Bezafibrate enhances proliferation and differentiation of osteoblastic MC3T3-E1 cells via AMPK and eNOS activation

Abstract

Aim: To investigate the effects of bezafibrate on the proliferation and differentiation of osteoblastic MC3T3-E1 cells, and to determine the signaling pathway underlying the effects. Methods: MC3T3-E1 cells, a mouse osteoblastic cell line, were used. Cell viability and proliferation were examined using MTT assay and colorimetric BrdU incorporation assay, respectively. NO production was evaluated using the Griess reagent. The mRNA expression of ALP, collagen I, osteocalcin, BMP-2, and Runx-2 was measured using real-time PCR. Western blot analysis was used to detect the expression of AMPK and eNOS proteins. Results: Bezafibrate increased the viability and proliferation of MC3T3-E1 cells in a dose-and time-dependent manner. Bezafibrate (100 μmol/L) significantly enhanced osteoblastic mineralization and expression of the differentiation markers ALP, collagen I and osteocalcin. Bezafibrate (100 μmol/L) increased phosphorylation of AMPK and eNOS, which led to an increase of NO production by 4.08-fold, and upregulating BMP-2 and Runx-2 mRNA expression. These effects could be blocked by AMPK inhibitor compound C (5 μmol/L), or the PPARβ inhibitor GSK0660 (0.5 μmol/L), but not by the PPARα inhibitor MK886 (10 μmol/L). Furthermore, GSK0660, compound C, or N G -nitro-L-arginine methyl ester hydrochloride (L-NAME, 1 mmol/L) could reverse the stimulatory effects of bezafibrate (100 μmol/L) on osteoblast proliferation and differentiation, whereas MK886 only inhibited bezafibrate-induced osteoblast proliferation. Conclusion: Bezafibrate stimulates proliferation and differentiation of MC3T3-E1 cells, mainly via a PPARβ-dependent mechanism. The drug might be beneficial for osteoporosis by promoting bone formation. AMPK is a heterotrimeric enzyme complex consisting of one catalytic α subunit, two regulatory β subunits and a γ subunit. AMPK is recognized as a regulator of energy homeostasis and is known to be expressed ubiquitously, including in bone. Pharmacological AMPK activators, 5-aminoimidazole-4-carboxamide-β-D-ribonucleoside (AICAR) or metformin, could promote the differentiation and mineralization of osteoblastic MC3T3-E1 cells [13,14] . It has recently been shown that AMPK activity regulates bone formation in vitro and the maintenance of bone mass in vivo [15] . These previous findings together suggested that AMPK could affect bone metabolism. AMPK has been shown to increase eNOS activity and contribute to NO production in endothelial cells [16] . NO is also a signaling molecule constitutively produced in bone cells. The source of NO production in bone cells is largely due to eNOS, which was constitutively expressed in bone. The eNOS isoform seems to play a key role in regulating osteoblast activity and bone formation since eNOS knockout mice have osteoporosis due to defective bone formation [17,18] . Because these data have been obtained from non-osteoblastic cells, it is still unclear whether bezafibrate could activate AMPK and eNOS in osteoblasts. A model commonly used to study osteogenic development is the MC3T3-E1 o...

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