Loss of HNRNPK During Cell Senescence Linked to Reduced Production of CDC20.

Cellular senescence is a complex biological response to sublethal damage. The RNA-binding protein HNRNPK was previously found to decrease prominently during senescence in human diploid fibroblasts. Here, analysis of the mechanisms leading to reduced HNRNPK abundance revealed that in cells undergoing senescence, HNRNPK mRNA levels declined transcriptionally and full-length HNRNPK protein was progressively lost, while the abundance of a truncated HNRNPK increased. The ensuing loss of full-length HNRNPK enhanced cell cycle arrest along with increased DNA damage. Analysis of the RNAs enriched after HNRNPK ribonucleoprotein immunoprecipitation (RIP) revealed a prominent target of HNRNPK, CDC20 mRNA, encoding a protein critical for progression through the G2/M phase of the cell division cycle. Silencing HNRNPK markedly decreased the levels of CDC20 mRNA via reduced transcription and stability of CDC20 mRNA, leading to lower CDC20 protein levels; conversely, overexpressing HNRNPK increased CDC20 production. Depletion of either HNRNPK or CDC20 impaired cell proliferation, with a concomitant reduction in the levels of CDK1, a key kinase for progression through G2/M. Given that overexpressing CDC20 in HNRNPK-silenced cells partly alleviated growth arrest, we propose that the reduction in HNRNPK levels in senescent cells contributed to inhibiting proliferation at least in part by suppressing CDC20 production.