Tumor metastasis: miR-9 under spotlight

miR-9 is a MYC-induced microRNA that suppresses E-cadherin expression and upregulates β-catenin signaling, leading to induced tumor cell migration and metastasis.

The list of microRNAs (miRs) involved in cancer metastasis is growing quickly. In their recent publication in Nature Cell Biology, Robert Weinberg and colleagues now demonstrate that MYC-induced expression of miR-9 downregulates E-cadherin expression, upregulates β-catenin signaling and VEGF expression, leading to increased cell migration and metastasis of breast cancer cells.

E-cadherin is an essential component of epithelial adherens junctions. Ectopic expression of miR-9 in epithelial cells reduced E-cadherin expression and resulted in mesenchymal-like morphologies. The conversion of epithelial cancer cells to cells with more mesenchymal characteristics has been implicated in promoting carcinoma invasion and metastasis. Consistently, a three-to-five fold increase in cancer cell motility was observed following miR-9-induced downregulation of E-cadherin expression. Conversely, expression of an E-cadherin construct lacking the miR-9 recognition site, and therefore resistant to miR-9-mediated suppression, reversed miR-9-induced cell migration and invasion in vitro. This result suggests that downregulation of E-cadherin is required for miR-9-mediated effects on cancer cells.

E-cadherin knock-down was previously shown to increase nuclear localization of β-catenin, which is normally sequestered at the adherens junctions and, when localized in the nucleus, activates gene transcription. In miR-9-expressing cancer cells, β-catenin was localized both to the cytoplasm and to the nucleus, and β-catenin signaling was activated. In addition, mRNA expression of the pro-angiogenic factor, VEGF, was also upregulated in miR-9-expressing cells, indicating that miR-9 could also affect tumor angiogenesis. Interestingly, expression of a short interfering RNA (siRNA) against E-cadherin or a constitutively active β-catenin was not sufficient to induce VEGF, suggesting that some additional miR-9 targets were involved in the upregulation of VEGF expression in cancer cells.

But, what is the mechanism leading to miR-9 expression in cancer cells? MYC is an oncogene known to directly regulate the expression of various miRs in cancer cells. Ectopic expression of MYC in breast cancer cells caused upregulation of miR-9 expression, and genome-wide chromatin immunoprecipitation (ChIP) experiments showed that both MYC and MYCN directly interact with the mir-9-3 locus to activate its expression.

Finally, high levels of miR-9 were also observed in clinical breast cancer samples from patients with metastatic tumors, and in vivo functional knock-down of miR-9 with a 'miRNA sponge' — an mRNA that acts by competing with natural targets for binding to a given miRNA — suppressed breast cancer cell metastasis to the lung in mice. Taken together, these data indicate that MYC-mediated miR-9 expression has a significant role in tumor metastasis, via regulation of E-cadherin, β-catenin and VEGF expression.

Iley Ozerlat
Cell Migration Gateway

Reference:

Ma L. et al.
miR-9, a MYC/MYCN-activated microRNA, regulates E-cadherin and cancer metastasis.
Nature Cell Biology 12, 247-256 (2010).
Full text | PDF | Subscribe to Nature Cell Biology

Therapeutics: Selective inhibitors gain traction

A selective TORC1 and TORC2 active site inhibitor has high efficacy and tolerability in models of acute leukaemia.

A great deal of evidence points to the crucial role of mTOR in several human malignancies, perhaps reflecting its regulation by upstream oncogenic PI3K signalling pathways and its regulation of downstream cell growth, survival and proliferation pathways. mTOR can form two complexes, mTOR complex 1 (TORC1) and TORC2. However, the realization that TORC2 is insensitive to the allosteric mTOR inhibitor rapamycin, and that rapamycin increases PI3K activity by suppressing a negative feedback loop, together with recent reports that rapamycin is an incomplete inhibitor of TORC1, have led researchers to explore alternative mTOR inhibitors.

Reporting in Nature Medicine, David Fruman and colleagues compared rapamycin with the selective mTOR active site inhibitor PP242. Consistent with previous findings, inhibition of the mTOR active site blocked TORC1 and TORC2 signalling without aberrantly affecting the negative feedback of PI3K in several mouse and human BCR–ABL (Philadelphia chromosome positive (Ph⁺)) leukaemic cell lines. PP242 also had a greater cytotoxic activity and antiproliferative potency towards leukaemia cells than rapamycin. With a view to improving treatment outcomes, drug combinations were assayed in newly diagnosed and relapsed or refractory Ph⁺ B precursor acute lymphoblastic leukaemia (B-ALL) patient samples. PP242, together with the second-generation tyrosine kinase inhibitor (TKI) dasatinib, exhibited significantly reduced colony formation compared with either drug alone or a combination of rapamycin and dasatinib.

Encouraged, the authors switched to in vivo models. In mice transplanted with p190 BCR–ABL-expressing mouse haematopoietic progenitor cells, PP242 dosing extended survival and enhanced the efficacy of the first-line BCR–ABL TKI imatinib more effectively than rapamycin. However, Ph⁺ patients with B-ALL can acquire resistance to this TKI. Therefore, on the basis of the earlier in vitro success with dasatinib, treatment with this drug was assessed in human Ph⁺ B-ALL cell lines and primary human Ph⁺ B-ALL xenograft models and found to synergize with PP242 resulting in disease regression. Together, these data suggest the potential therapeutic benefits of targeting both upstream tyrosine kinases and mTOR specifically through dasatinib–PP242 combination therapy.

The pan-PI3K–mTOR active site inhibitor PI-103 also exhibited strong anti-Ph⁺ B-ALL efficacy, so the authors sought to tease out the tolerability for rapamycin, PI-103 and PP242. As expected, rapamycin was immunosuppressive both in vitro and in vivo. But surprisingly, when administered alone and in combination with dasatinib, PP242 produced much less immunosuppression than PI-103.

Therefore, this promising blend of efficacy and tolerability in mouse models indicates that the further development of selective TORC1 and TORC2 inhibitors to treat leukaemia is warranted.

Kira Anthony
Pathway Interaction Database

Reference:
Janes, M. R. et al.
Effective and selective targeting of leukemia cells using a TORC1/2 kinase inhibitor
Nature Med. 13 Jan 2010
Full text | PDF | Subscribe to Nature Medicine