Transcriptional attenuation of PIM-1 gene

Anti-Pim-1 mAb inhibits activation and proliferation of T lymphocytes and prolongs mouse skin allograft survival

Pim-1 is an important signaling molecule mediating cell proliferation and survival. Our previous study identified a Pim-1 specific monoclonal antibody, P9, with significant inhibitory effect on cell proliferation. Herein, we report that P9 inhibited the activation and proliferation of PHA-stimulated human PBMC and induced them to undergo apoptosis. In contrast, P9 showed little effect on freshly isolated human blood T lymphocytes which poorly expressed Pim-1. P9 also detected an up-regulation of Pim-1 in mouse lymphocytes after mitogen stimulation, and showed similar selective inhibition on stimulated cells as observed with hPBMC. Furthermore, P9 inhibited the in vitro mixed lymphocyte reaction and P9 treatment significantly prolonged the survival of mouse skin allografts (P<0.001). It is concluded that Pim-1 expression correlates with lymphocyte proliferation and activation. P9 functions as a Pim-1 antagonist and is potential for immunosuppressive therapy.

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.

Cryptic promoter activity in the DNA sequence corresponding to the pim-1 5'-UTR

The serine/threonine kinase pim-1 mRNA contains a long and G/C rich 5'-untranslated region (5'-UTR). Previous work suggested that the pim-1 5'-UTR harbors an internal ribosomal entry site (IRES) allowing for internal initiation of translation. However, several previously reported eukaryotic IRES elements actually contain cryptic promoter activity. To test whether an IRES or a cryptic promoter is present in the pim-1 5'-UTR, the 5'-UTR was re-examined using stringent test procedures. Our results show the presence of strong promoter activity in the DNA sequence corresponding to the pim-1 5'-UTR. Both promoterless dicistronic test and northern blot analysis show transcripts being derived from the cryptic promoter in the pim-1 5'-UTR sequence. This cryptic promoter is active in all cell types tested, including Cos-7, NIH3T3, HEK293, Jurkat and K562 cells. When a dicistronic mRNA containing the pim-1 5'-UTR was translated in vitro or in vivo, no IRES activity could be detected. However, the control IRESs from both human rhinovirus and encephalomyocarditis virus exhibited strong IRES activities. In addition, both the RNase protection assay and the 5'-RACE assay detected endogenous pim-1 transcripts with shorter 5'-UTRs. Our data strongly suggest that the IRES activity reported earlier for the pim-1 5'-UTR sequence is due to cryptic promoter activity.

Protein phosphatase 2A regulates the stability of Pim protein kinases

The pim family of proto-oncogenes encodes three serine-threonine kinases that have been implicated in the development of malignancies in mice and in humans. Expression of the Pim protein kinases is tightly regulated at the transcriptional, post-transcriptional, and translational levels. Dysregulation of pim transcription and pim mRNA stability have been implicated in Pim-mediated transformation. The data presented herein demonstrate that expression of the Pim kinases is additionally regulated at the post-translational level, by the serine-threonine phosphatase protein phosphatase 2A (PP2A). The catalytic subunit of PP2A associates with the Pim kinases in vivo, and the Pim kinases are substrates of PP2A phosphatase activity in vitro. Furthermore, overexpression of PP2A reduces the levels of the Pim proteins, whereas inhibition of PP2A activity by the protein phosphatase inhibitor okadaic acid stabilizes the Pim proteins. Finally, the effects of PP2A on the expression of the Pim proteins can affect Pim function. Taken together, these data suggest that PP2A activity is important for the regulation of the stability and function of the Pim kinases.