No abstract

Malignant salivary gland tumors can arise from a t(11;19) translocation that fuses 42 residues from Mect1/Torc1, a cyclic AMP (cAMP)/cAMP-responsive element binding protein (CREB)-dependent transcriptional coactivator, with 982 residues from Maml2, a NOTCH receptor coactivator. To determine if the Mect1-Maml2 fusion oncogene mediates tumorigenicity by disrupting cAMP/CREB signaling, we have generated in-frame deletions within the CREB-binding domain of Mect1/Torc1 for testing transformation activity and have also developed a doxycycline-regulated Mect1-Maml2 mammalian expression vector for global gene expression profiling. We observed that small deletions within the CREB-binding domain completely abolished transforming activity in RK3E epithelial cells. Further, we have shown that the ectopic induction of Mect1-Maml2 in HeLa cells strongly activated the expression of a group of known cAMP/CREBregulated genes. In addition, we detected candidate cAMPresponsive element sites within 100 nucleotides of the transcriptional start sites of other genes activated by Mect1-Maml2 expression. In contrast, we did not observe alterations of known Notch-regulated target genes in these expression array profile experiments. We validated the results by reverse transcription-PCR in transfected HeLa, RK3E, and H2009 lung tumor cells and in mucoepidermoid cancer cells that endogenously express the fusion oncopeptide. Whereas overexpression of components of the cAMP pathway has been associated with a subset of human carcinomas, these data provide a direct genetic link between deregulation of cAMP/ CREB pathways and epithelial tumorigenesis and suggest future therapeutic strategies for this group of salivary gland tumors. (Cancer Res 2005; 65(16): 7137-44)

To study the molecular basis for the clinical phenotype of incomplete penetrance of familial retinoblastoma, we have examined the functional properties of three RB mutations identified in the germ line of five different families with low penetrance. RB mutants isolated from common adult cancers and from classic familial retinoblastoma (designated as classic RB mutations) are unstable and generally do not localize to the nucleus, do not undergo cyclin-dependent kinase (cdk)-mediated hyperphosphorylation, show absent protein ''pocket'' binding activity, and do not suppress colony growth of RB(؊) cells. In contrast, two low-penetrant alleles (661W and ''deletion of codon 480'') retained the ability to localize to the nucleus, showed normal cdk-mediated hyperphosphorylation in vivo, exhibited a binding pattern to simian virus 40 large T antigen using a quantitative yeast two-hybrid assay that was intermediate between classic mutants (null) and wild-type RB, and had absent E2F1 binding in vitro. A third, low-penetrant allele, ''deletion of RB exon 4,'' showed minimal hyperphosphorylation in vivo but demonstrated detectable E2F1 binding in vitro. In addition, each low-penetrant RB mutant retained the ability to suppress colony growth of RB(؊) tumor cells. These findings suggest two categories of mutant, low-penetrant RB alleles. Class 1 alleles correspond to promoter mutations, which are believed to result in reduced or deregulated levels of wild-type RB protein, whereas class 2 alleles result in mutant proteins that retain partial activity. Characterization of the different subtypes of class 2 lowpenetrant genes may help to define more precisely functional domains within the RB product required for tumor suppression.To study the molecular basis for the clinical phenotype of incomplete penetrance, we examined the functional properties of a series of retinoblastoma gene (RB) mutations identified in different families with incomplete penetrance. In contrast to classic familial retinoblastoma, which is characterized by a high penetrance for multifocal (bilateral) retinoblastoma tumors, certain kindreds have been reported where germ-line carriers of mutant RB alleles are either clinically unaffected or present with nonproliferative retinal scars (retinomas) or unifocal retinoblastoma (1). The ''gatekeeper'' gene for retinoblastoma tumors is the RB gene, which encodes a protein product (RB) that plays an important, though yet undefined, role in regulating the transit of dividing cells through the G 1 ͞S boundary of the cell cycle and in coordinating cellular differentiation via programmed cell death (apoptotic) pathways in selected tissues. The importance of the link between RB and the cell cycle is highlighted by the following observations: (i) RB is phosphorylated by members of the cyclin-dependent kinase (cdk) system, an enzyme family responsible for driving mitosis in all eukaryotic cells, (ii) cdk-mediated phosphorylation is tightly synchronized with the cell cycle such that RB undergoes hyperphosphorylation j...

Dephosphorylation of the RB protein has been reported to be associated with apoptosis. In contrast, we show that treatment of HL60 cells with etoposide or cytosine arabinoside or treatment of breast epithelial cells with a-FAS is associated with the cleavage of a 5 kDa fragment from the C-terminus of RB, resulting in a truncated product that we have designated as p100cl. This cleavage event coincides with the activation of cysteine proteases at the onset of apoptosis, is blocked by the addition of iodoacetamide to cells prior to the onset of apoptosis, and results in the expression of faster migrating protein species which can mimic dephosphorylated RB. The free 5 kDa fragment is detected only during apoptosis, predicts a cleavage site that we have mapped to a unique CPP32-like recognition sequence which is present at the C-terminus of all reported RB homologues, and results in a truncated RB protein with enhanced E2F binding anity. While the causality for this cleavage event in the apoptotic process is still under investigation, our ®ndings suggest distinct post-translational pathways for the RB product between cells examined during growth arrest (p105 hypophosphorylated RB) or apoptosis (p100cl).Keywords: apoptosis; chemotherapy; cysteine protease; FAS; retinoblastoma protein A working model for the RB tumor suppressor pathway proposes that the phosphorylation state of the RB protein serves as a thermostat which allows for a conformational tilt that either facilitates (hypophosphorylated state) or inhibits (hyperphosphorylated state) the ability of RB to non-covalently bind to a group of viral or cellular proteins (Ludlow et al., 1989;Weinberg, 1995). The best characterized RB (`pocket') cellular binding partner is the E2F/DP heterodimeric complex which can function to repress or activate the coordinated transcription of a set of genes required for DNA synthesis depending, respectively, on whether it is bound to or free from RB (Nevins, 1992;Weintraub et al., 1992). In addition to changes in the phosphorylation state of the full length RB protein, however, we have been studying other post-translational events involving RB. For example, investigators have recently reported that during apoptosis the RB protein undergoes rapid dephosphorylation which is followed by a cleavage event near the N-terminus of RB resulting in the expression of a truncated p68 species (Bing and Dou, 1996). Although it was suggested that this cleavage event was catalyzed by an ICE-like protease, no consensus cleavage recognition sequence was identi®ed. In contrast, we have determined that a single 5 kDa fragment is cleaved from the C-terminus of RB during either drug-mediated or a-FAS induced apoptosis in human cells derived from dierent lineages. This C-terminal cleavage: (i) coincides with the onset of apoptosis; (ii) is inhibited by iodoacetamide; (iii) maps to an amino acid sequence at RB residue no. 886 which is a consensus substrate for a CED3/CPP32b-like cysteine protease iv) mimics the migration pattern of hypophosphorylated RB a...

The stress70 protein chaperone family plays a central role in the processing of cytosolic and secretory proteins. We have cloned a human cDNA, designated Stch, that is conserved in rat tissues and which encodes a novel microsome‐associated member of the stress70 protein chaperone family. Stch mRNA is constitutively expressed in all human cell types and is induced by incubation with the calcium ionophore A23187, but not by exposure to heat shock. Inspection of the predicted amino acid sequence reveals that the STCH product contains a unique hydrophobic leader sequence and shares homology within the amino terminal domains of the stress70 gene family, but has a 50 residue insertion within the ATP‐binding domains and truncates the carboxyl terminal peptide‐binding region. Immunofluorescent and subcellular analyses show that STCH migrates predominantly as a 60 kDa species and is enriched in a membrane‐bound microsome fraction. In contrast to purified BiP and dnaK, however, STCH demonstrates ATPase activity that is independent of peptide stimulation. Stch, therefore, encodes a calcium‐inducible, microsome‐associated ATPase activity with properties similar to a proteolytically cleaved N‐terminal HSC70/BiP fragment. This truncated stress70 molecule may allow increased diversity in cellular responses to protein processing requirements.

Mucoepidermoid (MEC) salivary gland tumors arise from a t(11;19) rearrangement which generates a fusion oncogene, Mect1-Maml2, that functions to activate CREB-responsive target genes. To determine if sustained expression of Mect1-Maml2 is required for tumor cell growth, we first showed that ectopic expression of Mect1-Maml2 in rat epithelial RK3E cells is tumorigenic in vivo in nude mice and that excised xenografts continue to express the fusion oncogene. We then generated a hairpin RNAi vector that selectively suppressed the fusion peptide and showed that ectopic expression in either parotid or pulmonary MEC tumor cell lines containing the t(11;19) rearrangement resulted in at least 90% colony growth inhibition. In contrast, single nucleotide changes within this RNAi sequence abolished the ability to suppress Mect1-Maml2 protein and abolished all growth inhibition of these MEC tumor lines. In addition, the RNAi-specific vector had no effect on colony growth of non-MEC tumors including a lung tumor or two other salivary gland cell lines that do not express Mect1-Maml2. We also generated a mutant Mect1-Maml2 expression plasmid that carried silent nucleotide changes within the RNAi target sequence and observed that co-transfection of this mutant, but not wild-type Mect1-Maml2, could partially rescue RNAi growth inhibition in the MEC tumor line. The recent detection of acquired fusion oncogenes in epithelial solid tumors has suggested new possibilities for the diagnosis and therapy of these cancers. Our data show that the 'gain-of-function' activity from aberrant Mect1-Maml2 expression is a candidate therapeutic target for this group of malignant salivary gland tumors.

Activation of Crtc1 (also known as Mect1/Torc1) by a t(11;19) chromosomal rearrangement underlies the etiology of malignant salivary gland tumors. As LKB1 is a target for mutational inactivation in lung cancer and was recently shown to regulate hepatic Crtc2/CREB transcriptional activity in mice, we now present evidence suggesting disruption of an LKB1/Crtc pathway in cancer. Although Crtc1 is preferentially expressed in adult brain tissues, we observed elevated levels of steadystate Crtc1 in thoracic tumors. In addition, we show that somatic loss of LKB1 is associated with underphosphorylation of endogenous Crtc1, enhanced Crtc1 nuclear localization and enhanced expression of the Crtc prototypic target gene, NR4A2/Nurr1. Inhibition of NR4A2 was associated with growth suppression of LKB1 null tumors, but showed little effect on LKB1-wildtype cells. These data strengthen the role of dysregulated Crtc as a bona fide cancer gene, present a new element to the complex LKB1 tumorigenic axis, and suggest that Crtc genes may be aberrantly activated in a wider range of common adult malignancies.

The growth suppressor activities of the RB and p107 products are believed to be mediated by the reversible binding of a heterogeneous family of cellular proteins to a conserved T/E1A pocket domain that is present within both proteins. To study the functional role of these interactions, we examined the properties of cellular retinoblastoma binding protein 2 (RBP2) binding to RB, p1O7, and the related TATA-binding protein (TBP) product. We observed that although RBP2 bound exclusively to the T/E1A pocket of p107, it could interact with RB through independent T/E1A and non-T/ElA domains and with TBP only through the non-T/ElA domain. Consistent with this observation, we found that a mutation within the Leu-X-Cys-X-Glu motif of RBP2 resulted in loss of ability to precipitate p107, while RB-and TBP-binding activities were retained. We located the non-T/ElA binding site of RBP2 on a 15-kDa fragment that is independent from the Leu-X-Cys-X-Glu motif and encodes binding activity for RB and TBP but does not interact with p107. Despite the presence of a non-T/ElA binding site, however, recombinant RBP2 retained the ability to preferentially precipitate active hypophosphorylated RB from whole-cell lysates. In addition, we found that cotransfection of RBP2 can reverse in vivo RB-mediated suppression of E2F activity. These findings confirm the differential binding specificities of the related RB, p107, and TBP proteins and support the presence of multifunctional domains on the nuclear RBP2 product which may allow complex interactions with the cellular transcription machinery.An important step toward defining a mechanism underlying tumor suppressor activity of the Rb gene was the observation that the transforming products of adenovirus (ElA), simian virus 40 (large T), and human papillomavirus (E7) could precipitate wild-type RB protein (8,16,57,59). This, in turn, led to the identification of a family of cellular proteins that can reversibly bind to a discrete domain on RB, referred to as the T/ElA pocket by using the same specificity as the viral products (10, 12, 33-35, 39, 51, 52, 54). The subsequent observation that protein binding was inhibited following RB phosphorylation in the late G1 phase of the cell cycle suggested the hypothesis that RB, as well as the related p107 product, may regulate the functional activity of its binding partners by a cell cycle-dependent pattern of physical association (19,48,53).The binding domains within the RB protein have been delineated by a series of in vitro and naturally occurring mutants as two noncontiguous regions designated domains A and B which are interrupted by a spacer sequence (29,30,32) (Fig. 1A). This structure generates a hypothetical pocket conformation that (i) is conserved among a family of E1A-binding proteins, including RB, p107, and p130 (19,26,40,45); (ii) demonstrates binding to a series of viral and cellular proteins which can be blocked by short peptides containing a leucine-X-cysteine-X-glutamic acid motif (LXCXE, where X represents any amino acid); (iii) can ...