Enhanced Heterologous Expression of Biologically Active Human Granulocyte Colony Stimulating Factor in Transgenic Tobacco BY-2 Cells by Localization to Endoplasmic Reticulum

Tobacco Bright Yellow-2 (BY-2) cells, one of the best characterized cell lines is an attractive expression system for heterologous protein expression. However, the expression of foreign proteins is currently hampered by their low yield, which is partially the result of proteolytic degradation. Human granulocyte colony stimulating factor (hG-CSF) is a hematopoietic cytokine. Recombinant hG-CSF is successfully being used for the treatment of chemotherapy-induced neutropenia in cancer patients. Here, we describe a simple strategy for producing biologically active hG-CSF in tobacco BY-2 cells, localized in the apoplast of BY-2 cells, as well as targeted to the endoplasmic reticulum (ER). ER targeting significantly enhanced recombinant production which scaled to 17.89 mg/l from 4.19 mg/l when expressed in the apoplasts. Southern blotting confirmed the stable integration of hG-CSF in the BY-2 nuclear genome, and the expression of hG-CSF was analysed by Western blotting. Total soluble protein containing hG-CSF isolated from positive calli showed proliferative potential when tested on HL-60 cell lines by MTT assay. We also report the potential of a Fluorescence-activated cell sorting approach for an efficient sorting of the hG-CSF-expressing cell lines, which will enable the generation of homogenous high-producing cell lines.