Abstract C12: Overcoming resistance to inhibitors of the AKT protein kinases by targeting the Pim protein kinase pathway
Authors: Cen Bo , Mahajan Sandeep , Kraft Andrew S. 
Published: May 2013

Abstract
The AKT protein kinases are an important signal transduction target for the inhibition of prostate cancer growth. AKT is activated in 50-80% of cancers secondary to deletions, mutations and loss of heterozygosity of the PTEN phosphatase. Resistance to small molecule chemical inhibitors of protein kinases in human patients involves the induction of alternative signal transduction pathways. We demonstrate that the addition of the pan-AKT inhibitor GSK690693 (GlaxoSmithKline, Collegeville, PA) to prostate cancer cell lines, Du145, PC-3 and PC3-LN4 induced marked up regulation of multiple receptor tyrosine kinases, including c-Met, Her3, IGF-IR, INSR and EphA2. siRNAs that knock down the three AKT isoforms have a similar effect. Increases in these receptors have the potential to block the growth inhibitory action of this AKT kinase inhibitor. In addition to changes in these RTKs, treatment of prostate cancer cells with GSK690693 also markedly increased the levels of the Pim-1 protein kinase. Pim-1 has previously been identified as a protein kinase that regulates prostate cancer growth and progression. Importantly, down regulation of Pim-1 either through using cells derived from genetically engineered knock-out mice, knocking down the levels of Pim-1 with siRNAs or treating cells with Pim-1 inhibitory small molecules, blocked the ability of GSK690693 treatment to induce increases in RTKs. Likewise, over expression of Pim-1 in prostate cancer cell lines caused increases in multiple RTKs, with increases in c-Met being the most prominent. In comparison, in multiple prostate cancer cell lines knockdown of wild type levels of Pim-1 was sufficient to lead to decreased expression of endogenous RTKs. The Pim-1 induced c-Met expression enhanced the ability of prostate cancer cells to migrate when treated with the c-Met ligand HGF. While the observation in human prostate cancer tissue microarrays that Pim-1 and c-Met protein levels correlate suggested that in human cancers Pim-1 may also regulate c-Met protein levels. The ability of GSK690693 to induce Pim-1 was not influenced by the knockdown of the FOXO transcription factors nor did the application of GSK690693 change the half-life of the Pim-1 protein. These results suggest the possibility that this agent might mediate the cellular level of Pim-1 through translational controls. The addition of GSK690693 and the Pim-1 inhibitor SMI-4a (Vortex Biotechnology, Mt. Pleasant, SC) to PC3-LN4 cells caused a highly synergistic inhibition of growth both in tissue culture and in soft agar. When these agents (GSK690693 30 mg/kg intraperitoneally once daily and SMI-4a 60 mg/Kg orally twice daily) were administered to immune-compromised mice that had been previously injected subcutaneously with 5 × 106 PC3-LN4 cells, the combination markedly inhibited tumor growth where each agent given alone had only a partial inhibitory effect (about 25% inhibition). Based on these results we suggest that the Pim-1 protein kinase plays a critical role in the induction of RTKs by AKT inhibitors. Thus combination therapy of prostate cancer with a Pim and AKT inhibitor is likely to be a novel effective therapeutic strategy to overcome potential resistance mechanisms to AKT kinase inhibitors.
Citation Format: Bo Cen, Sandeep Mahajan, Andrew S. Kraft. Overcoming resistance to inhibitors of the AKT protein kinases by targeting the Pim protein kinase pathway [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr C12.