Kunxin Luo

The 7SK small nuclear RNA inhibits the CDK9/cyclin T1 kinase to control transcription.

The human positive transcription elongation factor P-TEFb, consisting of a CDK9/cyclin T1 heterodimer, functions as both a general and an HIV-1 Tat-specific transcription factor. P-TEFb activates transcription by phosphorylating RNA polymerase (Pol) II, leading to the formation of processive elongation complexes. As a Tat cofactor, P-TEFb stimulates HIV-1 transcription by interacting with Tat and the transactivating responsive (TAR) RNA structure located at the 5′ end of the nascent viral transcript. Here we identified 7SK, an abundant and evolutionarily conserved small nuclear RNA (snRNA) of unknown function, as a specific P-TEFb-associated factor. 7SK inhibits general and HIV-1 Tat-specific transcriptional activities of P-TEFb in vivo and in vitro by inhibiting the kinase activity of CDK9 and preventing recruitment of P-TEFb to the HIV-1 promoter. 7SK is efficiently dissociated from P-TEFb by treatment of cells with ultraviolet irradiation and actinomycin D. As these two agents have been shown to significantly enhance HIV-1 transcription and phosphorylation of Pol II (refs 6,7,8), our data provide a mechanistic explanation for their stimulatory effects. The 7SK/P-TEFb interaction may serve as a principal control point for the induction of cellular and HIV-1 viral gene expression during stress-related responses. Our studies demonstrate the involvement of an snRNA in controlling the activity of a Cdk–cyclin kinase.

A La-Related Protein Modulates 7SK snRNP Integrity to Suppress P-TEFb-Dependent Transcriptional Elongation and Tumorigenesis

The general transcription factor P-TEFb stimulates RNA polymerase II elongation and cotranscriptional processing of pre-mRNA. Contributing to a functional equilibrium important for growth control, a reservoir of P-TEFb is maintained in an inactive snRNP where 7SK snRNA is a central scaffold. Here, we identify PIP7S as a La-related protein stably associated with and required for 7SK snRNP integrity. PIP7S binds and stabilizes nearly all the nuclear 7SK via 3 -UUU-OH, leading to the sequestration and inactivation of P-TEFb. This function requires its La domain and intact C terminus. The latter is frequently deleted in human tumors due to microsatellite instability-associated mutations. Consistent with the tumor suppressor role of a Drosophila homolog of PIP7S, loss of PIP7S function shifts the P-TEFb equilibrium toward the active state, disrupts epithelial differentiation, and causes P-TEFb-dependent malignant transformation. Through PIP7S modulation of P-TEFb, our data thus link a general elongation factor to growth control and tumorigenesis.

SnoN in mammalian development, function and diseases.

SnoN (Ski-novel protein) was discovered as a nuclear proto-oncogene on the basis of its ability to induce transformation of chicken and quail embryonic fibroblasts. As a crucial negative regulator of transforming growth factor-β (TGF-β) signaling and also an activator of p53, it plays an important role in regulating cell proliferation, senescence, apoptosis, and differentiation. Recent studies of its expression patterns and functions in mouse models and mammalian cells have revealed important functions of SnoN in normal epithelial development and tumorigenesis. Evidence suggests that SnoN has both pro-oncogenic and anti-oncogenic functions by modulating multiple signaling pathways. These studies suggest that SnoN may have broad functions in the development and homeostasis of embryonic and postnatal tissues.

Transforming Growth Factor-β Regulator SnoN Modulates Mammary Gland Branching Morphogenesis, Postlactational Involution, and Mammary Tumorigenesis

SnoN is an important negative regulator of transforming growth factor-beta (TGF-beta) signaling that was originally identified as a transforming oncogene in chicken embryonic fibroblasts. Both pro-oncogenic and antioncogenic activities of SnoN have been reported, but its function in normal epithelial cells has not been defined. In the mouse mammary gland, SnoN is expressed at relatively low levels, but it is transiently upregulated at late gestation before being downregulated during lactation and early involution. To assess the effects of elevated levels of SnoN, we generated transgenic mice expressing a SnoN fragment under the control of the mouse mammary tumor virus promoter. In this model system, SnoN elevation increased side-branching and lobular-alveolar proliferation in virgin glands, while accelerating involution in postlactation glands. Increased proliferation stimulated by SnoN was insufficient to induce mammary tumorigenesis. In contrast, elevated levels of SnoN cooperated with polyoma middle T antigen to accelerate the formation of aggressive multifocal adenocarcinomas and to increase the formation of pulmonary metastases. Our studies define functions of SnoN in mammary epithelial cell proliferation and involution, and provide the first in vivo evidence of a pro-oncogenic role for SnoN in mammalian tumorigenesis.

SnoN regulates mammary gland alveologenesis and onset of lactation by promoting prolactin/Stat5 signaling

Mammary epithelial cells undergo structural and functional differentiation at late pregnancy and parturition to produce and secrete milk. Both TGF-β and prolactin pathways are crucial regulators of this process. However, how the activities of these two antagonistic pathways are orchestrated to initiate lactation has not been well defined. Here, we show that SnoN, a negative regulator of TGF-β signaling, coordinates TGF-β and prolactin signaling to control alveologenesis and lactogenesis. SnoN expression is induced at late pregnancy by the coordinated actions of TGF-β and prolactin. The elevated SnoN promotes Stat5 signaling by enhancing its stability, thereby sharply increasing the activity of prolactin signaling at the onset of lactation. SnoN–/– mice display severe defects in alveologenesis and lactogenesis, and mammary epithelial cells from these mice fail to undergo proper morphogenesis. These defects can be rescued by an active Stat5. Thus, our study has identified a new player in the regulation of milk production and revealed a novel function of SnoN in mammary alveologenesis and lactogenesis in vivo through promotion of Stat5 signaling.

Expression Profiles of SnoN in Normal and Cancerous Human Tissues Support Its Tumor Suppressor Role in Human Cancer

SnoN is a negative regulator of TGF-β signaling and also an activator of the tumor suppressor p53 in response to cellular stress. Its role in human cancer is complex and controversial with both pro-oncogenic and anti-oncogenic activities reported. To clarify its role in human cancer and provide clinical relevance to its signaling activities, we examined SnoN expression in normal and cancerous human esophageal, ovarian, pancreatic and breast tissues. In normal tissues, SnoN is expressed in both the epithelium and the surrounding stroma at a moderate level and is predominantly cytoplasmic. SnoN levels in all tumor epithelia examined are lower than or similar to that in the matched normal samples, consistent with its anti-tumorigenic activity in epithelial cells. In contrast, SnoN expression in the stroma is highly upregulated in the infiltrating inflammatory cells in high-grade esophageal and ovarian tumor samples, suggesting that SnoN may potentially promote malignant progression through modulating the tumor microenvironment in these tumor types. The overall levels of SnoN expression in these cancer tissues do not correlate with the p53 status. However, in human cancer cell lines with amplification of the snoN gene, a strong correlation between increased SnoN copy number and inactivation of p53 was detected, suggesting that the tumor suppressor SnoN-p53 pathway must be inactivated, either through downregulation of SnoN or inactivation of p53, in order to allow cancer cell to proliferate and survive. These data strongly suggest that SnoN can function as a tumor suppressor at early stages of tumorigenesis in human cancer tissues.

SnoN facilitates ALK1–Smad1/5 signaling during embryonic angiogenesis

SnoN directly binds ALK1 on the plasma membrane, enhancing Smad1/5 activation, and is required for normal angiogenesis.

SnoN Antagonizes the Hippo Kinase Complex to Promote TAZ Signaling during Breast Carcinogenesis

SnoN regulates multiple signaling pathways, including TGF-β/Smad and p53, and displays both pro-oncogenic and anti-oncogenic activities in human cancer. We have observed previously that both its intracellular localization and expression levels are sensitive to cell density, suggesting that it may crosstalk with Hippo signaling. Here we report that, indeed, SnoN interacts with multiple components of the Hippo pathway to inhibit the binding of Lats2 to TAZ and the subsequent phosphorylation of TAZ, leading to TAZ stabilization. Consistently, SnoN enhances the transcriptional and oncogenic activities of TAZ, and reducing SnoN decreases TAZ expression as well as malignant progression of breast cancer cells. Interestingly, SnoN itself is downregulated by Lats2 that is activated by the Scribble basolateral polarity protein. Thus, SnoN is a critical component of the Hippo regulatory network that receives signals from the tissue architecture andxa0polarity to coordinate the activity of intracellular signaling pathways.