Serum/glucose starvation strikingly reduces heterogeneous nuclear ribonucleoprotein A1 protein and its target, cyclin D1

Abstract

Our investigation to explore cellular alterations related to undernutrition in cancer cells revealed that the protein level of heterogenous nuclear ribonucleoprotein A1 (hnRNP A1) is drastically decreased by serum/glucose starvation. Its loss was reversible, serum/glucose starvation‐specific and universal throughout cell types and species. The hnRNP A1 mRNA level and hnRNP A1 mRNA/protein stability were not altered under this condition. CCND1 mRNA, which we newly identified as the binding target of hnRNP A1, was decreased by serum/glucose starvation. Under similar conditions, CCND1 protein was reduced in vitro and in vivo, whereas hnRNP A1 mRNA level and CCND1 mRNA level revealed no correlation in most clinical samples. Functional analyses revealed that CCND1 mRNA stability is certainly dependent on hnRNP A1 protein level and that RNA recognition motif‐1 (RRM1) in hnRNP A1 plays a central role in maintaining CCND1 mRNA stability and subsequent protein expression. The injection of RRM1‐deleted hnRNP A1‐expressing cancer cells in the mouse xenograft model did not form any tumours, and that of hnRNP A1‐expressing cancer cells retained CCND1 expression at the lesion adjacent to necrosis with a slight increase in tumour volume. Furthermore, RRM1 deletion caused growth suppression with the induction of apoptosis and autophagy, whereas CCND1 restoration completely recovered it. Our results indicate that serum/glucose starvation triggers entire hnRNP A1 protein loss, and its loss may play a role in CCND1 mRNA destabilization and CCND1‐mediated cellular event inhibition, i.e. growth promotion, apoptosis induction and autophagosome formation.