Endocrine Abstracts

Insulin is a major endocrine hormone able to regulate whole-body energy disposal and lipid metabolism. Particularly, insulin stimulates the renewal of the adipose tissue (AT) via activation of insulin receptors (IR) consisting of two splicing variants: a short isoform (IRa) involved in cell proliferation, and a long isoform (IRb) mainly implicated in metabolic responses. The contribution of these isoforms to the AT expansion is still poorly understood. We investigated the expression of both isoforms in human adipose-derived stem cells (ASC), in vitro differentiated adipocytes (dASC), and whole AT obtained from subjects with different BMI. Gene expression analysis was performed in subcutaneous AT (S-AT), visceral AT (V-AT), and ASC before and after differentiation into adipocytes (dASC) by qPCR. Adipogenesis was evaluated according to protein levels of IR and Nile red (NR) staining by flow cytometry. mRNA levels of IRa were more expressed than IRb in both S-ASC and V-ASC. S-ASC showed higher mRNA levels of total IR, IRa and IRb than V-ASC, as well as greater activation of insulin signaling. IRa expression significantly correlated with BMI (r = 0.53) and waist circumference (r = 0.51) in S-ASC and with BMI (r = -0.36) in V-ASC. Increased mRNA levels of total IR, IRa and IRb was observed after adipocyte differentiation in both S-dASC and V-dASC, with higher expression levels in S-dASC than V-dASC. Accordingly, IRa was more expressed than IRb in both S-AT and V-AT. Flow cytometric analysis revealed a time-dependent and tissue-specific increase of differentiated adipocytes stained with NR (NR+: 60% S-dASC, 23% V-dASC) but only a fraction of NR+ cells expressed IR, particularly in S-dASC (day 16, NR+IR+: 64% of NR+; day 30, NR+IR+: 77% of NR+). Conversely, V-dASC showed a high proportion of IR+ cells in the early phases of adipogenesis (day 16, NR+IR+: 82% of NR+), while a consistent reduction of the percentage of triglyceride-filled cells expressing IR was observed in the late phases (day 30, NR+IR+: 61% of NR+). In conclusion, mRNA levels of IR, particularly IRa, increase with BMI and adipocyte maturation especially in S-ASC. S-ASC display greater activation of insulin signaling and adipogenic phenotype as compared to V-ASC. A greater percentage of V-dASC expressing IR was observed during early adipogenesis but not in the late phase. IRa expression levels are associated with AT expansion typically in S-AT. The role of IR in V-AT expansion, particularly in the early phase of adipocyte commitment, needs further elucidation.