Can TGF-β1 and rhBMP-2 act in synergy to transform bone marrow stem cells to discogenic-type cells?

Introduction  The recombinant human bone morphogenic protein-2 (rhBMP-2) is known to increase the proteoglycan production and chondrogenic gene expression in the disc cells. The transforming growth factor-beta 1 (TGF-β₁) can transform the bone marrow stem cells (BMDCs) into the disc-like cells. Materials and methods  We carried out an experiment to determine if TGF-β₁ and rhBMP-2 can act in synergy on BMDCs by increasing the production of sulfated-glycosaminoglycan (sGAG) and affecting the mRNA expression of aggrecan, type I collagen, and type II collagen. The BMDCs were isolated from the iliac crest and femur of a New Zealand white rabbit (1 year). The BMDCs were culured in monolayer and treated for 6 days with TGF-β₁ 10 ng/ml (group 1), rhBMP-2 200 ng/ml (group 2), and both TGF-β₁ 10 ng/ml and rhBMP-2 200 ng/ml (group 3: the combined group) in Dulbecco’s modified Eagle medium/F-12 with 1% fetal bovine serum. After 6 days, the sGAG content in the media was quantified using 1,9-dimethylmethylene blue staining and the mRNA expression of aggrecan, type I collagen, type II collagen, Sox-9, BMP-2, and BMP-7 were measured with the real-time PCR. The same BMDCs were also cultured in the chamber slide at 3 × 10⁴ cells/chamber. After 6 days treatment, the treated cells were immunofluorescence stained with aggrecan, type I collagen, type II collagen, anti-BMP-2, anti-BMP-7 antibodies. After that, we compared the number of positive immunofluorescence stained cells with fluorescence microscope. The sGAG production and mRNA expression for each group were normalized against the same parameters for a non-treatment group. Results and discussion  The sGAG production was increased 1.15*, 1.34*, and 1.45* times in the TGF-β₁ 10 ng/ml group, the rhBMP-2 200 ng/ml group, and the combined group respectively. The mRNA expression of aggrecan was increased 1.28, 3.42*, and 5.34* times, the mRNA expression of type I collagen was increased 0.86, 1.09, 1.17 times, the mRNA expression of type II collagen was increased 3.58*, 3.77*, and 10.78* times, the mRNA expression of Sox-9 was increased 1.29, 2.45, 2.75* times, the mRNA expression of BMP-2 was increased 1.14, 2.07, 4.43* times, and the mRNA expression of BMP-7 was increased 1.16, 1.49, 1.97* times, respectively for each group (* indicates p < 0.05). On the immunofluorescence staining of antibodies, the average positively immunofluorescence stained cells number for aggrecan were 4.2, 15.8*, 10*, and 22* according to the non-treatment group, TGF-β₁ 10 ng/ml group, rhBMP-2 200 ng/ml group, and the combined group respectively. The average positively immunofluorescence stained cells number for type I collagen were 7, 14.2*, 9.2*, 17.4* and the average positively immunofluorescence stained cells number for type II collagen were 8.5, 28.25*, 20.25*, 42.25* and the average positively immunofluorescence stained cells number for anti-BMP-2 were 5, 16.75*, 8.75*, 27.25* and the average positively immunofluorescence stained cells number for anti-BMP-7 were 3.25, 7.5*, 8.75*, 15.25* (* indicates p < 0.05). Conclusions  Both TGF-β₁ and rhBMP-2 alone, can increase proteoglycan production in the BMDCs. However, if they were used in combination, there is a synergistic effect. Similarly, the mRNA expressions of both aggrecan, type II collagen, Sox-9, BMP-2, and BMP-7 except for type I collagen were increased significantly when TGF-β₁ and rhBMP-2 were combined. The positive immunofluorescence stained cell numbers for aggrecan, type I, II collagen, BMP-2 and BMP-7 were also increased after each TGF-β₁ and rhBMP-2 treatment, and also more increased significantly in the aggrecan, type I, II collagen, BMP-2, and 7 when they were used jointly.