Silkworm Storage Protein 1 Inhibits Autophagy-Mediated Apoptosis

Autophagy is a natural physiological process, and it induces the lysosomal degradation of intracellular components in response to environmental stresses, including nutrient starvation. Although an adequate autophagy level helps in cell survival, excessive autophagy triggered by stress such as starvation leads to autophagy-mediated apoptosis. Chinese hamster ovary (CHO) cells are widely used for producing biopharmaceuticals, including monoclonal antibodies. However, apoptosis induced by high stress levels, including nutrient deficiency, is a major problem in cell cultures grown in bioreactors, which should be overcome. Therefore, it is necessary to develop a method for suppressing excessive autophagy and for maintaining an appropriate autophagy level in cells. Therefore, we investigated the effect of silkworm storage protein 1 (SP1), an antiapoptotic protein, on autophagy-mediated apoptosis. SP1-expressing CHO cells were generated to assess the effect and molecular mechanism of SP1 in suppressing autophagy. These cells were cultured under starvation conditions by treatment with Earle's balanced salt solution (EBSS) to induce autophagy. We observed that SP1 significantly inhibited autophagy-mediated apoptosis by suppressing caspase-3 activation and reactive oxygen species generation. In addition, SP1 suppressed EBSS-induced conversion of LC3-I to LC3-II and the expression of autophagy-related protein 7. Notably, basal Beclin-1 level was significantly low in the SP1-expressing cells, indicating that SP1 regulated upstream events in the autophagy pathway. Together, these findings suggest that SP1 offers a new strategy for overcoming severe autophagy-mediated apoptosis in mammalian cells, and it can be used widely in biopharmaceutical production.