LARP7 suppresses P-TEFb activity to inhibit breast cancer progression and metastasis.
Bottom Line: The positive transcription elongation factor b (P-TEFb) stimulates transcriptional elongation by phosphorylating Pol II and antagonizing negative elongation factors.A reservoir of P-TEFb is sequestered in the inactive 7SK snRNP where 7SK snRNA and the La-related protein LARP7 are required for the integrity of this complex.Decreased levels of LARP7 and 7SK snRNA redistribute P-TEFb to the transcriptionally active super elongation complex, resulting in P-TEFb activation and increased transcription of EMT transcription factors, including Slug, FOXC2, ZEB2, and Twist1, to promote breast cancer EMT, invasion, and metastasis.
Affiliation: Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, United States.Show MeSH
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Mentions: In light of the above demonstrations that LARP7 inhibits EMT and that downregulation of LARP7 occurs in invasive, but not noninvasive breast cancer tissues and cell lines, we asked whether downregulation of LARP7 via shRNA could directly promote malignant progression of noninvasive breast cancer cells. To test this, we stably knocked down LARP7 in two noninvasive breast cancer cell lines T47D (Figure 3A) and BT474 (Figure 3—figure supplement 1A). This resulted in a reduced 7SK snRNA level and caused a significant increase in cell proliferation and anchorage-independent growth in soft agar (Figure 3B–D, Figure 3—figure supplement 1B–D). The LARP7 KD cells also exhibited dramatically increased EMT as evidenced by accelerated cell migration and invasion (Figure 3E,F, Figure 3—figure supplement 1E), downregulation of epithelial markers such as E-cadherin, DSP, and KRT19 and upregulation of mesenchymal markers Vimentin, N-cadherin, and matrix metalloproteinases (MMPs) (Figure 3H, Figure 3—figure supplement 1F). Since EMT has recently been linked to the expansion of cancer stem cells (CSCs) (Mani et al., 2008; Bessede et al., 2013), we also examined whether the LARP7 KD cells displayed an increased CSC feature in the two-round mammosphere formation assay. Indeed, the KD cells generated bigger and larger number of mammospheres (Figure 3G) and displayed elevated levels of various stem cell markers including Oct4, Sox2, and ALDH1 (Figure 3H).10.7554/eLife.02907.007Figure 3.Silencing LARP7 promotes malignant progression of T47D breast cancer cells.
Affiliation: Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, United States.