Endogenous expression of Müllerian inhibiting substance in early postnatal rat Sertoli cells requires multiple steroidogenic factor-1 and GATA-4-binding sites

Müllerian inhibiting substance (MIS) is a key element required to complete mammalian male sex differentiation. The expression pattern of MIS is tightly regulated in fetal, neonatal, and prepubertal testes and adult ovaries and is well conserved among mammalian species. Although several factors have been shown to be essential to MIS expression, its regulatory mechanisms are not fully understood. We have examined MIS promoter activity in 2-day postnatal primary cultures of rat Sertoli cells that continue to express endogenous MIS mRNA. Using this system, we found that the region between human MIS−269 and −192 is necessary for full MIS promoter activity. We identified by DNase I footprint and electrophoretic mobility-shift analyses a distal steroidogenic factor-1 (SF-1)-binding site that is essential for full promoter activity. Mutational analysis of this new distal SF-1 site and the previously identified proximal SF-1 site showed that both are necessary for transcriptional activation. Moreover, the proximal promoter also contains multiple GATA-4-binding sites that are essential for functional promoter activity. Thus multiple SF-1- and GATA-4-binding sites in the MIS promoter are required for normal tissue-specific and developmental expression of MIS.

Endogenous, hyperactive Rac3 controls proliferation of breast cancer cells by a p21-activated kinase-dependent pathway

Uncontrolled cell proliferation is a major feature of cancer. Experimental cellular models have implicated some members of the Rho GTPase family in this process. However, direct evidence for active Rho GTPases in tumors or cancer cell lines has never been provided. In this paper, we show that endogenous, hyperactive Rac3 is present in highly proliferative human breast cancer-derived cell lines and tumor tissues. Rac3 activity results from both its distinct subcellular localization at the membrane and altered regulatory factors affecting the guanine nucleotide state of Rac3. Associated with active Rac3 was deregulated, persistent kinase activity of two isoforms of the Rac effector p21-activated kinase (Pak) and of c-Jun N-terminal kinase (JNK). Introducing dominant-negative Rac3 and Pak1 fragments into a breast cancer cell line revealed that active Rac3 drives Pak and JNK kinase activities by two separate pathways. Only the Rac3–Pak pathway was critical for DNA synthesis, independently of JNK. These findings identify Rac3 as a consistently active Rho GTPase in human cancer cells and suggest an important role for Rac3 and Pak in tumor growth.

Mouse trp2, the homologue of the human trpc2 pseudogene, encodes mTrp2, a store depletion-activated capacitative Ca2+ entry channel

Capacitative Ca2+ entry (CCE) is Ca2+ entering after stimulation of inositol 1,4,5-trisphosphate (IP3) formation and initiation of Ca2+ store depletion. One hallmark of CCE is that it can also be triggered merely by store depletion, as occurs after inhibition of internal Ca2+ pumps with thapsigargin. Evidence has accumulated in support of a role of transient receptor potential (Trp) proteins as structural subunits of a class of Ca2+-permeable cation channels activated by agonists that stimulate IP3 formation—very likely through a direct interaction between the IP3 receptor and a Trp subunit of the Ca2+ entry channel. The role of Trp’s in Ca2+ entry triggered by store depletion alone is less clear. Only a few of the cloned Trp’s appear to enhance this type of Ca2+ entry, and when they do, the effect requires special conditions to be observed, which native CCE does not. Here we report the full-length cDNA of mouse trp2, the homologue of the human trp2 pseudogene. Mouse Trp2 is shown to be readily activated not only after stimulation with an agonist but also by store depletion in the absence of an agonist. In contrast to other Trp proteins, Trp2-mediated Ca2+ entry activated by store depletion is seen under the same conditions that reveal endogenous store depletion-activated Ca2+ entry, i.e., classical CCE. The findings support the general hypothesis that Trp proteins are subunits of store- and receptor-operated Ca2+ channels.

Molecular identification of human G-substrate, a possible downstream component of the cGMP-dependent protein kinase cascade in cerebellar Purkinje cells

G-substrate, an endogenous substrate for cGMP-dependent protein kinase, exists almost exclusively in cerebellar Purkinje cells, where it is possibly involved in the induction of long-term depression. A G-substrate cDNA was identified by screening expressed sequence tag databases from a human brain library. The deduced amino acid sequence of human G-substrate contained two putative phosphorylation sites (Thr-68 and Thr-119) with amino acid sequences [KPRRKDT(p)PALH] that were identical to those reported for rabbit G-substrate. G-substrate mRNA was expressed almost exclusively in the cerebellum as a single transcript. The human G-substrate gene was mapped to human chromosome 7p15 by radiation hybrid panel analysis. In vitro translation products of the cDNA showed an apparent molecular mass of 24 kDa on SDS/PAGE which was close to that of purified rabbit G-substrate (23 kDa). Bacterially expressed human G-substrate is a heat-stable and acid-soluble protein that cross-reacts with antibodies raised against rabbit G-substrate. Recombinant human G-substrate was phosphorylated efficiently by cGMP-dependent protein kinase exclusively at Thr residues, and it was recognized by antibodies specific for rabbit phospho-G-substrate. The amino acid sequences surrounding the sites of phosphorylation in G-substrate are related to those around Thr-34 and Thr-35 of the dopamine- and cAMP-regulated phosphoprotein DARPP-32 and inhibitor-1, respectively, two potent inhibitors of protein phosphatase 1. However, purified G-substrate phosphorylated by cGMP-dependent protein kinase inhibited protein phosphatase 2A more effectively than protein phosphatase 1, suggesting a distinct role as a protein phosphatase inhibitor.

Implications of EPHB6, EFNB2, and EFNB3 expressions in human neuroblastoma

Neuroblastoma (NB) is a common pediatric tumor that exhibits a wide range of biological and clinical heterogeneity. EPH (erythropoietin-producing hepatoma amplified sequence) family receptor tyrosine kinases and ligand ephrins play pivotal roles in neural and cardiovascular development. High-level expression of transcripts encoding EPHB6 receptors (EPHB6) and its ligands ephrin-B2 and ephrin-B3 (EFNB2, EFNB3) is associated with low-stage NB (stages 1, 2, and 4S) and high TrkA expression. In this study, we showed that EFNB2 and TrkA expressions were associated with both tumor stage and age, whereas EPHB6 and EFNB3 expressions were solely associated with tumor stage, suggesting that these genes were expressed in distinct subsets of NB. Kaplan-Meier and Cox regression analyses revealed that high-level expression of EPHB6, EFNB2, and EFNB3 predicted favorable NB outcome (P < 0.005), and their expression combined with TrkA expression predicted the disease outcome more accurately than each variable alone (P < 0.00005). Interestingly, if any one of the four genes (EPHB6, EFNB2, EFNB3, or TrkA) was expressed at high levels in NB, the patient survival was excellent (>90%). To address whether a good disease outcome of NB was a consequence of high-level expression of a “favorable NB gene,” we examined the effect of EPHB6 on NB cell lines. Transfection of EPHB6 cDNA into IMR5 and SY5Y expressing little endogenous EPHB6 resulted in inhibition of their clonogenicity in culture. Furthermore, transfection of EPHB6 suppressed the tumorigenicity of SY5Y in a mouse xenograft model, demonstrating that high-level expressions of favorable NB genes, such as EPHB6, can in fact suppress malignant phenotype of unfavorable NB.

Rapid induction of senescence in human cervical carcinoma cells

Expression of the bovine papillomavirus E2 regulatory protein in human cervical carcinoma cell lines repressed expression of the resident human papillomavirus E6 and E7 oncogenes and within a few days caused essentially all of the cells to synchronously display numerous phenotypic markers characteristic of cells undergoing replicative senescence. This process was accompanied by marked but in some cases transient alterations in the expression of cell cycle regulatory proteins and by decreased telomerase activity. We propose that the human papillomavirus E6 and E7 proteins actively prevent senescence from occurring in cervical carcinoma cells, and that once viral oncogene expression is extinguished, the senescence program is rapidly executed. Activation of endogenous senescence pathways in cancer cells may represent an alternative approach to treat human cancers.

Calcium-triggered acrosomal exocytosis in human spermatozoa requires the coordinated activation of Rab3A and N-ethylmaleimide-sensitive factor

The acrosome reaction of spermatozoa is a complex, calcium-dependent, regulated exocytosis. Fusion at multiple sites between the outer acrosomal membrane and the cell membrane causes the release of the acrosomal contents and the loss of the membranes surrounding the acrosome. However, very little is known about the molecules that mediate and regulate this unique fusion process. Here, we show that N-ethylmaleimide-sensitive factor (NSF), a protein essential for most fusion events, is present in the acrosome of several mammalian spermatozoa. Moreover, we demonstrate that calcium-dependent exocytosis of permeabilized sperm requires active NSF. Previously, we have shown that the addition of the active (GTP-bound) form of the small GTPase Rab3A triggers exocytosis in permeabilized spermatozoa. In the present report we show that Rab3A is necessary for calcium-dependent exocytosis. The activation of Rab3A protects NSF from N-ethylmaleimide inhibition and precludes the exchange of the endogenous protein with recombinant dominant negative mutants of NSF. Furthermore, Rab3A activation of acrosomal exocytosis requires active NSF. Our results suggest that, upon calcium stimulation, Rab3A switches to its active GTP-bound form, triggering the formation of a protein complex in which NSF is protected. This process is suggested to be an essential part of the molecular mechanism of membrane fusion leading to the release of the acrosomal contents.

Isoform-specific effects of human apolipoprotein E on brain function revealed in ApoE knockout mice: Increased susceptibility of females

Apolipoprotein E (apoE) mediates the redistribution of lipids among cells and is expressed at highest levels in brain and liver. Human apoE exists in three major isoforms encoded by distinct alleles (ɛ2, ɛ3, and ɛ4). Compared with APOE ɛ2 and ɛ3, APOE ɛ4 increases the risk of cognitive impairments, lowers the age of onset of Alzheimer’s disease (AD), and decreases the response to AD treatments. Besides age, inheritance of the APOE ɛ4 allele is the most important known risk factor for the development of sporadic AD, the most common form of this illness. Although numerous hypotheses have been advanced, it remains unclear how APOE ɛ4 might affect cognition and increase AD risk. To assess the effects of distinct human apoE isoforms on the brain, we have used the neuron-specific enolase (NSE) promoter to express human apoE3 or apoE4 at similar levels in neurons of transgenic mice lacking endogenous mouse apoE. Compared with NSE-apoE3 mice and wild-type controls, NSE-apoE4 mice showed impairments in learning a water maze task and in vertical exploratory behavior that increased with age and were seen primarily in females. These findings demonstrate that human apoE isoforms have differential effects on brain function in vivo and that the susceptibility to apoE4-induced deficits is critically influenced by age and gender. These results could be pertinent to cognitive impairments observed in human APOE ɛ4 carriers. NSE-apoE mice and similar models may facilitate the preclinical assessment of treatments for apoE-related cognitive deficits.

Mitochondrial mutational spectra in human cells and tissues

We have found that human organs such as colon, lung, and muscle, as well as their derived tumors, share nearly all mitochondrial hotspot point mutations. Seventeen hotspots, primarily G → A and A → G transitions, have been identified in the mitochondrial sequence of base pairs 10,030–10,130. Mutant fractions increase with the number of cell generations in a human B cell line, TK6, indicating that they are heritable changes. The mitochondrial point mutation rate appears to be more than two orders of magnitude higher than the nuclear point mutation rate in TK6 cells and in human tissues. The similarity of the hotspot sets in vivo and in vitro leads us to conclude that human mitochondrial point mutations in the sequence studied are primarily spontaneous in origin and arise either from DNA replication error or reactions of DNA with endogenous metabolites. The predominance of transition mutations and the high number of hotspots in this short sequence resembles spectra produced by DNA polymerases in vitro.

Inhibition of caspase 1 reduces human myocardial ischemic dysfunction via inhibition of IL-18 and IL-1β

The proinflammatory cytokine IL-18 was investigated for its role in human myocardial function. An ischemia/reperfusion (I/R) model of suprafused human atrial myocardium was used to assess myocardial contractile force. Addition of IL-18 binding protein (IL-18BP), the constitutive inhibitor of IL-18 activity, to the perifusate during and after I/R resulted in improved contractile function after I/R from 35% of control to 76% with IL-18BP. IL-18BP treatment also preserved intracellular tissue creatine kinase levels (by 420%). Steady-state mRNA levels for IL-18 were elevated after I/R, and the concentration of IL-18 in myocardial homogenates was increased (control, 5.8 pg/mg vs. I/R, 26 pg/mg; P < 0.01). Active IL-18 requires cleavage of its precursor form by the IL-1β-converting enzyme (caspase 1); inhibition of caspase 1 also attenuated the depression in contractile force after I/R (from 35% of control to 75.8% in treated atrial muscle; P < 0.01). Because caspase 1 also cleaves the precursor IL-1β, IL-1 receptor blockade was accomplished by using the IL-1 receptor antagonist. IL-1 receptor antagonist added to the perifusate also resulted in a reduction of ischemia-induced contractile dysfunction. These studies demonstrate that endogenous IL-18 and IL-1β play a significant role in I/R-induced human myocardial injury and that inhibition of caspase 1 reduces the processing of endogenous precursors of IL-18 and IL-1β and thereby prevents ischemia-induced myocardial dysfunction.

Stable expression in yeast of the mature form of human telomerase RNA depends on its association with the box H/ACA small nucleolar RNP proteins Cbf5p, Nhp2p and Nop10p

Telomerase is a ribonucleoprotein (RNP) particle required for the replication of telomeres. The RNA component, termed hTR, of human telomerase contains a domain structurally and functionally related to box H/ACA small nucleolar RNAs (snoRNAs). Furthermore, hTR is known to be associated with two core components of H/ACA snoRNPs, hGar1p and Dyskerin (the human counterpart of yeast Cbf5p). To assess the functional importance of the association of hTR with H/ACA snoRNP core proteins, we have attempted to express hTR in a genetically tractable system, Saccharomyces cerevisiae. Both mature non-polyadenylated and polyadenylated forms of hTR accumulate in yeast. The former is associated with all yeast H/ACA snoRNP core proteins, unlike TLC1 RNA, the endogenous RNA component of yeast telomerase. We show that the presence of the H/ACA snoRNP proteins Cbf5p, Nhp2p and Nop10p, but not Gar1p, is required for the accumulation of mature non-polyadenylated hTR in yeast, while accumulation of TLC1 RNA is not affected by the absence of any of these proteins. Our results demonstrate that yeast telomerase is unrelated to H/ACA snoRNPs. In addition, they show that the accumulation in yeast of the mature RNA component of human telomerase depends on its association with three of the four core H/ACA snoRNP proteins. It is likely that this is the case in human cells as well.

Mobilization of MHC class I molecules from late endosomes to the cell surface following activation of CD34-derived human Langerhans cells

Langerhans cells are a subset of dendritic cells (DCs) found in the human epidermis with unique morphological and molecular properties that enable their function as “sentinels” of the immune system. DCs are pivotal in the initiation and regulation of primary MHC class I restricted T lymphocyte immune responses and are able to present both endogenous and exogenous antigen onto class I molecules. Here, we study the MHC class I presentation pathway following activation of immature, CD34-derived human Langerhans cells by lipopolysaccharide (LPS). LPS induces an increase in all components of the MHC class I pathway including the transporter for antigen presentation (TAP), tapasin and ERp57, and the immunoproteasome subunits LMP2 and LMP7. Moreover, in CD34-derived Langerhans cells, the rapid increase in expression of MHC class I molecules seen at the cell surface following LPS activation is because of mobilization of MHC class I molecules from HLA-DM positive endosomal compartments, a pathway not seen in monocyte-derived DCs. Mobilization of class I from this compartment is primaquine sensitive and brefeldin A insensitive. These data demonstrate the regulation of the class I pathway in concert with the maturation of the CD34-derived Langerhans cells and suggest potential sites for antigen loading of class I proteins.

Candidate tumor suppressor HYAL2 is a glycosylphosphatidylinositol (GPI)-anchored cell-surface receptor for jaagsiekte sheep retrovirus, the envelope protein of which mediates oncogenic transformation

Jaagsiekte sheep retrovirus (JSRV) can induce rapid, multifocal lung cancer, but JSRV is a simple retrovirus having no known oncogenes. Here we show that the envelope (env) gene of JSRV has the unusual property that it can induce transformation in rat fibroblasts, and thus is likely to be responsible for oncogenesis in animals. Retrovirus entry into cells is mediated by Env interaction with particular cell-surface receptors, and we have used phenotypic screening of radiation hybrid cell lines to identify the candidate lung cancer tumor suppressor HYAL2/LUCA2 as the receptor for JSRV. HYAL2 was previously described as a lysosomal hyaluronidase, but we show that HYAL2 is actually a glycosylphosphatidylinositol (GPI)-anchored cell-surface protein. Furthermore, we could not detect hyaluronidase activity associated with or secreted by cells expressing HYAL2, whereas we could easily detect such activity from cells expressing the related serum hyaluronidase HYAL1. Although the function of HYAL2 is currently unknown, other GPI-anchored proteins are involved in signal transduction, and some mediate mitogenic responses, suggesting a potential role of HYAL2 in JSRV Env-mediated oncogenesis. Lung cancer induced by JSRV closely resembles human bronchiolo-alveolar carcinoma, a disease that is increasing in frequency and now accounts for ≈25% of all lung cancer. The finding that JSRV env is oncogenic and the identification of HYAL2 as the JSRV receptor provide tools for further investigation of the mechanism of JSRV oncogenesis and its relationship to human bronchiolo-alveolar carcinoma.

Direct transformation of rodent fibroblasts by jaagsiekte sheep retrovirus DNA

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary carcinoma, a unique animal model for human bronchioalveolar carcinoma. We previously isolated a JSRV proviral clone and showed that it was both infectious and oncogenic. Thus JSRV is necessary and sufficient for the development of ovine pulmonary carcinoma, but no data are available on the mechanisms of transformation. Inspection of the JSRV genome reveals standard retroviral genes, but no evidence for a viral oncogene. However, an alternate ORF in pol (orf-x) might be a candidate for a transforming gene. We tested whether the JSRV genome might encode a transforming gene by transfecting an expression plasmid for JSRV [pCMVJS21, driven by the cytomegalovirus (CMV) immediate early promoter] into mouse NIH 3T3 cells. Foci of transformed cells appeared in the transfected cultures 2–3 weeks posttransfection; cloned transformants showed anchorage independence for growth, and they expressed JSRV RNA. These results indicate that the JRSV genome contains information with direct transforming potential for NIH 3T3 cells. Transfection of a mutated version of pCMVJS21 in which the orf-x protein was terminated by two stop codons also gave transformed foci. Thus, orf-x was eliminated as the candidate transforming gene. In addition, another derivative of pCMVJS21 (pCMVJS21ΔGP) in which the gag, pol (and orf-x) coding sequences were deleted also gave transformed foci. These results indicate that the envelope gene carries the transforming potential. This is an unusual example of a native retroviral structural protein with transformation potential.

Expression of the telomerase catalytic subunit, hTERT, induces resistance to transforming growth factor β growth inhibition in p16INK4A(−) human mammary epithelial cells

Failures to arrest growth in response to senescence or transforming growth factor β (TGF-β) are key derangements associated with carcinoma progression. We report that activation of telomerase activity may overcome both inhibitory pathways. Ectopic expression of the human telomerase catalytic subunit, hTERT, in cultured human mammary epithelial cells (HMEC) lacking both telomerase activity and p16INK4A resulted in gaining the ability to maintain indefinite growth in the absence and presence of TGF-β. The ability to maintain growth in TGF-β was independent of telomere length and required catalytically active telomerase capable of telomere maintenance in vivo. The capacity of ectopic hTERT to induce TGF-β resistance may explain our previously described gain of TGF-β resistance after reactivation of endogenous telomerase activity in rare carcinogen-treated HMEC. In those HMEC that overcame senescence, both telomerase activity and TGF-β resistance were acquired gradually during a process we have termed conversion. This effect of hTERT may model a key change occurring during in vivo human breast carcinogenesis.