Optimization of the culture medium for recombinant protein production under the control of the αAmy3 promoter in a rice suspension-cultured cell expression system

Many recombinant proteins have been produced successfully in a rice suspension-cultured expression system based on the αAmy3 promoter and signal peptide. Here, a transgenic rice cell line, H11, which can produce recombinant human serum albumin (rHSA) under conditions of sugar starvation, was cultured in various common rice-cell media, including MS, AA, and N6D, to determine the optimal medium for the production of rHSA in rice suspension-cultured cells. The yields of rHSA secreted into the N6D medium were 23.2 mg/L at an initial cell density of 3% and 49.6 mg/L at an initial cell density of 24%, which was 1.8-fold and twofold higher, respectively, than the yields obtained in MS. Moreover, the yield of rHSA was also twofold higher in N6D than in AA medium. Thus, rice suspension cells cultured in N6D medium had the highest rHSA productivity compared with MS and AA media. Proline is known to play roles in plant tolerance to stress. Consequently, we examined the effect of proline in N6D on rHSA production. A twofold increase in the concentration of proline in N6D increased the yield of rHSA to 32.3 mg/L at an initial cell density of 3% after 16 days, an increase of 39.2% compared with standard N6D. A beneficial effect of proline was also demonstrated on the production of recombinant mouse granulocyte-macrophage colony-stimulating factor from the G10 transgenic cell line. Our study showed that the supplementation of N6D with additional proline can promote the production of recombinant proteins in the αAmy3 promoter/signal peptide-based rice suspension-cultured system.