ASCs can act as a viable source of free-radical scavengers, antioxidant chemicals, and chaperone/heat shock proteins. In injured regions such as ischemic sites, ASCs act in such a manner to clear the local environment of toxic substances, thereby improving recovery of surviving cells. Recent studies have demonstrated the capacity for bone marrow–derived MSCs to deliver mitochondria to injured cells and rescue aerobic metabolism. Comparable studies on ASCs may uncover a similar potential to contribute mitochondria.
Therapeutic benefits of ASCs also differ between autologous (derived from the same individual’s body) and allogenic (derived from genetically dissimilar individual) transplantation. While autologous ASCs can be beneficial from histocompatibility, infectious, and regulatory perspectives, it is rare for patients to provide their own therapeutic cells. Researchers have determined that a human’s ASCs that are passaged in cell culture, compared to freshly isolated cells, have reduced surface histocompatibility antigen expression and suppressed immune reactivity when cultured together with allogenic cells. While this implies that passaged ASCs may not produce a cytotoxic T-cell response when transplanted in vivo, comprehensive testing is required before clinical implementation. If proven correct, the use of allogenic ASCs in regenerative medicine holds the potential to lower costs of cell therapies, to improve availability of stem cells, and to reduce complications associated with organ and tissue failure.
Cardiac Disease and ASC Cardioplasty
In the United States, cardiovascular disease is responsible for 38% of deaths each year and remains the leading cause of death. The development of an effective cell-based therapy holds the potential to reduce medical expenses and improve patient outcomes in the context of heart failure and cardiac dysfunction. Experimental findings suggest a potential use of ASCs in cellular replacement therapy related to chronic, progressive cardiac disease and acute myocardial infarction (AMI). Studies have identified the ability of ASCs to differentiate to the cardiomyocyte lineage using 5-azacytidine in rabbits. In addition, human ASCs have shown a similar capacity for cardiac differentiation by reversibly permeabilizing their membranes then exposing the cells to cardiomyocyte extract from rats. In both cases, the cardiomyocyte lineage was evaluated based on positive immunostaining for a-actinin,
