Induction of p21 and p27 expression by amino acid deprivation of HepG2 human hepatoma cells involves mRNA stabilization.

mRNA abundance for a number of genes is increased by amino acid limitation. From an array screening study in HepG2 human hepatoma cells, it was established that one set of genes affected by amino acid availability is the set associated with cell-cycle control. The present study describes the increased expression of both mRNA and protein for the cyclin-dependent kinase inhibitors p21 and p27 in response to deprivation of HepG2 cells for a single essential amino acid, histidine. The increase in p21 and p27 mRNA content depended on de novo protein synthesis and involved a post-transcriptional mRNA stabilization component. For p21, increase in mRNA by histidine depletion appeared to be independent of p53 transactivation, and the absolute level of p53 protein was unaffected by this treatment. Histidine limitation caused an increase in the phosphorylation of ERK1/ERK2 (extracellular-signal-regulated kinase), and inhibition of the ERK signal transduction pathway resulted in a reduction in the starvation-dependent increase in p21 mRNA. Blockade of the phosphoinositide 3-kinase and mTOR (mammalian target of rapamycin) pathways also blunted the increase in p21 mRNA content. These results document the amino acid-dependent control of the synthesis of specific cell-cycle regulators and help to explain the block at G1 phase after amino acid limitation.