Extracellular matrix induced by TGFbeta impairs insulin signal transduction in 3T3-L1 preadipose cells

When 3T3-L1 preadipose cells are exposed to transforming growth factor beta (TGFbeta), they synthesize more extracellular matrix (ECM) and resist differentiation-inducing stimuli. The mechanism by which ECM suppresses adipose cell differentiation (adipogenesis) remains unknown. Since adipogenesis is an insulin/insulin-like growth factor-1 (IGF-1)-dependent process, we investigated whether TGFbeta-induced ECM inhibits insulin signaling. When preadipose cells were pretreated overnight with TGFbeta, we observed a 75% decrease in insulin-stimulated tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) compared to that in control cells. Culturing 3T3-L1 preadipose cells on fibronectin, a component of the ECM induced by TGFbeta, also inhibited insulin-dependent IRS-1 tyrosine phosphorylation and adipogenesis, supporting a role for ECM in mediating TGFbeta's inhibitory effect on insulin signaling. Since the insulin-stimulated association of phosphoinositide (PI) 3-kinase with IRS-1 depends on IRS-1 tyrosine phosphorylation, we measured the presence of the PI 3-kinase 85 kDa regulatory subunit in anti-IRS-1 immunoprecipitates. Following insulin stimulation, PI 3-kinase-IRS-1 association was reduced by 70% in TGFbeta pretreated vs. control preadipose cells. However, insulin-stimulated cellular production of PI(3,4,5)P3 was unaltered by TGFbeta pretreatment. This suggests that IRS-1-associated p85-type PI 3-kinase may represent a particular subset of total cellular PI 3-kinase that is specifically inhibited by TGFbeta. Reduction of insulin-stimulated association of IRS-1 with p85-type PI 3-kinase by TGFbeta may be one potential mechanism through which TGFbeta blocks 3T3-L1 adipose cell differentiation.

Evidence of adipocyte differentiation in human orbital fibroblasts in primary culture

Graves' ophthalmopathy (GO), in which orbital tissues are infiltrated with activated T lymphocytes and hyaluronan, can manifest an overabundance of adipose tissue in the human orbit. Little is known about adipogenesis in this anatomic region. We have investigated whether orbital fibroblasts in culture possess the capacity to undergo adipocytic differentiation. Orbital tissue from patients with or without GO was placed in primary culture, and proliferating fibroblasts were sub-passaged. Confluent fibroblasts were subjected to a differentiation protocol, involving a serum-free defined medium supplemented with insulin, triiodothyronine, carbaprostacyclin, thyrotropin, dexamethasone, and isobutylmethylxanthine. Control cells were maintained in serum-free medium supplemented only with insulin. After approximately 14 days, light microscopy revealed characteristic morphologic changes of adipocyte differentiation, including cell rounding and lipid droplet accumulation. Oil Red O staining could be demonstrated in those cells. Scanning electron microscopy revealed that the cells undergoing adipogenesis contain multiple, discrete lipid droplets. The overall percentage of fibroblasts undergoing differentiation was somewhat variable, but no more than 5-10%. The adipocytes arise in close proximity to each other, and often at the periphery of the culture surface. In contrast, dermal fibroblasts and perimysial fibroblasts from extraocular muscle fail to differentiate. We conclude that a subpopulation of orbital fibroblasts is capable of adipocyte differentiation. These in vitro observations may represent the in vivo process that contributes to excess orbital adipose tissue volume in GO.

Expression of a constitutively activated form of protein kinase B (c-Akt) in 3T3-L1 preadipose cells causes spontaneous differentiation

Insulin/IGF-1 is required for differentiation of 3T3-L1 adipose cells. Downstream targets of insulin/IGF-1 that lead to adipocyte differentiation appear to include Ras, phosphatidylinositol (PI) 3-kinase, Raf, and mitogen-activated protein kinase. We have tested whether protein kinase B (PKB), a serine/threonine kinase activated by PI 3-kinase, is sufficient for 3T3-L1 preadipose cell differentiation. A plasmid vector encoding a version of PKB that is constitutively activated (Gag-PKB) was expressed in 3T3-L1 preadipose cells (Gag-PKB cells). Spontaneous morphological changes indicative of adipocyte differentiation were observed in Gag-PKB cells. The cells assumed a spherical shape and they acquired characteristic lipid droplets that stained positively for Oil Red O. Northern blot analysis detected upregulation of LPL and aP2 mRNA, specific indicators of adipocyte differentiation. Our data demonstrate that constitutive activation of PKB is sufficient to trigger adipocyte differentiation.

The 3-phosphorylated phosphoinositide response of 3T3-L1 preadipose cells exposed to insulin, insulin-like growth factor-1, or platelet-derived growth factor

We compared the pattern of 3-phosphorylated phosphoinositides produced by confluent 3T3-L1 preadipose cells upon exposure to growth factors that either induce differentiation (insulin, insulin-like growth factor-1) or do not (platelet-derived growth factor). Following addition of insulin or insulin-like growth factor-1, PI(3,4,5)P3 rapidly rose, on average, to levels tenfold over basal. PI(3,4)P2 either did not change (after insulin) or slightly increased (1.5 fold). Time course studies with insulin demonstrated that the rise in PI(3,4,5)P3 peaked by 1 minute, and levels then remained steady over 30 minutes. Dose-response experiments showed that insulin at a concentration of 1 nM was sufficient for the PI(3,4,5)P3 response. Insulin failed to increase PI(3,4)P2 at any of the time points or at any of the doses used. In contrast, after addition of platelet-derived growth factor, both PI(3,4)P2 and PI(3,4,5)P3 rose concurrently and to comparable extents. These data suggest that one possible mechanism contributing to insulin/insulin-like growth factor-1-induced 3T3-L1 preadipose cell differentiation is a distinct pattern of 3-phosphorylated phosphoinositide accumulation, defined by a prominent increase in PI(3,4,5)P3 with no (in the case of insulin), or a minimal (in the case of IGF-1), rise in PI(3,4)P2.