Regulation of DNA synthesis in fat cells and stromal elements from rat adipose tissue

Abstract

A B S T R A C T The incorporation of tritiated thymidine into the deoxyribonucleic acid (DNA) of adipose fat and stromal cells was followed under a variety of conditions. After in vitro incubation of adipose slices or up to 2 days after in vivo injection of the isotope, all DNA radioactivity was in the stromal cell fraction. From 2 to 15 days after thymidine injection total tissue DNA radioactivity was constant, while between 2 and 5 days after injection label in fat cell DNA increased markedly. Thus new labeled fat cells, initially collected in the stromal pool, required 2-5 days after completion of DNA synthesis to accumulate sufficient lipid to be harvested in the fat cell fraction. Fasting before thymidine injection practically abolished DNA synthesis in primordial fat cells and reduced less drastically formation of stromal elements. However fasting sufficient to deplete lipid stores by 50% neither destroyed mature fat cells nor impaired their capacity to reaccumulate fat with refeeding. Other studies evaluated the role of new fat cell formation in the process of lipid accretion accompanying refeeding. These experiments indicated that at least during the early phase of rapid weight gain, accumulation of fat was due to deposition of triglyceride in existing cells rather than to accelerated formation of new fat cells. Studies with hypophysectomized rats demonstrated that pituitary ablation variably affected stromal DNA synthesis and nearly abolished the formation and (or) maturation of primordial fat cells. In these animals growth hormone markedly enhanced thymidine incorporation into stromal DNA but had no effect on fat cell preReceived for publication 10 April 1968 and in revised form 7 June 1968. cursors. In intact animals the predominant effect of growth hormone was also on the stromal fraction, although an action of the hormone of lesser magnitude on fat cell precursors was also evident. INTRODUCTIONWhile information concerning the accretion and mobilization of adipose lipid has accumulated rapidly over the past decade, there is still surprisingly little known about the regulation of fat cell formation, another mechanism that can influence adipose mass. Hindering work in this area is the difficulty in the histological recognition of primitive fat cells and the fact that only a part of total adipose tissue deoxyribonucleic acid (DNA) is contained in fat cells (1). The information that is available has been derived from the use of radioautography, chemical DNA analysis, and counting of adipose cells. These data indicate that the new fat cells are continually formed in the growing animal (2), that expansion of adipose mass by fat feeding is associated with an increase in total adipose tissue DNA (3), and that in human and in some experimental forms of obesity an increase in adipose cell number accompanies an increase in adipose cell size (4, 5). Although these data suggest an influence of diet on fat cell synthesis, the precise nutritional and hormonal factors that control the formation and maturation of ...

Experimental obesity in man: cellular character of the adipose tissue

“…The apparent reduction in cell number was found only in those individuals with extremely small cells. Similarly, it is possible that precursors of adipose cells containing little lipid may exist in the adipose depot and thus not be measured by the present techniques (15). If forced feeding induced the formation of substantial numbers of these precursor cells, an increase in cell number would go undetected.…”

Adipose tissue stem cells meet preadipocyte commitment: going back to the future

“…Subsequent studies confi rmed the existence of a mitotically active cell population in WAT ( 7 ), and that WAT expansion is associated with increases in total DNA content and adipocyte number ( 26 ). Following the development of methods for separating WAT into adipocyte and stromal vascular fractions (SVFs) ( 27 ), the source of these new adipocytes was determined by Hollenberg and Vost ( 28 ): using tritiated thymidine to monitor DNA synthesis in WAT, they detected temporal progression of radioactivity from the SVF to mature adipocytes, thereby inferring the presence of an adipocyte precursor cell in the SVF ( 28 ). Shortly thereafter, studies reported growth of fi broblast-like cells from the SVF that are capable of adipogenesis (29)(30)(31).…”

The developmental origins of adipose tissue

“…Tissue dissociation studies have implicated the adipose stromal vascular fraction (SVF) as a possible site of origin for adipose stem cells. The SVF is a heterogeneous mixture of cells that can be isolated from adipose tissue after dissociation and centrifugation, in which adipocytes float and the remaining cells pellet (Hollenberg and Vost, 1968). The SVF is composed of a myriad of cells, which includes fibroblasts, endothelial cells, hematopoietic cells and neural cells.…”

“…In the 1960s, it was shown in rodents that cells within adipose depots proliferate, that adipocytes are renewed and that adipocytes have a finite lifespan (Hellman and Hellerstrom, 1961;Hollenberg and Vost, 1968). Indeed, caloric excess stimulates the adipose stem cell compartment to proliferate and differentiate.…”