Regulating G protein-coupled receptors by topological inversion.

G protein-coupled receptors (GPCRs) are a family of proteins containing seven transmembrane helices, with the N- and C-terminus of the protein located at the extracellular space and cytosol, respectively. Here, we report that ceramide or related sphingolipids might invert the topology of many GPCRs that contain a GXXXN motif in their first transmembrane helix. The functional significance of this topological regulation is illustrated by the CCR5 chemokine receptor. In the absence of lipopolysaccharide (LPS), CCR5 adopts a topology consistent with that of GPCR, allowing mouse peritoneal macrophages to migrate toward its ligand CCL5. LPS stimulation results in increased production of dihydroceramide, which inverts the topology of CCR5, preventing macrophages from migrating toward CCL5. These results suggest that GPCRs may not always adopt the same topology and can be regulated through topological inversion. Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that major issues remain unresolved (see decision letter).

Identification of residues critical for topology inversion of the transmembrane protein TM4SF20 through regulated alternative translocation.

Adopting a proper topology is crucial for transmembrane proteins to perform their functions. We previously reported that ceramide regulates a transmembrane protein called TM4SF20 (transmembrane 4 L six family member 20) through topological inversion by altering the direction through which the protein is translocated across membranes during translation. This regulatory mechanism, denoted regulated alternative translocation (RAT), depends on a GXXXN motif present in the first transmembrane helix of TM4SF20. Here, using site-directed mutagenesis, we show that Asn-26 in the motif is crucial for RAT of TM4SF20, as it cannot be replaced even by Gln. In contrast, Gly-22 in the motif could be substituted by other small residues such as Ala and Ser without affecting RAT of TM4SF20. We further demonstrate that the GXXXN motif alone is insufficient to induce RAT of a transmembrane protein because TM4SF4, a relative of TM4SF20 that also contains the motif in the first transmembrane helix, did not undergo RAT. Using TM4SF40-TM4SF20 chimeras, we identified Pro-29 of TM4SF20 as another important element required for RAT of the protein. Substituting Pro-29 alone did not affect RAT of TM4SF20, whereas replacing Pro-29 together with either Leu-25 or Val-17 of TM4SF20 with the corresponding residues of TM4SF4 abolished RAT of TM4SF20. Because Val-17, Gly-22, Leu-25, Asn-26, and Pro-29 are predicted to reside along the same surface of the transmembrane helix, our results suggest that interactions with other proteins mediated by this surface during translocation may be critical for RAT of TM4SF20.

CREB3L1 as a potential biomarker predicting response of triple negative breast cancer to doxorubicin-based chemotherapy.

BACKGROUND: Doxorubicin-based chemotherapy is currently the most frequently used treatment for triple negative breast cancer (TNBC), yet the response rate is not high due to the lack of a biomarker allowing identification of responsive patients before the chemotherapy is initiated. We have demonstrated that doxorubicin inhibits proliferation of cancer cells through proteolytic activation of a transcription factor called CREB3L1 (cAMP response element binding protein 3-like 1), and that CREB3L1 expression in cancer cells is a key determinant of their sensitivity to doxorubicin when they are cultured in vitro or established as xenograft tumors in mice. The purpose of this study is to determine whether CREB3L1 expression in tumor cells of TNBC patients can be established as a biomarker to predict outcomes of doxorubicin-based chemotherapy. METHODS: We performed a retrospective analysis on breast core biopsy tissue samples taken from 18 TNBC patients before they were treated with doxorubicin-based chemotherapy. CREB3L1 expression in the cancer cells was analyzed by immunohistochemistry and quantified using the Immunoreactive Score (IRS). Outcomes of the chemotherapy were measured by the residual cancer burden (RCB) system. RESULTS: CREB3L1 expression levels in TNBC responsive to doxorubicin-based chemotherapy (RCB class 0-2) were significantly higher than that in resistant cancers (RCB class 3) (unpaired two-tailed t test, p = 0.0005; Statistical power 99.8 at 95% confidence level). All cancers expressing higher levels of CREB3L1 (IRS 4-12) responded to doxorubicin-based chemotherapy, whereas all cancers resisting the treatment expressed lower levels of CREB3L1 (IRS 0-3). CONCLUSIONS: These results suggest that CREB3L1 expression level may be used as a biomarker to identify TNBC patients who are more likely to benefit from doxorubicin-based chemotherapy.

Inverting the Topology of a Transmembrane Protein by Regulating the Translocation of the First Transmembrane Helix.

TM4SF20 (transmembrane 4 L6 family 20) is a polytopic membrane protein that inhibits proteolytic processing of CREB3L1 (cAMP response element-binding protein 3-like 1), a membrane-bound transcription factor that blocks cell division and activates collagen synthesis. Here we report that ceramide stimulates CREB3L1 cleavage by inverting the orientation of TM4SF20 in membranes. In the absence of ceramide, the N terminus of the first transmembrane helix of TM4SF20 is inserted into the endoplasmic reticulum (ER) lumen. This translocation requires TRAM2 (translocating chain-associated membrane protein 2), a membrane protein containing a putative ceramide-interacting domain. In the presence of ceramide, the N terminus of the first transmembrane domain of TM4SF20 is exposed to cytosol. Consequently, the membrane topology of TM4SF20 is inverted, and this form of TM4SF20 stimulates CREB3L1 cleavage. In the presence of ceramide, translocation of TM4SF20 is TRAM2-independent. We designate this mechanism-causing regulated inversion of the membrane topology as "regulated alternative translocation."

Identification of CREB3L1 as a Biomarker Predicting Doxorubicin Treatment Outcome.

BACKGROUND: Doxorubicin has been shown to inhibit proliferation of cancer cells through proteolytic activation of CREB3L1 (cAMP response element binding protein 3-like 1), a transcription factor synthesized as a membrane-bound precursor. Upon doxorubicin treatment, CREB3L1 is cleaved so that the N-terminal domain of the protein can reach the nucleus where it activates transcription of genes that inhibit cell proliferation. These results suggest that the level of CREB3L1 in cancer cells may determine their sensitivity to doxorubicin. METHODS: Mice transplanted with 6 lines of renal cell carcinoma (RCC) were injected with doxorubicin to observe the effect of the chemotherapy on tumor growth. Immunohistochemistry and bioinformatics analyses were performed to compare CREB3L1 levels in types of cancer known to respond to doxorubicin versus those resistant to doxorubicin. RESULTS: Higher levels of CREB3L1 protein are correlated with increased doxorubicin sensitivity of xenograft RCC tumors (p = 0.017 by Pearson analysis). From patient tumor biopsies we analyzed, CREB3L1 was expressed in 19% of RCC, which is generally resistant to doxorubicin, but in 70% of diffuse large B-cell lymphoma that is sensitive to doxorubicin. Doxorubicin is used as the standard treatment for cancers that express the highest levels of CREB3L1 such as osteosarcoma and malignant fibrous histiocytoma but is not generally used to treat those that express the lowest levels of CREB3L1 such as RCC. CONCLUSION: Identification of CREB3L1 as the biomarker for doxorubicin sensitivity may markedly improve the doxorubicin response rate by applying doxorubicin only to patients with cancers expressing CREB3L1.

Sustained induction of collagen synthesis by TGF-ß requires regulated intramembrane proteolysis of CREB3L1.

CREB3L1 (cAMP response element binding protein 3-like 1), a transcription factor synthesized as a membrane-bound precursor and activated through Regulated Intramembrane Proteolysis (RIP), is essential for collagen production by osteoblasts during bone development. Here, we show that TGF-ß (transforming growth factor-ß), a cytokine known to stimulate production of collagen during wound healing and fibrotic diseases, induces proteolytic activation of CREB3L1 in human A549 cells. This activation results from inhibition of expression of TM4SF20 (transmembrane 4 L6 family member 20), which normally inhibits RIP of CREB3L1. Cleavage of CREB3L1 releases its NH2-terminal domain from membranes, allowing it to enter the nucleus where it binds to Smad4 to activate transcription of genes encoding proteins required for assembly of collagen-containing extracellular matrix. Our findings raise the possibility that inhibition of RIP of CREB3L1 could prevent excess deposition of collagen in certain fibrotic diseases.

Epigenetic silencing of antiviral genes renders clones of Huh-7 cells permissive for hepatitis C virus replication.

Hepatitis C virus (HCV) does not replicate efficiently in wild-type human hepatoma Huh-7 cells, but it replicates robustly in certain subclones of Huh-7 cells. Previously, we demonstrated that silencing of cyclic AMP (cAMP) response element binding protein 3-like 1 (CREB3L1), a cellular transcription factor that inhibits HCV replication, allows HCV to replicate in HRP1 cells, a subclone of Huh-7 cells permissive for HCV replication. Here we show that silencing of myxovirus resistant 1 (MX1), a known interferon-induced antiviral gene, is responsible for HRP4 cells, another subclone of Huh-7 cells, being permissive for HCV replication. Both CREB3L1 and MX1 are epigenetically silenced through DNA methylation in HRP1 and HRP4 cells, respectively. We further demonstrate that Huh-7 cells exist as a mixed population of cells with distinct patterns of gene methylation and HCV replicates in subpopulations of Huh-7 cells that have antiviral genes epigenetically silenced by DNA hypermethylation. Our results demonstrate that understanding the mechanism through which subclones of Huh-7 cells become permissive for HCV replication is crucial for studying their interaction with HCV.

Doxorubicin blocks proliferation of cancer cells through proteolytic activation of CREB3L1.

Doxorubicin is used extensively for chemotherapy of diverse types of cancer, yet the mechanism through which it inhibits proliferation of cancer cells remains unclear. Here we report that doxorubicin stimulates de novo synthesis of ceramide, which in turn activates CREB3L1, a transcription factor synthesized as a membrane-bound precursor. Doxorubicin stimulates proteolytic cleavage of CREB3L1 by Site-1 Protease and Site-2 Protease, allowing the NH(2)-terminal domain of CREB3L1 to enter the nucleus where it activates transcription of genes encoding inhibitors of the cell cycle, including p21. Knockdown of CREB3L1 mRNA in human hepatoma Huh7 cells and immortalized human fibroblast SV589 cells conferred increased resistance to doxorubicin, whereas overexpression of CREB3L1 in human breast cancer MCF-7 cells markedly enhanced the sensitivity of these cells to doxorubicin. These results suggest that measurement of CREB3L1 expression may be a useful biomarker in identifying cancer cells sensitive to doxorubicin.DOI:http://dx.doi.org/10.7554/eLife.00090.001.

The membrane-bound transcription factor CREB3L1 is activated in response to virus infection to inhibit proliferation of virus-infected cells.

CREB3L1/OASIS is a cellular transcription factor synthesized as a membrane-bound precursor and activated by regulated intramembrane proteolysis in response to stimuli like ER stress. Comparing gene expression between Huh7 subclones that are permissive for hepatitis C virus (HCV) replication versus the nonpermissive parental Huh7 cells, we identified CREB3L1 as a host factor that inhibits proliferation of virus-infected cells. Upon infection with diverse DNA and RNA viruses, including murine γ-herpesvirus 68, HCV, West Nile virus (WNV), and Sendai virus, CREB3L1 was proteolytically cleaved, allowing its NH(2) terminus to enter the nucleus and induce multiple genes encoding inhibitors of the cell cycle to block cell proliferation. Consistent with this, we observed a necessity for CREB3L1 expression to be silenced in proliferating cells that harbor replicons of HCV or WNV. Our results indicate that CREB3L1 may play an important role in limiting virus spread by inhibiting proliferation of virus-infected cells.

Capacity for stochastic self-renewal and differentiation in mammalian spermatogonial stem cells.

Mammalian spermatogenesis is initiated and sustained by spermatogonial stem cells (SSCs) through self-renewal and differentiation. The basic question of whether SSCs have the potential to specify self-renewal and differentiation in a cell-autonomous manner has yet to be addressed. Here, we show that rat SSCs in ex vivo culture conditions consistently give rise to two distinct types of progeny: new SSCs and differentiating germ cells, even when they have been exposed to virtually identical microenvironments. Quantitative experimental measurements and mathematical modeling indicates that fate decision is stochastic, with constant probability. These results reveal an unexpected ability in a mammalian SSC to specify both self-renewal and differentiation through a self-directed mechanism, and further suggest that this mechanism operates according to stochastic principles. These findings provide an experimental basis for autonomous and stochastic fate choice as an alternative strategy for SSC fate bifurcation, which may also be relevant to other stem cell types.

Spermatogonial stem cell self-renewal requires OCT4, a factor downregulated during retinoic acid-induced differentiation.

The long-term production of billions of spermatozoa relies on the regulated proliferation and differentiation of spermatogonial stem cells (SSCs). To date only a few factors are known to function in SSCs to provide this regulation. Octamer-4 (OCT4) plays a critical role in pluripotency and cell survival of embryonic stem cells and primordial germ cells; however, it is not known whether it plays a similar function in SSCs. Here, we show that OCT4 is required for SSC maintenance in culture and for colonization activity following cell transplantation, using lentiviral-mediated short hairpin RNA expression to knock down OCT4 in an in vitro model for SSCs ("germline stem" [GS] cells). Expression of promyelocytic leukemia zinc-finger (PLZF), a factor known to be required for SSC self-renewal, was not affected by OCT4 knockdown, suggesting that OCT4 does not function upstream of PLZF. In addition to developing a method to test specific gene function in GS cells, we demonstrate that retinoic acid (RA) triggers GS cells to shift to a differentiated, premeiotic state lacking OCT4 and PLZF expression and colonization activity. Our data support a model in which OCT4 and PLZF maintain SSCs in an undifferentiated state and RA triggers spermatogonial differentiation through the direct or indirect downregulation of OCT4 and PLZF. The current study has important implications for the future use of GS cells as an in vitro model for spermatogonial stem cell biology or as a source of embryonic stem-like cells. Disclosure of potential conflicts of interest is found at the end of this article.

Sohlh2 knockout mice are male-sterile because of degeneration of differentiating type A spermatogonia.

The spermatogenesis and oogenesis-specific transcription factor Sohlh2 is normally expressed only in premeiotic germ cells. In this study, Sohlh2 and several other germ cell transcripts were found to be induced in mouse embryonic stem cells when cultured on a feeder cell line that overexpresses bone morphogenetic protein 4. To study the function of Sohlh2 in germ cells, we generated mice harboring null alleles of Sohlh2. Male Sohlh2-deficient mice were infertile because of a block in spermatogenesis. Although normal prior to birth, Sohlh2-null mice had reduced numbers of intermediate and type B spermatogonia by postnatal day 7. By day 10, development to the preleptotene spermatocyte stage was severely disrupted, rendering seminiferous tubules with only Sertoli cells, undifferentiated spermatogonia, and degenerating colonies of differentiating spermatogonia. Degenerating cells resembled type A2 spermatogonia and accumulated in M-phase prior to death. A similar phenotype was observed in Sohlh2-null mice on postnatal days 14, 21, 35, 49, 68, and 151. In adult Sohlh2-mutant mice, the ratio of undifferentiated type A spermatogonia (DAZL+/PLZF+) to differentiating type A spermatogonia (DAZL+/PLZF-) was twice normal levels. In culture, undifferentiated type A spermatogonia isolated from Sohlh2-null mice proliferated normally but linked the mutant phenotype to aberrant cell surface expression of the receptor-tyrosine kinase cKit. Thus, Sohlh2 is required for progression of differentiating type A spermatogonia into type B spermatogonia. One conclusion originating from these studies would be that testicular factors normally regulate the viability of differentiating spermatogonia by signaling through Sohlh2. This regulation would provide a crucial checkpoint to optimize the numbers of spermatocytes entering meiosis during each cycle of spermatogenesis. Disclosure of potential conflicts of interest is found at the end of this article.