Three-Dimensional Adipocyte Culture: The Next Frontier for Adipocyte Biology Discovery

It is now abundantly clear that adipocytes have extraordinary endocrine and metabolic functions that extend far beyond a simple role in energy storage. Indeed, adipocytes receive and respond to a variety of local and systemic metabolic, hormonal, and neuronal cues. To add further complexity, adipocytes not only exhibit a broad range of cellular diversity along the spectrum of white to beige/brite to brown (1) but also possess unique functional, secretory, and gene expression signatures that are highly dependent on their depot of origin. Numerous differences between adipose tissue depots, notably subcutaneous (sc) and visceral adipose tissues, have been documented (2). However, the mechanisms by which depotspecific differences influence adipocyte biology, and vice versa, remain poorly understood. Traditionally, studies designed to characterize adipocyte biology in isolation from other native tissue structures have relied on culturing preadipocytes under agonistic two-dimensional (2D) culture conditions involving adherence of adipocytes to tissue culture-treated polystyrene plates. Whereas such 2D approaches have been highly successful in elucidating the biology of sc adipocytes, these approaches have been suboptimal for recapitulating the biology of adipocytes from less robust sources such as visceral adipose tissue. This problem results largely because stem cells or preadipocytes isolated from visceral adipose tissue have impaired function with regard to both proliferation and differentiation in standard 2D culture (3, 4). The absence of robust methods to study visceral adipocytes in culture has impeded progress in understanding the full spectrum of biological differences among adipocytes from functionally different yet physiologicallyrelevant adipose tissue depots. Therefore, alternative methods are needed for culturing and studying adipocytes with intrinsic differences due to innate characteristics of the depot of origin. In this issue of Endocrinology, Emont et al (5) describe a facile method for culturing and studying visceral adipocytes in an optimized three-dimensional (3D) collagen hydrogel. Although various forms of 3D hydrogel systems for adipocyte culture have been studied (6, 7), the ability to culture and differentiate visceral adipocytes has remained challenging. Emont et al hypothesize that because visceral preadipocytes differentiate into large and unilocular lipid droplet-containing adipocytes, more 3D extracellular support is required than for sc preadipocytes, which are able to expand and differentiate on rigid polystyrene plates. Their data show that a 3D collagen hydrogel culture system may indeed be superior to a 2D culture system for adipocyte differentiation and culture. Specifically, expression of differentiation and metabolic markers improved substantially in 3D-cultured visceral adipocytes to levels comparable with sc adipocytes. Furthermore, the expression of several markers increased in 3D-cultured sc adipocytes as well. Perhaps more importantly, 3D-cultured adipocytes retained characteristics that more closely resembled their depot of origin. For example, sc adipocytes had higher expression of brown adipocyte-selective markers, and visceral adipocytes had higher expression of inflammatory markers. Finally, assessment of adrenergic-stimulated lipolysis confirmed that these 3Dcultured adipocytes not only maintain morphological characteristics and gene expression profiles of their depot of origin but also important functional characteristics.