Adhesion, proliferation, and adipogenesis in primary rat cell cultures: effects of collagenous substrata, fibronectin, and serum

Serum regulates adipogenesis of mesenchymal stem cells via MEK/ERK-dependent PPARgamma expression and phosphorylation

Mesenchymal stem cells (MSCs) provide us an excellent cellular model to uncover the molecular mechanisms underlying adipogenic differentiation of adult stem cells. PPARγ had been considered as an important molecular marker of cells undergoing adipogenic differentiation. Here, we demonstrated that expression and phosphorylation of PPARγ could be found in bone marrow–derived MSCs cultured in expansion medium without any adipogenic additives (dexamethasone, IBMX, insulin or indomethacin). Then, PPARγ was dephosphorylated in MSCs during the process of adipogenic differentiation. We then found that inhibition of MEK activation by specific inhibitor (PD98059) counteracted the PPARγ expression and phosphorylation. However, expression and phosphorylation of PPARγ did not present in MSCs cultured in medium with lower serum concentration. When these MSCs differentiated into adipocytes, no phosphorylation could be detected to accompany the expression of PPARγ. Moreover, exposure of MSCs to higher concentration of serum induced stronger PPARγ expression, and subsequently enhanced their adipogenesis. These data suggested that activation of the MEK/ERK signalling pathway by high serum concentration promoted PPARγ expression and phosphorylation, and subsequently enhanced adipogenic differentiation of MSCs.

Understanding adipocyte differentiation

The adipocyte plays a critical role in energy balance. Adipose tissue growth involves an increase in adipocyte size and the formation of new adipocytes from precursor cells. For the last 20 years, the cellular and molecular mechanisms of adipocyte differentiation have been extensively studied using preadipocyte culture systems. Committed preadipocytes undergo growth arrest and subsequent terminal differentiation into adipocytes. This is accompanied by a dramatic increase in expression of adipocyte genes including adipocyte fatty acid binding protein and lipid-metabolizing enzymes. Characterization of regulatory regions of adipose-specific genes has led to the identification of the transcription factors peroxisome proliferator-activated receptor-gamma (PPAR-gamma) and CCAAT/enhancer binding protein (C/EBP), which play a key role in the complex transcriptional cascade during adipocyte differentiation. Growth and differentiation of preadipocytes is controlled by communication between individual cells or between cells and the extracellular environment. Various hormones and growth factors that affect adipocyte differentiation in a positive or negative manner have been identified. In addition, components involved in cell-cell or cell-matrix interactions such as preadipocyte factor-1 and extracellular matrix proteins are also pivotal in regulating the differentiation process. Identification of these molecules has yielded clues to the biochemical pathways that ultimately result in transcriptional activation via PPAR-gamma and C/EBP. Studies on the regulation of the these transcription factors and the mode of action of various agents that influence adipocyte differentiation will reveal the physiological and pathophysiological mechanisms underlying adipose tissue development.

Regulation of matrix metalloproteinases following cellular transformation

During progression towards malignancy, many tumor cells display changes in their repertoire of matrix metalloproteinases (MMPs) and tissue inhibitors of matrix metalloproteinases (TIMPs). The recent finding that many members of the MMPs are regulated by protooncogenes may explain the frequent observation of changes in MMP gene expression during progression of many tumor types. While studies involving enzymatic assays of MMPs are usually confined to one or a few MMPs, reverse transcription-PCR (RT-PCR) permitted the analysis of seven members of the MMP family and two members of the TIMP family in several normal and transformed cell lines. RT-PCR permitted us to confirm the observation that MMP-9 is activated following transformation and also to observe the previously unreported activation of MMP-7 in SV40-transformed cells. It has previously been found that MMP-1, -2, -3, -8, and -9 are upregulated by phorbol esters; we have found that MMP-10 is also upregulated by phorbol esters. The phorbol ester upregulation of MMP-1, -3, and -10 was found to be abolished in cells transformed by SV40 virus. Several studies have shown that MMP-1 is upregulated by an integrin-mediated signal transduction pathway. This study demonstrates that MMP-3 and MMP-10 are also regulated by integrin-mediated signal transduction and that upregulation by this pathway is abolished following SV40 transformation. In summary, the more global view of MMP expression afforded by RT-PCR indicates that MMP-1, -3, and -10 are regulated by both integrin-mediated signal transduction and phorbol esters. While fibroblasts and transformed bone cells express several members of the MMP gene family, several other cell types do not express MMPs.

Extracellular matrix components secreted by microvascular endothelial cells stimulate preadipocyte differentiation in vitro

Paracrine interaction between preadipocytes and microvascular endothelial cells may play a role in the regulation of adipose tissue growth. We report here a study of the effect of extracellular matrix factors secreted by microvascular endothelial cells, derived from adipose tissue, on preadipocyte differentiation in primary culture. Extracellular matrix components (EC) were prepared by differential centrifugation of medium conditioned by microvascular endothelial cells (CM). Preadipocyte differentiation was assessed by enumerating cells containing Oil-Red-O-stainable neutral lipids and by assaying cellular triacylglycerol (TG) content and glycerol-3-phosphate dehydrogenase (GPDH) specific activity. Both supernatant (containing soluble components) and pelleted (containing large complexes of EC) fractions of CM stimulated preadipocyte differentiation. When the supernatant fraction was used, the proportion of cells containing visible lipid droplets was 29% +/- 3% of total preadipocytes in the presence of extracellular complexes, as compared with 6% +/- 1% under control conditions. This differentiation induction was associated with fourfold increases in TG content and GPDH specific activity. Neither the supernatant nor the pelleted fraction of EC affected the maximal differentiation induced by hormonal stimulation in serum-supplemented or serum-free media. The major EC, fibronectin, laminin, and collagen IV, had no effect on differentiation when added individually to culture medium. Collection of CM under hyperglycemic (18 mmol/L glucose) compared with control (6 mmol/L glucose) conditions reduced the stimulatory effect of extracellular complexes by twofold, suggesting decreased or altered production by endothelial cells. The present findings demonstrate that microvascular endothelial cells release EC that promote preadipocyte differentiation.(ABSTRACT TRUNCATED AT 250 WORDS)