Ectopic Overexpression of C/EBP-alpha in Primary Human Mesenchymal Stem Cells Is Sufficient To Induce Preadipocyte Determination but Not Terminal Differentiation without Hormonal Enhancement.

Background: Bone marrow Mesenchymal Stem Cells (MSC) have been identified as multipotent non-hematopoietic progenitor cells that can differentiate into osteoblasts, myocytes, chondrocytes and adipocytes. MSC also play an important role for the normal hematopoiesis and bone marrow insufficiency goes along with an increase of fat involution, which might origin from MSC. One important factor that regulates later stages of adipogenesis is the CCAAT/enhancer binding protein alpha (C/EBP-alpha). Whereas most insights into the regulation of adipogenesis come from the analysis of murine cell lines or human preadipoyctes under the influence of adipogenic hormonal growth medium, data on primary human MSC under the influence of a single adipogenic transcription factor are lacking - especially without hormonal enhancement. In this study, we examined, if ectopic expression of C/EBP-alpha is able to induce adipogenic differentiation of primary human MSC.

Methods / Results: Transient overexpression of C/EBP-alpha was performed in primary human MSC from bone marrow aspirates after two culture passages. MSC were cultured in DMEM medium (+10% FCS) without hormonal substitution. Total RNA was prepared from different time points (12h, 24h, 48h, 72h) and the expression of C/EBP-alpha as well as adipose genes (PPAR-gamma, FABP-4) was analyzed by semiquantitative RT-PCR. MSC transfected with the empty control vector had no detectable amounts of C/EBP-alpha, PPAR-gamma or FABP-4. In contrast, MSC under ectopic expression of C/EBP-alpha expressed FABP-4 after 48 hours with further increase over the time, whereas PPAR-gamma expression could be detected first after 72 hours. These MSC showed only slight morphologic changes without distinct intracellular lipid drops. Red Oil-O staining for fat cells was negative in all groups. Furthermore, permanent lentiviral overexpression of C/EBP-alpha for 7 to 14 days showed no significant morphological changes from MSC to adipocytes, when grown in normal culture medium. Adipogenic growth medium (Insulin 10ug/ml, Dexamethason 10uM, Indomethacin 200uM, Isobutylxanthine 500uM) was added after 3 days of C/EBP-alpha overexpression and led to terminal differentiated adipocytes after 3 further days, whereas in the control group the amount of differentiated adipocytes increased only after 7 days. The expression of C/EBP-alpha, PPAR-gamma and FABP-4 was determined again by semiquantitative RT-PCR and showed an earlier and higher upregulation of all factors in the C/EBP-alpha group compared to the control. Red Oil-O staining showed a significant higher amount of terminal differentiated adipocytes in the C/EBP-alpha group.

Conclusion: This work shows that ectopic expression of C/EBP-alpha in primary human Mesenchymal Stem Cells is able to determine MSC only to preadipocytes, which do not undergo morphologic terminal differentiation until other factors like PPAR-gamma are upregulated. Moreover, ectopic expression of C/EBP-alpha in primary human MSC accelerates the process of terminal adipogenic differentiation dramatically as soon as other factors are induced by hormonal enhancement.