Transcriptional regulation of Pim-1 kinase in vascular smooth muscle cells and its role for proliferation

The Ser/Thr-protein kinase Pim-1 has been discovered as a novel transducer of survival- and cell cycle promoting signals in the hematopoietic cell system. Although its significance for proliferation of vascular smooth muscle cells (VSMC) in vitro and neointima formation in vivo has been suggested recently, the mechanism has barely been characterized. This study aimed to foster the understanding of Pim-1 expression and regulation in murine VSMC in response to factors typically present within the atherosclerotic plaque. While oxidative stress, VEGF-A₁₆₅ and angiotensin II did not have any effect on Pim-1 expression, VSMC strongly increased (3-fold) Pim-1 mRNA upon stimulation with PDGF_bb, followed by its protein upregulation. Half life of Pim-1 RNA and protein were determined to be 25 min and 6 h, respectively. PDGF_bb induced a strong, 10-fold increase in BrdU-uptake, a marker of proliferation. This was effectively blocked by either Pim-1-specific inhibitor quercetagetin or adenovirally introduced Pim-1 shRNA. We further identified the signaling pathways linking PDGF_bb to Pim-1 in VSMC: as expected, we determined transcriptional stimulation of Pim-1 via Janus-activated kinase (Jak), but also an additional pathway involving protein kinase C (PKC) and the mitogen-activated protein kinase Mek1/2. Blockade of Akt signaling did, however, not interfere with Pim-1 upregulation, suggesting an independence of either survival system. PDGF_bb-induced proliferation of VSMC is partly attributed to transcriptionally upregulated Pim-1 and was assigned to distinct cell signaling. Our findings help to understand the fundamental processes of vasculoproliferative diseases thus opening avenues for its prevention and treatment.