Abstract 2409: Inhibition of PLK1 induces growth arrest and apoptosis in tumorigenic OS cells
Authors: Guijarro Maria V. , Nasri Elham , Levings Padraic P. , Ghivizzani Steven C. , Gibbs. C Parker 
Published: July 2016

Osteosarcoma (OS) remains a significant therapeutic challenge. At first diagnosis, patients without signs of cancer beyond the primary tumor show a 5-year survival rate of 60-80%; however, this falls to 20% in patients with evidence of metastatic disease. Toward development of treatments with greater therapeutic efficacy, we have been investigating the nature of intratumoral heterogeneity in OS. We have found that subpopulations of cells from primary OS biopsies are selectively capable of activating an exogenous reporter construct comprised of the human Oct4 promoter linked to the Green Fluorescent Protein (GFP). These Oct4/GFP+ cells are &gt;100 times tumorigenic than GFP- cells from within the same tumor when injected into NSG mice.

Our objective in this study was to investigate the effects of Polo-like kinase 1 (PLK1) among heterogeneous cell populations within individual OS tumors. PLK1 is a serine/threonine-specific kinase that plays an important role in mitosis and the maintenance of genomic stability. While PLK inhibition has previously been shown to reduce tumor growth in OS cell lines, drug resistance and modest clinical efficacy have been reported in several studies.

Microarray analysis revealed an upregulation of PLK1 of more than 1.7-fold in the Oct4/GFP+ cells relative to the GFP- population in all three OS cell lines tested, OS156, OS521 and OS28 (p value = 0.05). We next investigated the effects of the PLK inhibitors, BI2536 and BI6727, to determine the sensitivity of drug inhibition between 6 OS cell lines and among subpopulations within each tumor. IC50s showed that OS521, OS220 and OS28 OS cell lines were more resistant to the inhibitors than OS156, DCS0313 and D2712. Strikingly, all the Oct4/GFP+ populations of the tumors were more sensitive to drug treatment compared to GFP- cells. Cell cycle analysis showed that high doses (&gt;0.5 μM) of either of the inhibitors induced apoptosis whereas low doses (&lt;10 nM) induced G1 arrest. Within tumors, proliferation, clonability and migration were reduced in the GFP+ cell population relative to GFP- cells at doses of 10 nM. Consistent with these in vitro findings, tumor cell growth of Oct4/GFP+ pretreated cells injected in NSG mice was inhibited (by 50%) at a low dose of 10 nM (p = 0.005)

These findings offer evidence of the differential drug resistance as a consequence not only of the individual patient efficacy but also of the Oct4/GFP+ and GFP- populations from the same tumor, confirming that the intratumoral heterogeneity is a major obstacle for the treatment of OS, as the presence of minor populations that are insensitive to therapy can lead to disease relapse. Future studies will be conducted to understand what makes tumors resistant or vulnerable to PLK1 inhibition.

Citation Format: Maria V. Guijarro, Elham Nasri, Padraic P. Levings, Steven C. Ghivizzani, C Parker Gibbs. Inhibition of PLK1 induces growth arrest and apoptosis in tumorigenic OS cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2409.