365 Pim-1 kinase: Validated as a therapeutic cancer target for MYC-driven tumours

Provirus integration site for Moloney murine leukemia virus 1 (Pim-1) kinase belongs to a family of constitutively active serine/threonine kinases. The overexpression of Pim-1 can contribute to tumorigenesis, thereby establishing Pim-1 as a proto-oncogene. In B-cell lymphoma, colorectal cancer, pancreatic cancer and other tumours, the over expression of Pim-1 is linked to poor prognosis. Proviral integration experiments suggested the cooperation between Pim-1 and the protooncogene c-myc that was subsequently confirmed by double transgenic mice. Pim-1 acts synergistically with the transcription factor c-myc by phosphorylation and by interaction with c-myc on the chromatin level, leading to elevated trancription of c-myc target genes and cell proliferation. Pim-1 also promotes cell cycle progression. Pim-1-mediated phosphorylation of p27 leads to its binding to 14−3−3 protein, its nuclear export and proteasome-dependent degradation. The aim of our study was to analyze the effect of kinase-inhibition of Pim-1 on tumour growth in a mouse model of haematological malignancy. To this end, we constructed a conditional knock-in mouse model in which a single amino acid exchange can be induced by expression of the Cre-recombinase, leading to expression of an inactive form of Pim-1 kinase. These Pim-1 kinase-dead (KD) mice do not display phenotypic abnormalities or reduced live span even after homozygous exchange by CMV-Cre expression of the wild type form of Pim-1 against the mutated form. As expected, elevated levels of the cell cycle inhibitor p27 can be observed in the organs of Pim-1 kinase-dead mice by immunohistochemistry (IHC). In order to study the effect of the Pim-1 kinase inhibition on tumour development driven by c-myc, we crossed the Pim-1 KD mice with the Em-cmyc transgenic mouse line, which expresses high levels of the oncoprotein c-myc leading to the development of a rapidly growing B-cell lymphoma that resembles the human Burkitt-lymphoma. Even the heterozygous switch to the kinase-dead form of Pim-1 doubles the survival of the Em-c-myc mice; the homozygous switch brings survival back close to normal. On the molecular level, we observed elevated p27-levels in tumours of heteroand homozygous Pim-1 kinase-dead mice carrying the Em-c-myc transgene. The tumours of these mice also displayed areas with reduced levels of c-myc (Western blot and IHC). Based upon these results, we set-up an allogenic Em-c-myc mouse model that allowed us to study the effect of kinase inhibitors in an efficient and standardized way. We can show that novel, selective and potent Pim-1 kinase inhibitors that have been developed in our department are able to efficiently inhibit c-myc-driven tumour growth, as proposed by the genetic ablation of Pim-1 kinase activity. In summary, the presented study proofs inhibition of Pim-1 kinase activity to be a promising strategy to inhibit cancer growth driven by c-myc, one of the most frequently deregulated oncogenes in human cancer.