| Silent information regulation2homolog1(Sirt1) is a nicotinamide adenine dinucleotide(NAD~+)-dependent protein deacetylases, which can regulate cell differentiation, energymetabolism and senescence in organisams. NAD~+is one of the substrates for Sirt1mediateddeacetylation, which is consumed by Sirt1and cleaved into nicotinamide (NAM) andOacetyl-ADP-ribose. NAM, in return, functions as a potent endogenous inhibitor, which canbe catalyzed by nicotinamide phosphoribosyltransferase (Nampt) and form into NAD~+. As therate-limited enzyme in NAD~+biosynthesis, Nampt is the key point between the NAD~+biosynthesis and Sirt1activity in mammalian, which has been the focus in the fields of cellenergy metabolism and aging.The adult bone is continuously maintained by a dynamic balance-bone formation byosteoblasts and bone resorption by osteoclasts. The balance will be disrupted by decreasedosteoblastic activities and increased osteoclastic activities in aging. The imbalance finallyresults in senile osteoporosis. With the acceleration of aging in world population, osteoporosisas one of senile degenerative diseases has been increasingly emphasized. Senile osteoporosisis characterized by a progressive decline in bone mass and an accumulation of marrow fat.The marrow adipocytes and osteoblasts share common progenitors, mesenchymal stem cells(MSCs), which have ability of multi-lineage differentiation. Therefore, we hypothesize thatloss of bone mass, namely senile osteoporosis is likely due to a disruption of the energybalance in aging which can result in the imbalance of osteogenic and adipogenicdifferentiation at the stem cells level. Furthermore, Nampt plays a vital role in the regulationon MSCs differentiation.In the present study, the murine mesenchymal stem cell line C3H10T1/2and preosteoblastcell line MC3T3-E1were used. To study the regulation of Nampt on mesenchymal stem cells differentiation, FK866-specific inhibitor of Nampt and Nampt shRNA Lentiviral transductionparticles were utilized in osteoblast differentiation, and then Sirt1expression and activity andintracellular NAD concentration and NAM were detected. In addition, the effects of FK866and aging on osteoblast differentiation of mouse bone marrow stromal cells from C57BL/6mice were investigated. The results are as follows:Nampt specific inhibitor, FK866, in osteoblast differentiation reduced mineralization whileincreased adipocytes formation in C3H101/2. Consistent with the effect of FK866, adipocyteformation was significantly enhanced after knock-down of Nampt in C3H10T1/2cells.Real-time PCR analysis showed that the expression of the adipocyte specific transcriptionfactor PPARγ was significantly higher in Nampt deficient cells while the osteoblast keytranscription factor Runx2, as well as the osteoblast marker genes, osteocalcin and OPG, weresignificantly downregulated. Inhibition of Nampt activity, by its specific inhibitor FK866orgene silencing, also attenuated osteoblast differentiation and matrix mineralization inMC3T3-E1. These effects including decreased osteogenesis or increased adipogenesiscorrelated with a relatively lower Sirt1activity and a lower intracellular NAD~+concentration.Meanwhile, in mouse bone marrow stromal cells, treatment with Nampt inhibitor, FK866,increased adipocytes formation while reduced mineralization. What’s more, the bone marrowstromal cells derived from15-month-old mice developed fewer bone nodules and moreadipocytes than cells derived from4-week-old mice, which was associated with a relativelylower intracellular NAD~+concentration and a lower Sirt1activity.In summary, Nampt has regulatory effects on MSCs differentiation. Senile osteoporosiscould be due to a disruption of the energy balance in aging which can lead to the imbalance ofosteogenic and adipogenic differentiation of mesenchymal stem cells. It will provide not onlythe innovative ideas for the regulatory mechanisms on stem cell differentiation, but also theimportant theoretic basis for treatment of senile osteoporosis. |
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