Transient arrest in a quiescent state allows ovarian cancer cells to survive suboptimal growth conditions and is mediated by both Mirk/dyrk1b and p130/Rb2

Abstract

Some ovarian cancer cells in vivo are in a reversible quiescent state where they can contribute to cancer spread under favorable growth conditions. The serine/threonine kinase Mirk/dyrk1B was expressed in each of seven ovarian cancer cell lines and in 21 of 28 resected human ovarian cancers, and upregulated in 60% of the cancers. Some ovarian cancer cells were found in a G0 quiescent state, with the highest fraction in a line with an amplified Mirk gene. Suboptimal culture conditions increased the G0 fraction in SKOV3 and TOV21G, but not OVCAR4 cultures. Less than half as many OVCAR4 cells survived under suboptimal culture conditions as shown by total cell numbers, dye exclusion viability studies, and assay of cleaved apoptotic marker proteins. G0 arrest in TOV21G and SKOV3 cells led to increased levels of Mirk, the CDK inhibitor p27, p130/Rb2, and p130/Rb2 complexed with E2F4. The G0 arrest was transient, and cells exited G0 when fresh nutrients were supplied. Depletion of p130/Rb2 reduced the G0 fraction, increased cell sensitivity to serum-free culture and to cisplatin, and reduced Mirk levels. Mirk contributed to G0 arrest by destabilization of cyclin D1. In TOV21G cells, but not in normal diploid fibroblasts, Mirk depletion led to increased apoptosis and loss of viability. Because Mirk is expressed at low levels in most normal adult tissues, the elevated Mirk protein levels in ovarian cancers may present a novel therapeutic target, in particular for quiescent tumor cells which are difficult to eradicate by conventional therapies targeting dividing cells.