Fibrillin-1 Inhibits Early Adipogenic Commitment Via αvβ3 Integrin Signaling

Extracellular fibrillin-1 is present in many tissues, including adipose tissue. Mutations in fibrillin-1 cause Marfan syndrome, paradoxically presenting with either low body fat at younger ages or frequently increased adiposity in older individuals. The latter correlated well with data obtained with the Fbn1^mgR/mgR mice, which show that global fibrillin-1 deficiency leads to sex-dependent obesity by negatively regulating early adipogenic stem cell differentiation. However, the underlying cell receptors and mechanisms remained unknown. Here, we used recombinant fibrillin-1 fragments harboring either an active RGD integrin-binding motif (rFBN1-N-RGD) or an inactive RGA motif (rFBN1-N-RGA) and tested their roles in adipogenic differentiation with primary mesenchymal progenitors isolated from bone marrow or fat tissue, as well as with adipogenic 3T3-L1 cells. rFBN1-N-RGD inhibited adipogenic differentiation in all cell models when present during the early commitment phase but not during the maturation phase, whereas the rFBN1-N-RGA control was inactive. siRNA-mediated gene silencing or pharmacologic inhibition of integrin αvβ3, but not α5β1, rescued rFBN1-N-RGD-dependent inhibition of adipogenic differentiation. Fibrillin-1 engagement of integrin αvβ3 triggered activation of the downstream focal adhesion kinase (Fak)-Src kinase (Src) signaling cascade, driving extracellular signal-regulated kinase (Erk) activation. This cascade suppressed early adipogenic transcriptional programs by restraining Cebpα and Pparγ activation, thereby maintaining progenitor cells in a committed but undifferentiated state. Pharmacological inhibition of Fak, Src and Erk restored the anti-adipogenic effect of rFBN1-N-RGD. These data establish the integrin αvβ3-Fak-Src-Erk axis as an important signaling pathway by which fibrillin-1 imposes an adipogenic checkpoint during the early adipogenic commitment phase, thereby limiting excessive differentiation.