An mTOR feedback loop mediates the 'flare' ('rebound') response to MET tyrosine kinase inhibition.

Targeted therapy significantly impairs tumour growth but suffers from limitations, among which the 'flare' ('rebound') effect. Among cancers driven by tyrosine kinase receptors, those relying on alterations of the MET oncogene benefit from treatment by specific inhibitors. Previously, we reported that discontinuation of MET tyrosine kinase receptor inhibition causes 'rebound' activation of the oncogene, with a post-treatment transient hyperphosphorylation phase that culminates into a dramatic increase in cancer cell proliferation. The molecular mechanisms behind the 'MET burst' after treatment cessation are unknown but critically important for patients. Here we identify a positive feedback loop mediated by the AKT/mTOR pathway leading to (a) enhanced MET translation by activating p70S6K and 4EBP1 and (b) MET hyper-phosphorylation by inactivation of the tyrosine-phosphatase PTP1B. The latter effect is due to m-TOR-driven PTP1B phosphorylation of the inhibitory residues Ser50 and Ser378. These data provide in vitro evidence for the use of mTOR inhibitors to prevent the 'flare effect' in MET targeted therapy, with potential applicative ramifications for patient clinical management.

Direct cooperation between androgen receptor and E2F1 reveals a common regulation mechanism for androgen-responsive genes in prostate cells.

We have studied the regulation of ATAD2 gene expression by androgens in prostate cells. ATAD2 is a coactivator of the androgen receptor (AR) and the MYC protein. We showed that ATAD2 expression is directly regulated by AR via an AR binding sequence (ARBS) located in the distal enhancer of its regulatory region. The gene is also regulated by the E2F1 transcription factor. Using knockdown and chromatin immunoprecipitation technique approaches, we could demonstrate that AR and E2F1 functionally collaborate and physically interact between each other. From the analysis of chromatin conformation, we conclude that this cooperation results from a chromatin looping over the ATAD2 promoter region between the ARBS and E2F1 binding site in an androgen-dependent manner. Furthermore, we could show that several genes overexpressed in prostate cancer and potentially involved in several aspects of tumor development have an ARBS and an E2F1 binding site in their regulatory regions and exhibit the same mechanism of regulation by both transcription factors as ATAD2.

Cell cycle regulated expression of NCoR might control cyclic expression of androgen responsive genes in an immortalized prostate cell line.

In this work we have studied the mechanisms of regulation of expression of androgen receptor (AR) target genes. We have used an immortalized non-tumorigenic prostate cell line RWPE-1-AR(tag) constitutively expressing an exogenous AR as a model. We observed that all studied AR target genes exhibited a specific expression during the G1 phase of the cell cycle despite the constitutive expression of AR. Importantly, we found that the expression of NCoR, an AR co-repressor, was downregulated during the G1 phase and expressed as mRNA and protein specifically during the S phase. The role of NCoR in repressing androgen-induced expression of AR target genes in S phase was further demonstrated by altering expression of NCoR during the cell cycle through knockdown or induced overexpression. Using two alternative techniques we show that AR binds directly to target DNA in the chromatin only during the G1 phase. These data support the hypothesis that NCoR might control a cell cycle dependent regulation of expression AR target genes in prostate cells.