Differential cellular responses by oncogenic levels of c-Myc expression in long-term confluent retinal pigment epithelial cells.

c-Myc is a highly pleiotropic transcription factor known to control cell cycle progression, apoptosis, and cellular transformation. Normally, ectopic expression of c-Myc is associated with promoting cell proliferation or triggering cell death via activating p53. However, it is not clear how the levels of c-Myc lead to different cellular responses. Here, we generated a series of stable RPE cell clones expressing c-Myc at different levels, and found that consistent low level of c-Myc induced cellular senescence by activating AP4 in post-confluent RPE cells, while the cells underwent cell death at high level of c-Myc. In addition, high level of c-Myc could override the effect of AP4 on cellular senescence. Further knockdown of AP4 abrogated senescence-like phenotype in cells expressing low level of c-Myc, and accelerated cell death in cells with medium level of c-Myc, indicating that AP4 was required for cellular senescence induced by low level of c-Myc.

JNK-associated Leucine Zipper Protein Functions as a Docking Platform for Polo-like Kinase 1 and Regulation of the Associating Transcription Factor Forkhead Box Protein K1.

JLP (JNK-associated leucine zipper protein) is a scaffolding protein that interacts with various signaling proteins associated with coordinated regulation of cellular process such as endocytosis, motility, neurite outgrowth, cell proliferation, and apoptosis. Here we identified PLK1 (Polo-like kinase 1) as a novel interaction partner of JLP through mass spectrometric approaches. Our results indicate that JLP is phospho-primed by PLK1 on Thr-351, which is recognized by the Polo box domain of PLK1 leading to phosphorylation of JLP at additional sites. Stable isotope labeling by amino acids in cell culture and quantitative LC-MS/MS analysis was performed to identify PLK1-dependent JLP-interacting proteins. Treatment of cells with the PLK1 kinase inhibitor BI2536 suppressed binding of the Forkhead box protein K1 (FOXK1) transcriptional repressor to JLP. JLP was found to interact with PLK1 and FOXK1 during mitosis. Moreover, knockdown of PLK1 affected the interaction between JLP and FOXK1. FOXK1 is a known transcriptional repressor of the CDK inhibitor p21/WAF1, and knockdown of JLP resulted in increased FOXK1 protein levels and a reduction of p21 transcript levels. Our results suggest a novel mechanism by which FOXK1 protein levels and activity are regulated by associating with JLP and PLK1.

Transport of c-MYC by Kinesin-1 for proteasomal degradation in the cytoplasm.

c-MYC is an oncogenic transcription factor that is degraded by the proteasome pathway. However, the mechanism that regulates delivery of c-MYC to the proteasome for degradation is not well characterized. Here, the results show that the motor protein complex Kinesin-1 transports c-MYC to the cytoplasm for proteasomal degradation. Inhibition of Kinesin-1 function enhanced ubiquitination of c-MYC and induced aggregation of c-MYC in the cytoplasm. Transport studies showed that the c-MYC aggregates moved from the nucleus to the cytoplasm and KIF5B is responsible for the transport in the cytoplasm. Furthermore, inhibition of the proteasomal degradation process also resulted in an accumulation of c-MYC aggregates in the cytoplasm. Moreover, Kinesin-1 was shown to interact with c-MYC and the proteasome subunit S6a. Inhibition of Kinesin-1 function also reduced c-MYC-dependent transformation activities. Taken together, the results strongly suggest that Kinesin-1 transports c-MYC for proteasomal degradation in the cytoplasm and the proper degradation of c-MYC mediated by Kinesin-1 transport is important for transformation activities of c-MYC. In addition, the results indicate that Kinesin-1 transport mechanism is important for degradation of a number of other proteins as well.