Link between leptin and dickkopf-related protein 2 expression under hypoxia. Human osteoarthritis osteoblasts were incubated for 24 hours under hypoxia (2% O2) in the presence of 50 nM 1,25-dihydroxyvitamin D3 (D3) to optimize leptin and dickkopf-related protein 2 (DKK2) production (n =5 or 6 experiments). The RNA silencing protocol described in the Methods section was followed for siDKK2 and siLept. *Statistically significant vs siScr D3. siScr, Scrambled silencing RNA.