Uncarboxylated osteocalcin reverses the high glucose‑induced inhibition of the osteogenic differentiation of MC3T3E1 cells via the GPRC6A/cAMP/PKA/AMPK signaling pathway.

Diabetic osteoporosis is a serious complication of diabetes affecting human bones. Uncarboxylated osteocalcin (GluOC), a small molecular protein specifically synthesized and secreted from osteoblasts, is of importance in regulating energy metabolism. In previous studies, the authors demonstrated that high glucose inhibited osteoblastic differentiation, but promoted adipocytic differentiation. GluOC promoted osteogenic and inhibited adipogenic differentiation under high glucose conditions. However, the corresponding receptors and signaling pathways through which GluOC exerts its effects on MC3T3E1 cells remain elusive. Thus, in the present study, Cell Counting kit‑8 assays and western blot analysis were performed to assess the proliferation of MC3T3E1 cells. Alizarin Red S or Oil Red O staining, as well as reverse transcription‑quantitative PCR analysis were performed to examine osteogenic and adipogenic differentiation. The cells were transfected with short interfering RNA or inhibitors to investigate the possible signaling pathways involved. The results revealed that G‑protein coupled receptor, class C, group 6, subtype A (GPRC6A) receptor expression was markedly increased following the addition of GluOC to the MC3T3E1 cells. GPRC6A silencing decreased osteogenic gene expression, while it increased adipogenic gene expression. Furthermore, GluOC promoted osteoblast differentiation via the subsequent activation of the cyclic AMP (cAMP)/protein kinase A(PKA)/AMP‑activated protein kinase (AMPK) signaling pathway in MC3T3E1 cells. On the whole, the results of the present study suggest that GluOC reverses the high glucose‑induced inhibition of osteogenic differentiation via the GPRC6A/cAMP/PKA/AMPK signaling pathway in MC3T3E1 cells, and thus may prove to be beneficial in the treatment of diabetic osteoporosis.