Regulation of mutlipotential mesenchymal stomal cell differentiation

As aging occurs, red hematopoietic bone marrow (BM) converts to yellow adipocytic marrow. Potential consequences are anemias, aplasias, osteoporosis and other hematopoietic deficiencies. Alcohol consumption exacerbates these hematopoietic disorders likely, by hastening the conversion of red hematopoietic marrow to yellow adipocytic marrow. Recent studies conducted by our group using human subjects and cadavers demonstrated that increased BM fat, during aging, is associated with a decline in HSC numbers. Mutlipotential mesenchymal stromal cells (MSC) and their differentiated progeny, including osteoblasts and adipocytes, are responsible in regulating the HSC; osteoblasts are thought to be favorable whereas adipocytes are thought to provide unfavorable microenvironment to HSC. Although there is evidence for MSC differentiation, the mechanisms underlying these differentiation and the regulatory events are not completely understood. In the present investigation, regulation of MSC differentiation, mainly into osteoblasts versus adipocytes has been elucidated. OMA-AD cells were assessed based on their morphology, doubling time, surface phenotype, differentiation potential, radioprotection, immunosuppression and consequently established as a bonafide MSC cell line. Therefore, OMA-AD cells represented primary MSC in all the further experiments. When exposed to various concentrations of ethanol, MSC differentiated into adipocytes. Conversely, leptin, a hormone secreted by adipocytes, induced rapid and faster differentiation of MSC into osteoblasts. There is a hint that MSC may be a common precursor to osteoblast and adipocytes and differentiation into one lineage occurring at the expense of the other. To evaluate the possibility of 'trans' or 'competitive' differentiation between osteoblasts and adipocytes, MSC were exposed to ethanol and subsequently to leptin. Cultures exhibited presence of adipocytes as well as osteoblasts, posing a possibility of transdifferentiation. Furthermore, signaling pathways involved in MSC differentiation were screened by using inhibitors to the respective pathways. The Wnt pathway was involved in osteoblast differentiaion whereas Notch, Sonic Hedgehog and PPARγ were involved in adipocytic differentiation of MSC. In conclusion, these studies provide important information regarding regulation of MSC differentiation which may prove instrumental in devising strategies to impove osteopenic and hematopoietic disorders, by manipulating MSC differentiation pathways to favor osteoblastogenesis versus adipogenesis.