TORC1 ensures membrane trafficking of Tat2 tryptophan permease via a novel transcriptional activator Vhr2 in budding yeast.

The target of rapamycin complex 1 (TORC1) protein kinase is activated by nutrients and controls nutrient uptake via the membrane trafficking of various nutrient permeases. However, its molecular mechanisms remain elusive. Cholesterol (ergosterol in yeast) in conjunction with sphingolipids forms tight-packing microdomains, "lipid rafts", which are critical for intracellular protein sorting. Here we show that a novel target of rapamycin (TOR)-interacting transcriptional activator Vhr2 is required for full expression of some ERG genes for ergosterol biogenesis and for proper sorting of the tryptophan permease Tat2 in budding yeast. Loss of Vhr2 caused sterol biogenesis disturbance and Tat2 missorting. TORC1 activity maintained VHR2 transcript and protein levels, and total sterol levels. Vhr2 was not involved in regulation of the TORC1-downstream protein kinase Npr1, which regulates Tat2 sorting. This study suggests that TORC1 regulates nutrient uptake via sterol biogenesis.

c-JUN promotes BCR-ABL-induced lymphoid leukemia by inhibiting methylation of the 5' region of Cdk6.

The transcription factor c-JUN and its upstream kinase JNK1 have been implicated in BCR-ABL-induced leukemogenesis. JNK1 has been shown to regulate BCL2 expression, thereby altering leukemogenesis, but the impact of c-JUN remained unclear. In this study, we show that JNK1 and c-JUN promote leukemogenesis via separate pathways, because lack of c-JUN impairs proliferation of p185(BCR-ABL)-transformed cells without affecting their viability. The decreased proliferation of c-Jun(Δ/Δ) cells is associated with the loss of cyclin-dependent kinase 6 (CDK6) expression. In c-Jun(Δ/Δ) cells, CDK6 expression becomes down-regulated upon BCR-ABL-induced transformation, which correlates with CpG island methylation within the 5' region of Cdk6. We verified the impact of Cdk6 deficiency using Cdk6(-/-) mice that developed BCR-ABL-induced B-lymphoid leukemia with significantly increased latency and an attenuated disease phenotype. In addition, we show that reexpression of CDK6 in BCR-ABL-transformed c-Jun(Δ/Δ) cells reconstitutes proliferation and tumor formation in Nu/Nu mice. In summary, our study reveals a novel function for the activating protein 1 (AP-1) transcription factor c-JUN in leukemogenesis by antagonizing promoter methylation. Moreover, we identify CDK6 as relevant and critical target of AP-1-regulated DNA methylation on BCR-ABL-induced transformation, thereby accelerating leukemogenesis.