Developmental signaling pathways regulating mammary stem cells and contributing to the etiology of triple-negative breast cancer.

Cancer has been considered as temporal and spatial aberrations of normal development in tissues. Similarities between mammary embryonic development and cell transformation suggest that the underlying processes required for mammary gland development are also those perturbed during various stages of mammary tumorigenesis and breast cancer (BC) development. The master regulators of embryonic development Cripto-1, Notch/CSL, and Wnt/ß-catenin play key roles in modulating mammary gland morphogenesis and cell fate specification in the embryo through fetal mammary stem cells (fMaSC) and in the adult organism particularly within the adult mammary stem cells (aMaSC), which determine mammary progenitor cell lineages that generate the basal/myoepithelial and luminal compartments of the adult mammary gland. Together with recognized transcription factors and embryonic stem cell markers, these embryonic regulatory molecules can be inappropriately augmented during tumorigenesis to support the tumor-initiating cell (TIC)/cancer stem cell (CSC) compartment, and the effects of their deregulation may contribute for the etiology of BC, in particular the most aggressive subtype of BC, triple-negative breast cancer (TNBC). This in depth review will present evidence of the involvement of Cripto-1, Notch/CSL, and Wnt/ß-catenin in the normal mammary gland morphogenesis and tumorigenesis, from fMaSC/aMaSC regulation to TIC generation and maintenance in TNBC. Specific therapies for treating TNBC by targeting these embryonic pathways in TICs will be further discussed, providing new opportunities to destroy not only the bulk tumor, but also TICs that initiate and promote the metastatic spread and recurrence of this aggressive subtype of BC.

Cripto-1 ablation disrupts alveolar development in the mouse mammary gland through a progesterone receptor-mediated pathway.

Cripto-1, a member of the epidermal growth factor-Cripto-1/FRL-1/Cryptic family, is critical for early embryonic development. Together with its ligand Nodal, Cripto-1 has been found to be associated with the undifferentiated status of mouse and human embryonic stem cells. Several studies have clearly shown that Cripto-1 is involved in regulating branching morphogenesis and epithelial-mesenchymal transition of the mammary gland both in vitro and in vivo and together with the cofactor GRP78 is critical for the maintenance of mammary stem cells ex vivo. Our previous studies showed that mammary-specific overexpression of human Cripto-1 exhibited dramatic morphological alterations in nulliparous mice mammary glands. The present study shows a novel mechanism for Cripto-1 regulation of mammary gland development through direct effects on progesterone receptor expression and pathways regulated by progesterone in the mammary gland. We demonstrate a strict temporal regulation of mouse Cripto-1 (mCripto-1) expression that occurs during mammary gland development and a stage-specific function of mCripto-1 signaling during mammary gland development. Our data suggest that Cripto-1, like the progesterone receptor, is not required for the initial ductal growth but is essential for subsequent side branching and alveologenesis during the initial stages of pregnancy. Dissection of the mechanism by which this occurs indicates that mCripto-1 activates receptor activator NF-κB/receptor activator NF-κB ligand, and NF-κB signaling pathways.

The multifaceted role of the embryonic gene Cripto-1 in cancer, stem cells and epithelial-mesenchymal transition.

Cripto-1 (CR-1)/Teratocarcinoma-derived growth factor1 (TDGF-1) is a cell surface glycosylphosphatidylinositol (GPI)-linked glycoprotein that can function either in cis (autocrine) or in trans (paracrine). The cell membrane cis form is found in lipid rafts and endosomes while the trans acting form lacking the GPI anchor is soluble. As a member of the epidermal growth factor (EGF)/Cripto-1-FRL-1-Cryptic (CFC) family, CR-1 functions as an obligatory co-receptor for the transforming growth factor-ß (TGF-ß) family members, Nodal and growth and differentiation factors 1 and 3 (GDF1/3) by activating Alk4/Alk7 signaling pathways that involve Smads 2, 3 and 4. In addition, CR-1 can activate non-Smad-dependent signaling elements such as PI3K, Akt and MAPK. Both of these pathways depend upon the 78kDa glucose regulated protein (GRP78). Finally, CR-1 can facilitate signaling through the canonical Wnt/ß-catenin and Notch/Cbf-1 pathways by functioning as a chaperone protein for LRP5/6 and Notch, respectively. CR-1 is essential for early embryonic development and maintains embryonic stem cell pluripotentiality. CR-1 performs an essential role in the etiology and progression of several types of human tumors where it is expressed in a population of cancer stem cells (CSCs) and facilitates epithelial-mesenchymal transition (EMT). In this context, CR-1 can significantly enhance tumor cell migration, invasion and angiogenesis. Collectively, these facts suggest that CR-1 may be an attractive target in the diagnosis, prognosis and therapy of several types of human cancer.

Differential inhibitory effects of cyanovirin-N, griffithsin, and scytovirin on entry mediated by envelopes of gammaretroviruses and deltaretroviruses.

The antiviral lectins griffithsin (GRFT), cyanovirin-N (CV-N), and scytovirin (SVN), which inhibit several enveloped viruses, including lentiviruses, were examined for their ability to inhibit entry mediated by Env proteins of delta- and gammaretroviruses. The glycoproteins from human T-cell leukemia virus type 1 (HTLV-1) were resistant to the antiviral effects of all three lectins. For gammaretroviruses, CV-N inhibited entry mediated by some but not all of the envelopes examined, whereas GRFT and SVN displayed only little or no effect.

Cripto-1: an extracellular protein - connecting the sequestered biological dots.

Cripto-1 (CR-1) is a multifunctional embryonic protein that is re-expressed during inflammation, wound repair, and malignant transformation. CR-1 can function either as a tethered co-receptor or shed as a free ligand underpinning its flexible role in cell physiology. CR-1 has been shown to mediate cell growth, migration, invasion, and induce epithelial to mesenchymal transition (EMT). The main signaling pathways mediating CR-1 effects include Nodal-dependent (Smad2/3) and Nodal-independent (Src/p44/42/Akt) signaling transduction pathways. In addition, there are several naturally occurring binding partner proteins (BPPs) for CR-1 that can either agonize or antagonize its bioactivity. We will review the collective role of CR-1 as an extracellular protein, discuss caveats to consider in developing a quantitation assay, define possible mechanistic avenues applicable for drug discovery, and report on our experimental approaches to overcome these problematic issues.

The receptor complex associated with human T-cell lymphotropic virus type 3 (HTLV-3) Env-mediated binding and entry is distinct from, but overlaps with, the receptor complexes of HTLV-1 and HTLV-2.

Little is known about the transmission or tropism of the newly discovered human retrovirus, human T-cell lymphotropic virus type 3 (HTLV-3). Here, we examine the entry requirements of HTLV-3 using independently expressed Env proteins. We observed that HTLV-3 surface glycoprotein (SU) binds efficiently to both activated CD4(+) and CD8(+) T cells. This contrasts with both HTLV-1 SU, which primarily binds to activated CD4(+) T cells, and HTLV-2 SU, which primarily binds to activated CD8(+) T cells. Binding studies with heparan sulfate proteoglycans (HSPGs) and neuropilin-1 (NRP-1), two molecules important for HTLV-1 entry, revealed that these molecules also enhance HTLV-3 SU binding. However, unlike HTLV-1 SU, HTLV-3 SU can bind efficiently in the absence of both HSPGs and NRP-1. Studies of entry performed with HTLV-3 Env-pseudotyped viruses together with SU binding studies revealed that, for HTLV-1, glucose transporter 1 (GLUT-1) functions at a postbinding step during HTLV-3 Env-mediated entry. Further studies revealed that HTLV-3 SU binds efficiently to naive CD4(+) T cells, which do not bind either HTLV-1 or HTLV-2 SU and do not express detectable levels of HSPGs, NRP-1, and GLUT-1. These results indicate that the complex of receptor molecules used by HTLV-3 to bind to primary T lymphocytes differs from that of both HTLV-1 and HTLV-2.

Crosstalk between the Smad and the Mitogen-Activated Protein Kinase Pathways is Essential for Erythroid Differentiation of Erythroleukemia Cells Induced by TGF-ß, Activin, Hydroxyurea and Butyrate.

The role of crosstalk between the Smad and the MAPK signaling pathways in activin-, transforming growth factor-ß (TGF-ß)-, hydroxyurea (HU) - and butyrate-dependent erythroid differentiation of K562 leukemic cells was studied. Treatment with all four inducers caused transient phosphorylation of Smad2/3 and MAPK proteins including ERK, p38 and JNK. Use of specific inhibitors of p38, ERK and JNK MAPK proteins, and TGF-ß type I receptor indicated that differentiation induced by each of these agents involves activation of Smad2/3 and p38 MAPK, and inhibition of ERK MAPK. Also, treatment of cells with an inhibitor of protein serine/threonine phosphatase, okadaic acid (OA), induced phosphorylation of Smad2/3, and p38 MAPK, coincident with its induction of erythroid differentiation. Specific inhibition of TGF-ß type I receptor kinase activity not only abolished TGF-ß/activin effects but also prevented Smad2/3 activation and erythroid differentiation induced by OA, HU and butyrate. The TGF-ß type I receptor kinase inhibitor blocked OA-induced differentiation but not p38 MAPK phosphorylation demonstrating that signals from both pathways are needed. As previously observed, addition of ERK1/2 MAPK inhibitors upregulated Smad2/3 phosphorylation and enhanced differentiation, but these effects were dependent on signals from the TGF-ß type I receptor. These data indicate that activation of both Smad2/3 and p38 MAPK signaling pathways is a prerequisite to induce erythroid differentiation of erythroleukemia cells by activin, TGF-ß, HU, OA and butyrate.

Distribution of xenotropic murine leukemia virus-related virus (XMRV) infection in chronic fatigue syndrome and prostate cancer.

In 2006, sequences described as xenotropic murine leukemia virus-related virus (XMRV) were discovered in prostate cancer patients. In October 2009, we published the first direct isolation of infectious XMRV from humans and the detection of infectious XMRV in patients with chronic fatigue syndrome. In that study, a combination of classic retroviral methods were used including: DNA polymerase chain reaction and reverse transcriptase polymerase chain reaction for gag and env, full length genomic sequencing, immunoblotting for viral protein expression in activated peripheral blood mononuclear cells, passage of infectious virus in both plasma and peripheral blood mononuclear cells to indicator cell lines, and detection of antibodies to XMRV in plasma. A combination of these methods has since allowed us to confirm infection by XMRV in 85% of the 101 patients that were originally studied. Since 2009, seven studies, predominantly using DNA polymerase chain reaction of blood products or tumor tissue, have reported failures to detect XMRV infection in patients with either prostate cancer or chronic fatigue syndrome. A review of the current literature on XMRV supports the importance of applying multiple independent techniques in order to determine the presence of this virus. Detection methods based upon the biological and molecular amplification of XMRV, which is usually present at low levels in unstimulated blood cells and plasma, are more sensitive than assays for the virus by DNA polymerase chain reaction of unstimulated peripheral blood mononuclear cells. When we examined patient blood samples that had originally tested negative by DNA polymerase chain reaction by more sensitive methods, we observed that they were infected with XMRV; thus, the DNA polymerase chain reaction tests provided false negative results. Therefore, we conclude that molecular analyses using DNA from unstimulated peripheral blood mononuclear cells or from whole blood are not yet sufficient as stand-alone assays for the identification of XMRV-infected individuals. Complementary methods are reviewed, that if rigorously followed, will likely show a more accurate snapshot of the actual distribution of XMRV infection in humans.

Cell-free HTLV-1 infects dendritic cells leading to transmission and transformation of CD4(+) T cells.

Cell-free human T-lymphotropic virus type 1 (HTLV-1) virions are poorly infectious in vitro for their primary target cells, CD4(+) T cells. Here, we show that HTLV-1 can efficiently infect myeloid and plasmacytoid dendritic cells (DCs). Moreover, DCs exposed to HTLV-1, both before and after being productively infected, can rapidly, efficiently and reproducibly transfer virus to autologous primary CD4(+) T cells. This DC-mediated transfer of HTLV-1 involves heparan sulfate proteoglycans and neuropilin-1 and results in long-term productive infection and interleukin-2-independent transformation of the CD4(+) T cells. These studies, along with observations of HTLV-1-infected DCs in the peripheral blood of infected individuals, indicate that DCs have a central role in HTLV-1 transmission, dissemination and persistence in vivo. In addition to altering the current paradigm concerning how HTLV-1 transmission occurs, these studies suggest that impairment of DC function after HTLV-1 infection plays a part in pathogenesis.

Levels of Smad7 regulate Smad and mitogen activated kinases (MAPKs) signaling and controls erythroid and megakaryocytic differentiation of erythroleukemia cells.

Smad and MAPK signaling cascades are involved in erythroid and megakaryocytic differentiation. The inhibitory Smad for TGF-beta/activin signaling, Smad7, may directly or indirectly affect these signaling pathways. By modulating Smad7 expression, we attempted to delineate the relevance of Smad7 during erythro-megakaryocytic (E/M) differentiation of human erythroleukemia cells. Smad7 transcripts were detected at low levels in different erythroleukemia cell lines (TF-1, HEL and K562). Reduction of expression of endogenous Smad7 by RNA interference enhanced erythroid differentiation of K562 cells in response to physiological doses of activin-A/TGF-beta1. Stable over-expression of Smad7 in K562 cells (K562/7) prevented activation of Smad2/3 and MAPK (ERK1/2, p38 and JNK1/2) proteins by activin-A/TGF-beta1 and subsequent induction of erythroid differentiation. High levels of Smad7 also interfered with hydroxyurea- and butyrate-, but not hemin-induced erythroid differentiation. Interestingly, K562/7 cells were found to harbor a significant proportion (about 35%) of large ploy nucleated cells compared to fewer than 12% in control cells. K562/7 cells treated with phorbol 12-myristate 13-acetate (PMA), showed a great shift in ploidy towards high ploidy classes (> or =8N) accompanied with an increase in the expression of the maturation marker CD42b. We showed here that: (a) low levels of endogenous Smad7 in erythroleukemia cells are physiologically relevant, and (b) high levels of Smad7 interferes with TGF-beta/activin-induced Smad/MAPK signaling and erythro-differentiation and promotes megakaryocytic differentiation, possibly by blocking autocrine TGF-beta.

Human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 use different receptor complexes to enter T cells.

Studies using adherent cell lines have shown that glucose transporter-1 (GLUT-1) can function as a receptor for human T-cell leukemia virus type 1 (HTLV). In primary CD4(+) T cells, heparan sulfate proteoglycans (HSPGs) are required for efficient entry of HTLV-1. Here, the roles of HSPGs and GLUT-1 in HTLV-1 and HTLV-2 Env-mediated binding and entry into primary T cells were studied. Examination of the cell surface of activated primary T cells revealed that CD4(+) T cells, the primary target of HTLV-1, expressed significantly higher levels of HSPGs than CD8(+) T cells. Conversely, CD8(+) T cells, the primary target of HTLV-2, expressed GLUT-1 at dramatically higher levels than CD4(+) T cells. Under these conditions, the HTLV-2 surface glycoprotein (SU) binding and viral entry were markedly higher on CD8(+) T cells while HTLV-1 SU binding and viral entry were higher on CD4(+) T cells. Binding studies with HTLV-1/HTLV-2 SU recombinants showed that preferential binding to CD4(+) T cells expressing high levels of HSPGs mapped to the C-terminal portion of SU. Transfection studies revealed that overexpression of GLUT-1 in CD4(+) T cells increased HTLV-2 entry, while expression of HSPGs on CD8(+) T cells increased entry of HTLV-1. These studies demonstrate that HTLV-1 and HTLV-2 differ in their T-cell entry requirements and suggest that the differences in the in vitro cellular tropism for transformation and in vivo pathobiology of these viruses reflect different interactions between their Env proteins and molecules on CD4(+) and CD8(+) T cells involved in entry.

Heparan sulfate proteoglycans mediate attachment and entry of human T-cell leukemia virus type 1 virions into CD4+ T cells.

Heparan sulfate proteoglycans (HSPGs) are used by a number of viruses to facilitate entry into host cells. For the retrovirus human T-cell leukemia virus type 1 (HTLV-1), it has recently been reported that HSPGs are critical for efficient binding of soluble HTLV-1 SU and the entry of HTLV pseudotyped viruses into non-T cells. However, the primary in vivo targets of HTLV-1, CD4(+) T cells, have been reported to express low or undetectable levels of HSPGs. For this study, we reexamined the expression of HSPGs in CD4(+) T cells and examined their role in HTLV-1 attachment and entry. We observed that while quiescent primary CD4(+) T cells do not express detectable levels of HSPGs, HSPGs are expressed on primary CD4(+) T cells following immune activation. Enzymatic modification of HSPGs on the surfaces of either established CD4(+) T-cell lines or primary CD4(+) T cells dramatically reduced the binding of both soluble HTLV-1 SU and HTLV-1 virions. HSPGs also affected the efficiency of HTLV-1 entry, since blocking the interaction with HSPGs markedly reduced both the internalization of HTLV-1 virions and the titer of HTLV-1 pseudotyped viral infection in CD4(+) T cells. Thus, HSPGs play a critical role in the binding and entry of HTLV-1 into CD4(+) T cells.

Induction of human T cell leukemia virus type I receptors on quiescent naive T lymphocytes by TGF-beta.

The retrovirus human T cell leukemia virus (HTLV) type I (HTLV-I) is primarily transmitted by breast-feeding or sexual contact, by cell-to-cell contact between T cells. TGF-beta, which has been shown to enhance transmission of HTLV-I in vitro, is found at high levels in breast milk and semen. In this study, the ability of TGF-beta to regulate expression of molecules involved in HTLV-I binding and entry was examined. Previous studies using a soluble form of the HTLV-I envelope protein SU have shown that quiescent human T cells do not express cell surface molecules that specifically bind SU. After T cell activation, HTLV SU binding proteins are rapidly induced. In this study, we report that TGF-beta induces expression of proteins that bind soluble HTLV SU and HTLV virions on naive CD4(+) T lymphocytes. The induction of these proteins occurred without cell cycle entry or expression of activation markers, involved TGF-beta-induced intracellular signaling, and required de novo transcription and translation. Treatment of naive CD4(+) T lymphocytes with TGF-beta induced expression of GLUT-1, which has recently been reported to function as a receptor for HTLV. Treatment of a TGF-beta-sensitive human myeloid cell line increased the titer of both HTLV-I- and HTLV-II-pseudotyped viruses. Although earlier studies suggested that HTLV SU binding proteins might be an early marker of T cell activation and/or cell proliferation, we report in this study that TGF-beta induces binding of HTLV virions and expression of glucose transporter type 1 in primary CD4(+) T lymphocytes that remain quiescent.

Levels of phospho-Smad2/3 are sensors of the interplay between effects of TGF-beta and retinoic acid on monocytic and granulocytic differentiation of HL-60 cells.

We have investigated the role of Smad family proteins, known to be important cytoplasmic mediators of signals from the transforming growth factor-beta (TGF-beta) receptor serine/threonine kinases, in TGF-beta-dependent differentiation of hematopoietic cells, using as a model the human promyelocytic leukemia cell line, HL-60. TGF-beta-dependent differentiation of these cells to monocytes, but not retinoic acid-dependent differentiation to granulocytes, was accompanied by rapid phosphorylation and nuclear translocation of Smad2 and Smad3. Vitamin D(3) also induced phosphorylation of Smad2/3 and monocytic differentiation; however the effects were indirect, dependent on its ability to induce expression of TGF-beta1. Simultaneous treatment of these cells with TGF-beta1 and all-trans-retinoic acid (ATRA), which leads to almost equal numbers of granulocytes and monocytes, significantly reduced the level of phospho-Smad2/3 and its nuclear accumulation, compared with that in cells treated with TGF-beta1 alone. TGF-beta1 and ATRA activate P42/44 mitogen-activated protein (MAP) kinase with nearly identical kinetics, ruling out its involvement in these effects on Smad phosphorylation. Addition of the inhibitor-of-protein serine/threonine phosphatases, okadaic acid, blocks the ATRA-mediated reduction in TGF-beta-induced phospho-Smad2 and shifts the differentiation toward monocytic end points. In HL-60R mutant cells, which harbor a defective retinoic acid receptor-alpha (RAR-alpha), ATRA is unable to reduce levels of TGF-beta-induced phospho-Smad2/3, coincident with its inability to differentiate these cells along granulocytic pathways. Together, these data suggest a new level of cross-talk between ATRA and TGF-beta, whereby a putative RAR-alpha-dependent phosphatase activity limits the levels of phospho-Smad2/3 induced by TGF-beta, ultimately reducing the levels of nuclear Smad complexes mediating the TGF-beta-dependent differentiation of the cells to monocytic end points.

Tuberous sclerosis complex 2 gene product interacts with human SMAD proteins. A molecular link of two tumor suppressor pathways.

Tuberin (TSC2) is a tumor suppressor gene. At the cellular level, tuberin is required as a critical regulator of cell growth, neuronal differentiation, and tumor suppression. Here we report a critical role for tuberin in late stage myeloid cell differentiation. Tuberin strongly augments transforming growth factor (TGF)-beta1 signal transduction pathways, including SMAD activation. We also demonstrate that the amino-terminal region of tuberin interacts specifically with the MH2 domain of SMAD2 and SMAD3 proteins to regulate TGF-beta1-responsive genes such as p21(CIP). Inhibition of tuberin expression by Tsc2 antisense greatly reduces the ability of TGF-beta to transcriptionally regulate p21(CIP), p27(KIP), and cyclin A leading to an abrogation of the antiproliferative effects of TGF-beta1. Also, inhibition of tuberin expression during stimulation of monocytic differentiation with vitamin D(3) and TGF-beta1 significantly impaired myeloid cell growth inhibition and differentiation. Together, the data demonstrate the presence of a novel activation process following TGF-beta1 stimulation that requires tuberin-dependent activity.