Loss of pim1 imposes a hyperadhesive phenotype on endothelial cells.

BACKGROUND: PIM1 is a constitutively active serine-threonine kinase regulating cell survival and proliferation. Increased PIM1 expression has been correlated with cancer metastasis by facilitating migration and anti-adhesion. Endothelial cells play a pivotal role in these processes by contributing a barrier to the blood stream. Here, we investigated whether PIM1 regulates mouse aortic endothelial cell (MAEC) monolayer integrity. METHODS: Pim1-/-MAEC were isolated from Pim1 knockout mice and used in trypsinization-, wound closure assays, electrical cell-substrate sensing, immunostaining, cDNA transfection and as RNA source for microarray analysis. RESULTS: Pim1-/-MAEC displayed decreased migration, slowed cell detachment and increased electrical resistance across the endothelial monolayer. Reintroduction of Pim1- cDNA into Pim1-/-MAEC significantly restored wildtype adhesive characteristics. Pim1-/--MAEC displayed enhanced focal adhesion and adherens junction structures containing vinculin and ß-catenin, respectively. Junctional molecules such as Cadherin 13 and matrix components such as Collagen 6a3 were highly upregulated in Pim1-/- cells. Intriguingly, extracellular matrix deposited by Pim1-/- cells alone was sufficient to induce the hyperadhesive phenotype in wildtype endothelial cells. CONCLUSION: Loss of Pim1 induces a strong adhesive phenotype by enhancing endothelial cell-cell and cell-matrix adhesion by the deposition of a specific extracellular matrix. Targeting PIM1 function therefore might be important to promote endothelial barrier integrity.

Nuclear PIM1 confers resistance to rapamycin-impaired endothelial proliferation.

The PIM serine/threonine kinases and the mTOR/AKT pathway integrate growth factor signaling and promote cell proliferation and survival. They both share phosphorylation targets and have overlapping functions, which can partially substitute for each other. In cancer cells PIM kinases have been reported to produce resistance to mTOR inhibition by rapamycin. Tumor growth depends highly on blood vessel infiltration into the malignant tissue and therefore on endothelial cell proliferation. We therefore investigated how the PIM1 kinase modulates growth inhibitory effects of rapamycin in mouse aortic endothelial cells (MAEC). We found that proliferation of MAEC lacking Pim1 was significantly more sensitive to rapamycin inhibition, compared to wildtype cells. Inhibition of mTOR and AKT in normal MAEC resulted in significantly elevated PIM1 protein levels in the cytosol and in the nucleus. We observed that truncation of the C-terminal part of Pim1 beyond Ser 276 resulted in almost exclusive nuclear localization of the protein. Re-expression of this Pim1 deletion mutant significantly increased the proliferation of Pim1(-/-) cells when compared to expression of the wildtype Pim1 cDNA. Finally, overexpression of the nuclear localization mutant and the wildtype Pim1 resulted in complete resistance to growth inhibition by rapamycin. Thus, mTOR inhibition-induced nuclear accumulation of PIM1 or expression of a nuclear C-terminal PIM1 truncation mutant is sufficient to increase endothelial cell proliferation, suggesting that nuclear localization of PIM1 is important for resistance of MAEC to rapamycin-mediated inhibition of proliferation.