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The dysregulation of miRNAs has been increasingly recognized as a critical mediator of cancer development and progression. Here, we show that frequent deletion of the locus is associated with poor prognosis in primary breast cancer. Forced expression of miR-135a decreased breast cancer progression, while inhibition of miR-135a with a specific miRNA sponge elicited opposing effects, suggestive of a tumor suppressive role of miR-135a in breast cancer. Estrogen receptor alpha (ERα) bound the promoter of for its transcriptional activation, whereas tamoxifen treatment inhibited expression of miR-135a in ERα breast cancer cells. miR-135a directly targeted and, forming a negative feedback loop to inhibit ERα signaling. This regulatory feedback between miR-135a and ERα demonstrated that miR-135a regulated the response to tamoxifen. The tamoxifen-mediated decrease in miR-135a expression increased the expression of miR-135a targets to reduce tamoxifen sensitivity. Consistently, miR-135a expression was downregulated in ERα breast cancer cells with acquired tamoxifen resistance, while forced expression of miR-135a partially resensitized these cells to tamoxifen. Tamoxifen resistance mediated by the loss of miR-135a was shown to be partially dependent on the activation of the ERK1/2 and AKT pathways by miR-135a-targeted genes. Taken together, these results indicate that deletion of the locus and decreased miR-135a expression promote ERα breast cancer progression and tamoxifen resistance. Loss of miR-135a in breast cancer disrupts an estrogen receptor-induced negative feedback loop, perpetuating disease progression and resistance to therapy. http://cancerres.aacrjournals.org/content/canres/78/17/4915/F1.large.jpg .

Hypoxia has been reported to regulate the cancer stem cell (CSC) population yet the underlying mechanism is poorly characterized. Herein, we show that Artemin (ARTN), a member of the glial cell derived neurotrophic factor family of ligands, is a hypoxia-responsive factor and is essential for hypoxia-induced CSC expansion in hepatocellular carcinoma (HCC). Clinically, elevated expression of ARTN in HCC was associated with larger tumor size, faster relapse and shorter survival. In vitro, HCC cells with forced expression of ARTN exhibited reduced apoptosis, increased proliferation, epithelial-mesenchymal transition (EMT) and enhanced motility. Additionally, ARTN dramatically increased xenograft tumor size and metastasis in vivo. Moreover, ARTN also enhanced tumorsphere formation and the tumor initiating capacity of HCC cells, consequent to expansion of the CD133+ CSC population. ARTN transcription was directly activated by hypoxia-induced factor-1α (HIF-1α) and hypoxia induced ARTN promoted EMT and increased the CSC population via AKT signaling. We herein identify a novel HIF-1α/ARTN axis promoting CSC-like behavior in hypoxic environments which implicates ARTN as a valuable therapeutic target for HCC.

The serine/threonine protein kinase Akt regulates a wide range of cellular functions via phosphorylation of various substrates distributed throughout the cell, including at the plasma membrane and endomembrane compartments. Disruption of compartmentalized Akt signaling underlies the pathology of many diseases such as cancer and diabetes. However, the specific spatial organization of Akt activity and the underlying regulatory mechanisms, particularly the mechanism controlling its activity at the lysosome, are not clearly understood. We developed a highly sensitive excitation-ratiometric Akt activity reporter (ExRai-AktAR2), enabling the capture of minute changes in Akt activity dynamics at subcellular compartments. In conjunction with super-resolution expansion microscopy, we found that growth factor stimulation leads to increased colocalization of Akt with lysosomes and accumulation of lysosomal Akt activity. We further showed that 3-phosphoinositides (3-PIs) accumulate on the lysosomal surface, in a manner dependent on dynamin-mediated endocytosis. Importantly, lysosomal 3-PIs are needed for growth-factor-induced activities of Akt and mechanistic target of rapamycin complex 1 (mTORC1) on the lysosomal surface, as targeted depletion of 3-PIs has detrimental effects. Thus, 3-PIs, a class of critical lipid second messengers that are typically found in the plasma membrane, unexpectedly accumulate on the lysosomal membrane in response to growth factor stimulation, to direct the multifaceted kinase Akt to organize lysosome-specific signaling.

The mechanistic target of rapamycin complex 1 (mTORC1) integrates growth, nutrient and energy status cues to control cell growth and metabolism. While mTORC1 activation at the lysosome is well characterized, it is not clear how this complex is regulated at other subcellular locations. Here, we combine location-selective kinase inhibition, live-cell imaging and biochemical assays to probe the regulation of growth factor-induced mTORC1 activity in the nucleus. Using a nuclear targeted Akt Substrate-based Tandem Occupancy Peptide Sponge (Akt-STOPS) that we developed for specific inhibition of Akt, a critical upstream kinase, we show that growth factor-stimulated nuclear mTORC1 activity requires nuclear Akt activity. Further mechanistic dissection suggests that nuclear Akt activity mediates growth factor-induced nuclear translocation of Raptor, a regulatory scaffolding component in mTORC1, and localization of Raptor to the nucleus results in nuclear mTORC1 activity in the absence of growth factor stimulation. Taken together, these results reveal a mode of regulation of mTORC1 that is distinct from its lysosomal activation, which controls mTORC1 activity in the nuclear compartment.

No abstract

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mTOR complex 1 (mTORC1) is a key signaling hub that regulates many fundamental cellular processes, including macromolecule synthesis, proliferation and autophagy. Abnormalities in mTORC1 signaling pathway are implicated in pathological conditions such as the progression of cancer and diabetes. Diverse signals can regulate mTORC1 activity and spatiotemporal regulation of mTORC1 has been suggested to play a crucial role in achieving high signaling specificity. Previously, a fluorescent mTORC1 activity reporter (TORCAR) revealed nuclear mTORC1 activity in live cells. However, the specific mechanisms that regulate nuclear mTORC1 activity and the functionalities of nuclear mTORC1 are unclear. In this study, we investigated whether and how Rheb, a key mTORC1 activator, is involved in regulating growth factor induced nuclear mTORC1 activity. First, we showed knockdown of Rheb largely suppressed growth factor stimulated nuclear TORCAR response. We further showed that Tuberous sclerosis complex subunit 2 (TSC2), a GAP for Rheb, was absent from the nucleus, and overexpressing TSC2 exclusively in the nucleus significantly inhibited nuclear TORCAR response, suggesting that Rheb is constitutively active in the nucleus. Our experimental data established a model of nuclear mTORC1 regulation and suggested Rheb is indispensable for nuclear mTORC1 activity. Our ongoing experiments include using phosphoproteomic experiments to identify the substrates of nuclear mTORC1 and RNAseq experiments to examine the emerging role of nuclear mTORC1 in controlling transcription in the nucleus.