Short communication: Milk basic protein promotes proliferation and inhibits differentiation of mouse chondrogenic...

It has been reported that the intake of milk basic protein (MBP) increases bone density by promoting bone formation and suppressing bone resorption. However, few studies have been done on MBP in cartilage, the tissue adjacent to bone. We therefore investigated the effect of MBP on a chondrocyte cell line, ATDC5. In a proliferative assay using the WST-1 method, the addition of 10, 100, and 1,000 µg/mL of MBP to ATDC5 cells significantly increased the cell number by about 1.2-, 1.5-, and 1.7-fold, respectively, compared with the control cells. The cell cycle analysis using flow-cytometry revealed that the proportion of S- and G₂/M-phase cells was increased but that of G₀/G₁ phase was decreased in a dose-dependent manner with MBP addition. We measured the alkaline phosphatase activity of MBP-treated ATDC5 cells to examine the differentiation stage of the cells. Alkaline phosphatase activity was suppressed in a dose-dependent manner with MBP addition and was especially drastic at higher doses of MBP (100 and 1,000 µg/mL). The Alizarin Red S staining intensity, the indicator for calcification of cells, was lower in the MBP-treated (100 µg/mL) cells than in nontreated control cells. In the reverse-transcription PCR experiment, the mRNA level of SRY-box containing gene 9 (Sox9) and type II collagen (Col2) was significantly increased in the MBP-treated cells compared with the control cells. A significant decrease of the mRNA level of runt-related transcription factor 2 (Runx2) and type X collagen (Col10) was also observed in the MBP-treated cells. These results suggested that MBP promoted the proliferation of chondrocytes by suppressing their differentiation toward calcification.