Abstract
Despite extensive investigation, the mechanisms underlying adipogenesis are not fully understood. We previously identified proliferative cells in adipose tissue expressing adipocyte-specific genes, which were named small proliferative adipocytes (SPA). In this study, we investigated the characteristics and roles of SPA in adipose tissue. Epididymal and inguinal fat was digested by collagenase, and then SPA were separated by centrifugation from stromal vascular cells (SVC) and mature white adipocytes. To clarify the feature of gene expression in SPA, microarray and real-time PCR were performed. The expression of adipocyte-specific genes and several neuronal genes was increased in the order of SVC < SPA < mature white adipocytes. In addition, proliferin was detected only in SPA. SPA differentiated more effectively into lipid-laden cells than SVC. Moreover, differentiated SPA expressed uncoupling protein 1 and mitochondria-related genes more than differentiated SVC. Treatment of SPA with pioglitazone and CL316243, a specific β3-adrenergic receptor agonist, differentiated SPA into beige-like cells. Therefore, SPA are able to differentiate into beige cells. SPA isolated from epididymal fat (epididymal SPA), but not SPA from inguinal fat (inguinal SPA), expressed a marker of visceral adipocyte precursor, WT1. However, no significant differences were detected in the expression levels of adipocyte-specific genes or neuronal genes between epididymal and inguinal SPA. The ability to differentiate into lipid-laden cells in epididymal SPA was a little superior to that in inguinal SPA, whereas the ability to differentiate into beige-like cells was greater in inguinal SPA than epididymal SPA. In conclusion, SPA may be progenitors of beige cells.
Highlights
Numerous investigations have contributed to the elucidation of adipocyte differentiation
Fraction D was obtained by centrifugation of dispersed cells at 226 g for 3 min, which was regarded as a total non-adipocyte cell fraction for flow cytometry
The results showed that PLF was expressed in D-small proliferative adipocytes (SPA), but not in mature white adipocytes as shown in Fig. 3B, clearly indicating that differentiated SPA (D-SPA) differ from mature white adipocytes
Summary
Numerous investigations have contributed to the elucidation of adipocyte differentiation. It has been proposed that adipogenesis, which comprises the whole process of adipocyte formation, consists of at least two steps, determination of adipose progenitor cells (APC) as the initiation of differentiation and emergence of adiposespecific genes as the terminal differentiation. Studies using fluorescence-activated cell sorting and lineage tracing have characterized APC in white adipose tissue. A recent study revealed functional heterogeneity of stromal vascular cells (SVC).
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