Inhibition of CKS1B Expression Affects Myeloma Cell Growth In Vivo.

Abstract

CKS1B is the regulatory component of the SCF-Skp2 ubiquitin ligase that ubiquinates the cyclin-dependent kinase inhibitor p27kip1 targeting it for proteasomal degradation. Overexpression of CKS1B is associated with poor prognosis in malignancies such as oral squamous cell carcinomas, colorectal carcinoma, breast cancer, prostate cancer, and others. Recently, high expression levels of CKS1B were associated with an aggressive course of Multiple Myeloma (Shaughnessy J. Hematology . 2005; 10 Suppl 1:117). We investigated whether inhibition of CKS1B in myeloma cell lines with multiple copies of 1q21 and high expression of CKS1B affects in vivo tumor growth. Myeloma cell lines were transduced with lentivirus conditionally expressing (tet on) CKS1B shRNA and lentivirus constitutively expressing luciferase (for in vivo experiments), or with lentivirus constitutively expressing CKS1B shRNA (for in vitro work). Cells with inducible expression were injected into the human bones of SCID-hu mice, and, once tumors established, expression was induced with doxycycline in the drinking water. Changes in tumor sizes were followed by weekly luminescence imaging. Cells with constitutive shRNA expression were used to determine effects on cell cycle by propidium iodide flow cytometry. Effects of shRNA expression on cytoplasmic and nuclear CKS1B and p27kip1 levels were determined by western blot analysis. Induction of CKS1B shRNA expression in JJN3 and OCI-MY5 cells in SCID-hu mice inhibited tumor growth by 57% and 81% within one week, compared to non-targeting construct. As expected, viable cells recovered from these tumors had low levels of CKS1B and higher levels of p27kip1 proteins. Similar to observations in the MM cell lines JJN3 and OCI-MY5 (see Abstract by Zhan and colleagues), constitutive expression of shRNA in CAG and XG1 cells effectively reduced cytoplasmic and nuclear CKS1B protein levels. This was associated with increased levels of p27kip1 protein, marked (51%) increases of cells in the G1 phase of the cell cycle, and reduced S and G2 (33% and 52%, respectively), compatible with a G1 arrest, and a 76% increase in the number of sub G1 (apoptotic) cells. In contrast to these observations, cells constitutively over-expressing p27kip1 had no change in G1, 50% fewer cells in S, and a 150% increase in G2, indicating G1 and G2 arrests, with no change in the sub G1 fraction. Interestingly, cells over-expressing p27kip1 had lower levels of CKS1B proteins, compatible with the reported proteasomal degradation of the molecule during the later phases of the cell cycle (Hattori T et al, Genes Cells. 2003; 8:889). These observations suggest that the poor prognosis associated with 1q21 amplifications and high CKS1B expression in myeloma reflects an elevated proliferative state of the tumor cells that results in rapid regrowth of minimal residual disease.