Amino acid deprivation-induced expression of asparagine synthetase regulates the growth and survival of cultured silkworm cells

Expression of Bombyx mori Asparagine synthetase (BmASNS), one gene that encodes an enzyme catalyzing asparagine biosynthesis, is transcriptionally induced following amino acid deprivation. Previous transcriptional analysis of the BmASNS gene showed the involvement of Polycomb proteins, epigenetic repressors, in suppressing BmASNS expression in a cell cycle-dependent manner. However, the role of BmAsns protein in these cellular processes remains unclear. The present study thus exploited the potential function of BmAsns protein in cultured silkworm cells. Our results showed that ectopic overexpression of BmASNS gene effectively inhibited cell growth in silkworm cells, whereas its overexpression could rescue cell growth upon amino acid deprivation treatment. We found that the cells expressing BmAsns protein were capable of influencing the formation of autophagic vacuoles stimulated by amino acid deprivation. We speculated that the recovery of cell growth by overexpressed BmAsns protein is due to the rapid turnover of autophagic vacuoles in the cells. To further assess the effects of BmAsns on cell development, we used RNA interference to silence BmASNS expression in silkworm cells in the presence or absence of amino acids. Our results revealed a significant change of cell proliferation as well as cell cycle distribution after knockdown of BmASNS. Importantly, silkworm cells lacking BmASNS under the condition of amino acid deprivation showed severely impaired proliferation. Altogether, we concluded that the up-regulated expression of BmASNS would be able to protect cells from impairment induced by amino acid deprivation, which in turn facilitates cell growth and survival.