SSeCKS, a major protein kinase C substrate with tumor suppressor activity, regulates G(1)-->S progression by controlling the expression and cellular compartmentalization of cyclin D

Loss of the SSeCKS/Gravin/AKAP12 gene results in prostatic hyperplasia

SSeCKS/Gravin/AKAP12 (SSeCKS) is a kinase scaffolding protein that encodes metastasis-suppressor activity through the suppression of Src-mediated oncogenic signaling and vascular endothelial growth factor expression. SSeCKS expression is down-regulated in Src- and Ras-transformed fibroblasts, in human cancer cell lines and in several types of human cancer, including prostate. Normal human and mouse prostates express abundant SSeCKS in secretory epithelial cells and, to a lesser extent, in the surrounding mesenchyme. Here, we show that the loss of SSeCKS results in prostatic hyperplasia in the anterior and ventral lobes as well as increased levels of apoptosis throughout the prostate. Dysplastic foci were observed less frequently but were associated with the loss of E-cadherin staining and the loss of high molecular weight cytokeratin-positive basal epithelial cells. SSeCKS-null prostate tissues expressed significantly higher relative levels of AKT(poS473) compared with wild-type controls, suggesting that SSeCKS attenuates phosphatidylinositol-3-OH kinase signaling. The data suggest that SSeCKS-null mice have increased susceptibility for oncogenic transformation in the prostate.

A-kinase anchoring protein 12 regulates the completion of cytokinesis

A-kinase anchoring protein 12 (AKAP12) gene is frequently inactivated in human gastric cancer and in several other cancers due to promoter hypermethylation. However, the biological function of AKAP12 in tumorigenesis remains to be identified. Aneuploidy, a hallmark of cancer cells, is often caused by abnormal cell division. In the present study, AKAP12 was found to localize to the cell periphery during interphase and to the actomyosin contractile ring during cytokinesis. Furthermore, AKAP12 depletion using small interfering RNA increased the number of multinucleated cells, and disrupted the completion of cytokinesis. Interestingly, the inhibition of myosin light chain kinase (MLCK), a key regulator of actomyosin contractility, removed AKAP12 from the cell periphery during interphase and from the contractile ring during cytokinesis, suggesting that AKAP12 might be a downstream effector of MLCK. Our findings implicate AKAP12 in the regulation of cytokinesis progression, and suggest a novel role for AKAP12 tumor suppressor.

Spatiotemporal patterns of SSeCKS expression after rat spinal cord injury

Src suppressed C kinase substrate (SSeCKS) was identified as a PKC substrate/PKC-binding protein, which plays a role in mitogenic regulatory activity and has a function in the control of cell signaling and cytoskeletal arrangement. However its distribution and function in the central nervous system (CNS) lesion remain unclear. In this study, we mainly investigated the mRNA and protein expression and cellular localization of SSeCKS during spinal cord injury (SCI). Real-time PCR and Western blot analysis revealed that SSeCKS was present in normal whole spinal cord. It gradually increased, reached a peak at 3 days for its mRNA level and 5 days for its protein level after SCI, and then declined during the following days. In ventral horn, the expression of SSeCKS underwent a temporal pattern that was similar with the whole spinal cord in both mRNA and protein level. However, in dorsal horn, the mRNA and protein for SSeCKS expression were significantly increased at 1 day for its mRNA level and 3 days for its protein level, and then gradually declined to the baseline level, ultimately up-regulated again from 7 to 14 days. The protein expression of SSeCKS was further analysed by immunohistochemistry. The positively stained areas for SSeCKS changed with the similar pattern to that of protein expression detected by immunoblotting analysis. Double immunofluorescence staining showed that SSeCKS immunoreactivity (IR) was found in neurons, astrocytes, oligodendrocytes of spinal cord tissues within 5 mm from the lesion site. Importantly, injury-induced expression of SSeCKS was co-labeled by active caspase-3 (apoptotic marker), Tau-1 (the marker for pathological oligodendrocyte) and beta-1,4-galactosyltransferase 1 (GalT). All the results suggested that SSeCKS might play important roles in spinal cord pathophysiology and further research is needed to have a good understanding of its function and mechanism.

Metastasis suppressors and the tumor microenvironment

The most dangerous attribute of cancer cells is their ability to metastasize. Throughout the process of metastasis, tumor cells interact with other tumor cells, host cells and extracellular molecules. This brief review explores how a new class of molecules – metastasis suppressors – regulate tumor cell–microenvironmental interactions. Data are presented which demonstrate that metastasis suppressors act at multiple steps of the metastatic cascade. A brief discussion for how metastasis suppressor regulation of cellular interactions might be exploited is presented.

AKAP12, a gene with tumour suppressor properties, is a target of promoter DNA methylation in childhood myeloid malignancies

A-kinase anchor protein 12 (AKAP12) is a scaffold protein that participates in mitotic regulation and other signalling processes and probably exerts tumour suppressor function. We hypothesized that epigenetic repression of the AKAP12 gene might occur in malignant myeloid disorders. This study demonstrated that the 5' CpG island of AKAP12 was unmethylated in normal haematopoietic progenitors and granulocytes but exhibited profound methylation in Kasumi-1 and SKNO-1 leukaemic myeloblasts. Correspondingly, AKAP12 was expressed in normal progenitors but transcriptionally silent in leukaemic blasts. Re-expression of AKAP12 in Kasumi-1 and SKNO-1 cells was accomplished by treatment with MS275 alone or in combination with zebularine, indicating epigenetic mechanisms of gene repression. AKAP12 hypermethylation was found in one case of refractory anaemia with excess blasts (RAEB) and two cases of acute myeloid leukaemia (AML) in a panel of 21 blood or bone marrow samples from children with malignant myeloid disorders including refractory cytopenia, RAEB, juvenile myelomonocytic leukaemia and AML. While AKAP12 function has not been previously linked to leukaemogenesis, our results raise the possibility that epigenetic silencing of AKAP12 is involved in myeloid malignancies.

v-Src-mediated Down-regulation of SSeCKS Metastasis Suppressor Gene Promoter by the Recruitment of HDAC1 into a USF1-Sp1-Sp3 Complex

SSeCKS (Src-suppressed C kinase substrate), also called gravin/AKAP12, is a large scaffolding protein with metastasis suppressor activity. Two major isoforms of SSeCKS are expressed in most cell and tissue types under the control of two independent promoters, designated alpha and beta, separated by 68 kb. SSeCKS transcript and protein levels are severely decreased in Src- and Ras-transformed fibroblasts and in many epithelial tumors. By dissecting its promoters with progressive deletion analysis, we identified the sequence between -106 and -49 in the alpha proximal promoter as the minimal v-Src-responsive element, which contains E- and GC-boxes bound by USF1 and Sp1/Sp3, respectively. Both E- and GC-boxes are crucial for v-Src-responsive and basal promoter activities. v-Src does not alter USF1 binding levels at the E-box, but it increases Sp1/Sp3 binding to the GC-box despite no change in their cellular protein abundance. SSeCKS alpha and beta transcript levels in v-Src/3T3 cells can be restored by treatment with the histone deacetylase inhibitor, trichostatin A, but not with the DNA demethylation agent, 5-azacytidine. Chromatin changes are found only on the alpha promoter even though the beta proximal promoter contains a similar E- and GC-box arrangement. Recruitment of HDAC1 is necessary and sufficient to cause repression of alpha proximal promoter activity, and the addition of Sp1 and/or Sp3 potentiates the repression. Our data suggest that suppression of the beta promoter is facilitated by Src-induced changes in the alpha promoter chromatinization mediated by a USF1-Sp1-Sp3 complex.

Lipopolysaccharide induces expression of SSeCKS in rat lung microvascular endothelial cell

Src-suppressed C kinase substrate (SSeCKS) plays a role in membrane-cytoskeletal remodeling to regulate mitogenesis, cell differentiation, and motility. Previous study showed that lipopolysaccharide (LPS) induced a selective and strong expression of SSeCKS in the vascular endothelial cells of lung. Here we show that LPS stimulation elevated expression of SSeCKS mRNA and protein in Rat pulmonary microvascular endothelial cell (RPMVEC). LPS potentiated SSeCKS phosphorylation in a time- and dose-dependent manner, and partly induced translocation of SSeCKS from the cytosol to the membrane after LPS challenge. The PKC inhibitor, Calphostin C, significantly decreased LPS-induced phosphorylation of SSeCKS, inhibited SSeCKS translocation and actin cytoskeleton reorganization after LPS challenge, suggesting that PKC may play a role in LPS-induced SSeCKS translocation and actin rearrangement. We conclude that SSeCKS is located downstream of PKC and that SSeCKS and PKC are both necessary for LPS-induced stress fiber formation.

Src suppressed C kinase substrate regulates the lipopolysaccharide-induced TNF-α biosynthesis in rat astrocytes

The protein kinase C (PKC) is known to be a critical component in the signaling cascades that lead to astrocyte-activation. To further understand the mechanism of PKC signaling in astrocyte-activation, we investigated the effect of SSeCKS, a PKC substrate, on LPS-induced cytokine expression in astrocytes by RT-PCR and enzyme-linked immunosorbent assay. Exposure of the cells to LPS induced rapid translocation of SSeCKS to the perinuclear sides, ERK activation and pronounced TNF-alpha production, which can be inhibited by the PKC inhibitor Gö6983. By using siRNA knockdown of SSeCKS expression, LPS-induced signaling events were partly inhibited, including ERK activation, inducible TNF-alpha biosynthesis and secretion. These results suggest that SSeCKS is involved in the LPS-induced TNF-alpha expression in astrocytes mediated by PKC.

AKAP12 induces apoptotic cell death in human fibrosarcoma cells by regulating CDKI-cyclin D1 and caspase-3 activity

AKAP12 (A-Kinase anchoring protein 12) is a protein kinase C substrate and a potential tumor suppressor. AKAP12 is down-regulated by several oncogenes and strongly suppressed in various cancers including prostate, ovarian and breast cancers. AKAP12 acts as a regulator of mitogenesis by anchoring key signal proteins such as PKA, PKC, and cyclins. In this study, AKAP12 was found to suppress tumor cell viability by inducing apoptosis via caspase-3 in HT1080 cells. This AKAP12-induced apoptosis was associated with a decreased expression of Bcl-2 and increased expression of Bax. Moreover, AKAP12-transfectant strongly induced the expression of Cip1/p21 and Kip1/p27, but resulted in a decrease in cyclin D1 involved in G(1) progression. Accordingly, these results suggest that AKAP12 may play an important role in tumor growth suppression by inducing apoptosis with the regulation of multiple molecules in the cell cycle progression.

Neoplastic hepatocyte growth associated with cyclin D1 redistribution from the cytoplasm to the nucleus in mouse hepatocarcinogenesis

Cyclin D1 overexpression is a frequent change in hepatocellular carcinomas (HCCs). Our present study demonstrated that cyclin D1 overexpression with abundant cyclin E, cdk4, cdk2, and p27Kip1 (p27) occurred in neoplastic hepatocytes from the early stage of mouse hepatocarcinogenesis. While cyclin D1 expression was mainly found in the cytoplasm of the tumor cells, it shifted to the nucleus in association with cell proliferation after the animals were subjected to a partial hepatectomy (PH), and then returned once more to the cytoplasm when the cells became quiescent. Inhibition of PI3 kinase (PI3K) by Ly294002 in mouse HCC cells in vitro suppressed the nuclear shift of cyclin D1 as well as cell proliferation, while PI3K activation by PTEN suppression failed to induce nuclear shift of cyclin D1, suggesting that PI3K activation is essential but not sufficient for the cyclin D1 nuclear shift. While MEK-ERK1/2 inhibition by PD98059 and mTOR inhibition by rapamycin affected the cyclin D1 nuclear shift and cell proliferation to a lesser extent, both these inhibitors reduced cyclin D1 levels. Finally, although p27, cdk4 and calmodulin (CaM) were detected in the cyclin D1 immunoprecipitates from both quiescent and proliferating HCC cells, Hsc70 and SSeCKS were detected only in the immunoprecipitate from quiescent cells, and p21Waf1/Cip1 (p21) was detected only in that from proliferating cells, suggesting that the cyclin D1 complex is different in quiescent and proliferating cells. These observations indicate that the nuclear/cytoplasmic localization of cyclin D1 plays an important role in the proliferation/quiescence of neoplastic hepatocytes.

SSeCKS metastasis-suppressing activity in MatLyLu prostate cancer cells correlates with vascular endothelial growth factor inhibition

SSeCKS, a Src-suppressed protein kinase C substrate with metastasis suppressor activity, is the rodent orthologue of human gravin/AKAP12, a scaffolding protein for protein kinase A and protein kinase C. We show here that the tetracycline-regulated reexpression of SSeCKS in MatLyLu (MLL) prostate cancer cells suppressed formation of macroscopic lung metastases in both spontaneous and experimental models of in vivo metastasis while having minimal inhibitory effects on the growth of primary-site s.c. tumors. SSeCKS decreased angiogenesis in vitro and in vivo by suppressing vascular endothelial growth factor (VEGF) expression in MLL tumor cells as well as in stromal cells. The forced reexpression of VEGF(165) and VEGF(121) isoforms was sufficient to reverse aspects of SSeCKS metastasis-suppressor activity in both the experimental and spontaneous models. SSeCKS reexpression in MLL cells resulted in the down-regulation of proangiogenic genes, such as osteopontin, tenascin C, KGF, angiopoietin, HIF-1alpha, and PDGFRbeta, and the up-regulation of antiangiogenic genes, such as vasostatin and collagen 18a1, a precursor of endostatin. These results suggest that SSeCKS suppresses formation of metastatic lesions by inhibiting VEGF expression and by inducing soluble antiangiogenic factors.

Gene expression property of high-density three-dimensional tissue of HepG2 cells formed in radial-flow bioreactor

In our previous study, we examined three-dimensional culture using 5-ml radial-flow bioreactor (RFB) and showed that genes encoding cell cycle related proteins were suppressed in a stable phase. In this study, we analyzed the gene expression profiles of RFB-cultivated HepG2 cells and found that vascular endothelial growth factor (VEGF) production was strongly induced in the stable phase compared with the growth phase or static two-dimensional culture. When human umbilical vein endothelial cells (HUVECs) were grown under the conditioned medium of the stable phase, it was found that the formation of new blood vessels was induced in the angiogenesis model. DNA microarray analysis showed that the expression levels of both genes related to cell cycle arrest and which are known as tumor markers have increased in the stable phase. This result suggests that HepG2 cells in the stable phase maintain an active tumor phenotype. In addition, the expression of genes induced in the hypoxic condition was also induced in the stable phase. When the culture was carried out under a higher dissolved oxygen (DO) concentration, VEGF production did not decrease significantly and the new blood-vessel-forming ability of the conditioned medium was not suppressed. This suggests that the induction of VEGF production in a stable phase is not affected by DO during the tested level. These results suggest that the RFB cell culture system may be used to assess tumor progression mechanism under three-dimensional condition in vitro.

SSeCKS/Gravin/AKAP12 attenuates expression of proliferative and angiogenic genes during suppression of v-Src-induced oncogenesis

Background

SSeCKS is a major protein kinase C substrate with kinase scaffolding and metastasis-suppressor activity whose expression is severely downregulated in Src- and Ras-transformed fibroblast and epithelial cells and in human prostate, breast, and gastric cancers. We previously used NIH3T3 cells with tetracycline-regulated SSeCKS expression plus a temperature-sensitive v-Src allele to show that SSeCKS re-expression inhibited parameters of v-Src-induced oncogenic growth without attenuating in vivo Src kinase activity.

Methods

We use cDNA microarrays and semi-quantitative RT-PCR analysis to identify changes in gene expression correlating with i) SSeCKS expression in the absence of v-Src activity, ii) activation of v-Src activity alone, and iii) SSeCKS re-expression in the presence of active v-Src.

Results

SSeCKS re-expression resulted in the attenuation of critical Src-induced proliferative and pro-angiogenic gene expression including Afp, Hif-1α, Cdc20a and Pdgfr-β, and conversely, SSeCKS induced several cell cycle regulatory genes such as Ptpn11, Gadd45a, Ptplad1, Cdkn2d (p19), and Rbbp7.

Conclusion

Our data provide further evidence that SSeCKS can suppress Src-induced oncogenesis by modulating gene expression downstream of Src kinase activity.

Medium-sized peptides as built in carriers for biologically active compounds

A growing number of oligopeptides of natural and/or synthetic origin have been described and considered as targeting structures for delivery bioactive compounds into various cell types. This review will outline the discovery of peptide sequences and the corresponding mid-sized oligopeptides with membrane translocating properties and also summarize de novo designed structures possessing similar features. Conjugates and chimera constructs derived from these sequences with covalently attached bioactive peptide, epitope, oligonucleotide, PNA, drug, reporter molecule will be reviewed. A brief note will refer to the present understanding on the uptake mechanism at the end of each section.

SSeCKS/Gravin/AKAP12 Metastasis Suppressor Inhibits Podosome Formation via RhoA- and Cdc42-Dependent Pathways

Podosomes are poorly understood actin-rich structures notably found in cancer cell lines or in v-Src-transformed cells that are thought to facilitate some of the invasive properties involved in tumor metastasis. The enrichment of the Tks5/Fish protein, a v-Src substrate, is required for formation of podosomes. We showed previously that the tetracycline-regulated reexpression of the Src-suppressed C kinase substrate (SSeCKS, also known as Gravin/AKAP12) inhibited variables of v-Src-induced oncogenic growth in NIH3T3, correlating with the induction of normal actin cytoskeletal structures and cell morphology but not with gross inhibition of Src phosphorylation activity in the cell. Here, we show that SSeCKS reexpression at physiologic levels suppresses podosome formation, correlating with decreases in Matrigel invasiveness, whereas there is no effect on total cellular tyrosine phosphorylation or on the phosphorylation of Tks5/Fish. Activated forms of RhoA and Cdc42 were capable of rescuing podosome formation in v-Src cells reexpressing SSeCKS, and this correlated with the ability of SSeCKS to inhibit RhoA and Cdc42 activity levels by >5-fold. Interestingly, although activated Rac I had little effect on podosome formation, it could partner with activated RhoA to reverse the cell flattening induced by SSeCKS. These data suggest that v-Src-induced Tks5 tyrosine phosphorylation is insufficient for podosome formation in the absence of RhoA- and/or Cdc42-mediated cytoskeletal remodeling. Additionally, they strengthen the notion that SSeCKS suppresses Src-induced oncogenesis by reestablishing actin-based cytoskeletal architecture.

Enhanced v-Src-induced oncogenic transformation in the absence of focal adhesion kinase is mediated by phosphatidylinositol 3-kinase

We showed previously [K. Moissoglu, I.H. Gelman, J. Biol. Chem. 278 (2003) 47946-47959] that oncogenic v-Src could induce 7- to 10-fold greater anchorage-independent growth (AIG) in FAK-null mouse embryo fibroblasts (MEF) compared to those expressing FAK. Here, we demonstrate that the enhanced AIG (eAIG) correlates with increased activation levels of phosphatidylinositol 3-kinase (PI3K) and not with changes in the protein levels of the p85 regulatory subunit of PI3K, PDK1 or PTEN- modulators, and/or mediators of PI3K activity. eAIG could be blunted selectively by treatment with the PI3K inhibitor, LY294002, or by overexpression of either the PI3K antagonist, PTEN, dominant-interfering alleles of PI3K or a downstream PI3K mediator, AKT, but not by the MEK inhibitor, PD98059, dominant-interfering alleles of MEK or the signal transducer and activator of transcription (STAT)-3. In contrast, RNAi-mediated knockdown of FAK resulted in increased v-Src-induced AIG. Expression of a constitutively active PI3K allele was sufficient to induce higher levels of AIG, whereas overexpression of v-Src produced only larger-sized colonies in soft agar. Interestingly, FAK was required for full activation of PI3K by PDGF whereas the activation of PI3K by insulin was significantly increased in FAK-/- cells. Thus, although FAK is dispensable for v-Src-induced oncogenic transformation in vitro, it may exert either positive or negative effects on signaling or motility depending on which pathways are activated in cancer cells.

Cross-species sequence analysis reveals multiple charged residue-rich domains that regulate nuclear/cytoplasmic partitioning and membrane localization of a kinase anchoring protein 12 (SSeCKS/Gravin)

A kinase anchoring proteins (AKAPs) assemble and compartmentalize multiprotein signaling complexes at discrete subcellular locales and thus confer specificity to transduction cascades using ubiquitous signaling enzymes, such as protein kinase A. Intrinsic targeting domains in each AKAP determine the subcellular localization of these complexes and, along with protein-protein interaction domains, form the core of AKAP function. As a foundational step toward elucidating the relationship between location and function, we have used cross-species sequence analysis and deletion mapping to facilitate the identification of the targeting determinants of AKAP12 (also known as SSeCKS or Gravin). Three charged residue-rich regions were identified that regulate two aspects of AKAP12 localization, nuclear/cytoplasmic partitioning and perinuclear/cell periphery targeting. Using deletion mapping and green fluorescent protein chimeras, we uncovered a heretofore unrecognized nuclear localization potential. Five nuclear localization signals, including a novel class of this type of signal termed X2-NLS, are found in the central region of AKAP12 and are important for nuclear targeting. However, this nuclear localization is suppressed by the negatively charged C terminus that mediates nuclear exclusion. In this condition, the distribution of AKAP12 is regulated by an N-terminal targeting domain that simultaneously directs perinuclear and peripheral AKAP12 localization. Three basic residue-rich regions in the N-terminal targeting region have similarity to the MARCKS proteins and were found to control AKAP12 localization to ganglioside-rich regions at the cell periphery. Our data suggest that AKAP12 localization is regulated by a hierarchy of targeting domains and that the localization of AKAP12-assembled signaling complexes may be dynamically regulated.

Delivery of bioactive molecules into the cell: the Trojan horse approach

In recent years, vast amounts of data on the mechanisms of neural de- and regeneration have accumulated. However, only in disproportionally few cases has this led to efficient therapies for human patients. Part of the problem is to deliver cell death-averting genes or gene products across the blood-brain barrier (BBB) and cellular membranes. The discovery of Antennapedia (Antp)-mediated transduction of heterologous proteins into cells in 1992 and other "Trojan horse peptides" raised hopes that often-frustrating attempts to deliver proteins would now be history. The demonstration that proteins fused to the Tat protein transduction domain (PTD) are capable of crossing the BBB may revolutionize molecular research and neurobiological therapy. However, it was only recently that PTD-mediated delivery of proteins with therapeutic potential has been achieved in models of neural degeneration in nerve trauma and ischemia. Several groups have published the first positive results using protein transduction domains for the delivery of therapeutic proteins in relevant animal models of human neurological disorders. Here, we give an extensive review of peptide-mediated protein transduction from its early beginnings to new advances, discuss their application, with particular focus on a critical evaluation of the limitations of the method, as well as alternative approaches. Besides applications in neurobiology, a large number of reports using PTD in other systems are included as well. Because each protein requires an individual purification scheme that yields sufficient quantities of soluble, transducible material, the neurobiologist will benefit from the experiences of other researchers in the growing field of protein transduction.

AKAP12/Gravin is inactivated by epigenetic mechanism in human gastric carcinoma and shows growth suppressor activity

AKAP12/Gravin, one of the A-kinase anchoring proteins (AKAPs), functions as a kinase scaffold protein and as a dynamic regulator of the beta2-adrenergic receptor complex. However, the biological role of AKAP12 in cancer development is not well understood. The AKAP12 gene encodes two major isoforms of 305 and 287 kDa (designated AKAP12A and AKAP12B, respectively, in this report). We found that these two isoforms are independently expressed and that they are probably under the control of two different promoters. Moreover, both isoforms were absent from the majority of human gastric cancer cells. The results from methylation-specific PCR (MSP) and bisulfite sequencing revealed that the 5' CpG islands of both AKAP12A and AKAP12B are frequently hypermethylated in gastric cancer cells. Treatment with DNA methyltransferase inhibitor and/or histone deacetylase inhibitor efficiently restored the expression of AKAP12 isoforms, confirming that DNA methylation is directly involved in the transcriptional silencing of AKAP12 in gastric cancer cells. Hypermethylation of AKAP12A CpG island was also detected in 56% (10 of 18) of primary gastric tumors. The restoration of AKAP12A in AKAP12-nonexpressing cells reduced colony formation and induced apoptotic cell death. In conclusion, our results suggest that AKAP12A may function as an important negative regulator of the survival pathway in human gastric cancer.

Murine coronavirus nonstructural protein p28 arrests cell cycle in G0/G1 phase

Murine coronavirus mouse hepatitis virus (MHV) gene 1 encodes several nonstructural proteins. The functions are unknown for most of these nonstructural proteins, including p28, which is encoded at the 5' end of the MHV genome. Transient expression of cloned p28 in several different cultured cells inhibited cell growth, indicating that p28 expression suppressed cell proliferation. Expressed p28 was exclusively localized in the cytoplasm. Cell cycle analysis by flow cytometry demonstrated that p28 expression induced G(0)/G(1) cell cycle arrest. Characterization of various cellular proteins that are involved in regulating cell cycle progression demonstrated that p28 expression resulted in an accumulation of hypophosphorylated retinoblastoma protein (pRb), tumor suppressor p53, and cyclin-dependent kinase (Cdk) inhibitor p21(Cip1). Expression of p28 did not alter the amount of p53 transcripts yet increased the amount of p21(Cip1) transcripts, suggesting that p28 expression increased p53 stability and that p21(Cip1) was transcriptionally activated in a p53-dependent manner. Our present data suggest the following model of p28-induced G(0)/G(1) cell cycle arrest. Expressed cytoplasmic p28 induces the stabilization of p53, and accumulated p53 causes transcriptional upregulation of p21(Cip1). The increased amount of p21(Cip1) suppresses cyclin E/Cdk2 activity, resulting in the inhibition of pRb hyperphosphorylation. Accumulation of hypophosphorylated pRb thus prevents cell cycle progression from G(0)/G(1) to S phase.

Multiple promoters direct expression of three AKAP12 isoforms with distinct subcellular and tissue distribution profiles

A Kinase Anchoring Protein 12 (AKAP12; also known as src-suppressed C kinase substrate (SSeCKS) and Gravin) is a multivalent anchoring protein with tumor suppressor activity. Although expression of AKAP12 has been examined in a number of contexts, its expression control remains to be elucidated. Herein, we characterize the genomic organization of the AKAP12 locus, its regulatory regions, and the spatial distribution of the proteins encoded by the AKAP12 gene. Using comparative genomics and various wet-lab assays, we show that the AKAP12 locus is organized as three separate transcription units that are governed by non-redundant promoters coordinating distinct tissue expression profiles. The proteins encoded by the three AKAP12 isoforms (designated alpha, beta, and gamma) share >95% amino acid sequence identity but differ at their N termini. Analysis of the targeting of each isoform reveals distinct spatial distribution profiles. An N-terminal myristoylation motif present in AKAP12alpha is shown to be necessary and sufficient for targeted expression of this AKAP12 isoform to the endoplasmic reticulum, a novel subcellular compartment for AKAP12. Our results demonstrate heretofore unrecognized complexity within the AKAP12 locus and suggest a mechanism for genetic control of signaling specificity through distinct regulation of alternately targeted anchoring protein isoforms.

Identification of a novel putative non-selenocysteine containing phospholipid hydroperoxide glutathione peroxidase (NPGPx) essential for alleviating oxidative stress generated from polyunsaturated fatty acids in breast cancer cells

A dramatic reduction in the expression of a novel phospholipid hydroperoxide glutathione peroxidase (PHGPx), which incorporates cysteine instead of selenocysteine in the conserved catalytic motif was observed in a microarray analysis using cDNAs amplified from mRNA of Brca1-null mouse embryonic fibroblasts. This non-selenocysteine PHGPx named NPGPx is a cytoplasmic protein with molecular mass of approximately 22 kDa and has little detectable glutathione peroxidase activity in vitro. Ectopic expression of NPGPx in Brca1-null cells that were sensitive to oxidative stress induced by hydrogen peroxide conferred a similar resistance level to that of the wild-type cells, suggesting the importance of this protein in reducing oxidative stress. Expression of NPGPx was found in many tissues, including developing mammary gland. However, the majority of breast cancer cell lines studied (11 of 12) expressed very low or undetectable levels of NPGPx irrespective of BRCA1 status. Re-expression of NPGPx in breast cancer lines, MCF-7 and HCC1937, which have very little or no endogenous NPGPx, induced resistance to eicosapentaenoic acid (an omega-3 type of polyunsaturated fatty acid)-mediated cell death. Conversely, inhibition of the expression of NPGPx by the specific small interfering RNA in HS578T breast cancer cells that originally express substantial amounts of endogenous NPGPx increased their sensitivity to eicosapentaenoic acid-mediated cell death. Thus, NPGPx plays an essential role in breast cancer cells in alleviating oxidative stress generated from polyunsaturated fatty acid metabolism.

Transcription repression in oncogenic transformation: common targets of epigenetic repression in cells transformed by Fos, Ras or Dnmt1

Fos and Ras function in both dependent and independent signal transduction pathways, and sustained activity of either oncogene is sufficient to induce cell transformation and tumorigenesis. Increased DNA (cytosine-5) methyltransferse (Dnmt1) activity is involved in the mechanism of transformation by both oncogenes, suggesting that inappropriate epigenetic transcription regulation may be a common route of oncogenesis, and that cell transformation may model aspects of the epigenetic deregulation that often occurs in tumors. Here, we have taken a microarray-based gene expression approach to identify differentially expressed genes in cells transformed by c-fos, v-fos, ras or Dnmt1. The cohort of genes differentially expressed in all four transformation systems includes an over-representation of repressed genes, many of which have been functionally implicated in the suppression of transformation or tumorigenesis. Furthermore, we identified four potential tumor suppressor genes subject to epigenetic transcriptional repression in transformed cells. The results emphasize the role of transcription repression in oncogenesis, and they provide insights into the potential common epigenetic mechanisms impacting cell transformation.

Kinetics of senescence-associated changes of gene expression in an epithelial, temperature-sensitive SV40 large T antigen model

Replicative senescence limits the number of times primary cells can divide and is therefore regarded as a potential checkpoint for cancer progression. The majority of studies examining changes of gene expression upon senescence have been made with stationary senescent cells. We wanted to study the transition from normal growth to senescence in detail and identify early regulators of senescence by analyzing early changes in global gene expression, using Affymetrix microarrays. For this purpose, we used a murine epithelial senescence model, where senescence is abrogated by SV40 large T antigen and can be induced by using a temperature-sensitive form of SV40 large T antigen (SV40ts58). Comparisons were made to wild-type SV40 large T antigen-expressing cells and to cells expressing SV40ts58 large T antigen grown to confluence. After removal of genes that are similarly regulated in wild-type and temperature-sensitive SV40 large T antigen-expressing cells, 60% of the remaining genes were shared between cells arrested by inactivation of SV40 T antigen and by confluence. We identified 125 up-regulated and 39 down-regulated candidate genes/expressed sequence tags that are regulated upon SV40 T antigen inactivation and not during heat shock or confluence and classified these based on their kinetic profiles. Our study identified genes that fall into different functional clusters, such as transforming growth factor-beta-related genes and transcription factors, and included genes not identified previously as senescence associated. The genes are candidates as early regulators of the senescence checkpoint and may be potential molecular targets for novel anticancer drugs.

Cell-cycle targeted therapies

Eukaryotic organisms depend on an intricate and evolutionary conserved cell cycle to control cell division. The cell cycle is regulated by a number of important protein families which are common targets for mutational inactivation or overexpression in human tumours. The cyclin D and E families and their cyclin-dependent kinase partners initiate the phosphorylation of the retinoblastoma tumour suppressor protein and subsequent transition through the cell cycle. Cyclin/cdk activity and therefore control of cell division is restrained by two families of cyclin dependent kinase inhibitors. A greater understanding of the cell cycle has led to the development of a number of compounds with the potential to restore control of cell division in human cancers. This review will introduce the protein families that regulate the cell cycle, their aberrations in malignant progression and pharmacological strategies targeting this important process.

DNA oligonucleotide microarray technology identifies fisp-12 among other potential fibrogenic genes following murine unilateral ureteral obstruction (UUO): Modulation during epithelial-mesenchymal transition

BACKGROUND

Tubulointerstitial inflammation and fibrosis are pathologic hallmarks of end-stage renal disease (ESRD). Here we have used DNA microarray technology to monitor the transcriptomic responses to murine unilateral ureteral obstruction (UUO) with a view to identifying molecular modulators of tubulointerstitial fibrosis.

METHODS

Using Affymetrix Mu74Av2 microarrays, gene expression 4 and 10 days postobstruction was investigated relative to control contralateral kidneys. Candidate profibrogenic genes were further investigated in epithelial cells undergoing epithelial to mesenchymal transition (EMT) in vitro.

RESULTS

mRNA levels for 1091 gene/EST sequences, of a total of 12,488 displayed on the microarray, were altered twofold or greater by days 4 and 10 postobstruction compared to contralateral control kidneys. Genes were categorised into functional groups, including modulators of cytoskeletal and extracellular matrix metabolism, cell growth, signalling, and transcription/translational events. Among the potentially profibrogenic genes, whose mRNA levels were increased after UUO, were fibroblast-inducible secreted protein (fisp-12), the murine homologue of connective tissue growth factor (CTGF), collagen XVIIIalpha1, secreted protein acidic and rich in cysteine (SPARC), and src-suppressed C-kinase substrate (SSeCKS). A sustained increase in fisp-12 mRNA level was observed during EMT induced by transforming growth factor-beta1 (TGF-beta1) and epidermal growth factor (EGF).

CONCLUSION

Altered gene expression in murine UUO has been demonstrated. Increased expression of fisp-12, SPARC, and SSeCKS has been shown in response to TGF-beta1 treatment and during EMT, suggesting that these genes may offer potential therapeutic targets against tubulointerstitial fibrosis.

The association of cyclin A and cyclin kinase inhibitor p21 in response to γ-irradiation requires the CDK2 binding region, but not the Cy motif

The cyclin kinase inhibitor p21 associates with and inhibits cyclin-CDKs to retard the progress of the cell cycle in response to DNA damage. The recognition sites for cyclin binding on the various cell cycle-related molecules have been identified as RXL motifs. In the case of p21, the dependence of the Cy1 (18CRRL) or Cy2 (154KRRL) motifs on cyclin E, but not on cyclin A has been demonstrated by in vitro experiments. In this study, to clarify the mechanism of p21 association with cyclin A, we constructed a p21 expression system in mammalian cells. After transfection with an expression vector containing cDNA of various p21-mutants, cells were irradiated with 10 Gy of gamma-rays to introduce DNA damage, followed by quantification of the p21-cyclin A association. The p21-mutant constructs were single or multiple deletions in Cy1, Cy2, and the CDK2 binding region, and a nonphosphorylatable alanine mutant of the C-terminal phosphorylation site. We demonstrated that the association of p21 and cyclin A in response to gamma-irradiation requires the CDK binding region, 49-71 aa, but not the Cy motifs. We believe the mechanism by which p21 inhibits cyclin-CDKs is distinct in each phase of the cell cycle. Furthermore, the increase in the association of p21 and cyclin A was not correlated with the levels of p21. This suggests that DNA damage triggers a signal to the p21 region between 21 and 96 aa to allow cyclin A association.

v-Src rescues actin-based cytoskeletal architecture and cell motility and induces enhanced anchorage independence during oncogenic transformation of focal adhesion kinase-null fibroblasts

The ability of the focal adhesion kinase (FAK) to integrate signals from extracellular matrix and growth factor receptors requires the integrity of Tyr397, a major autophosphorylation site that mediates the Src homology 2-dependent binding of Src family kinases. However, the precise roles played by FAK in specific Src-induced pathways, especially as they relate to oncogenic transformation, remain unclear. Here, we investigate the role of FAK in v-Src-induced oncogenic transformation by transducing temperature-sensitive v-Src (ts72v-Src) into p53-null FAK+/+ or FAK-/- mouse embryo fibroblasts (MEF). At the permissive temperature (PT), ts72v-Src induced abundant tyrosine phosphorylation, morphological transformation and cytoskeletal rearrangement in FAK-/- MEF, including the restoration of cell polarity, typical focal adhesion complexes, and longitudinal F-actin stress fibers. v-Src rescued the haptotactic, linear directional, and invasive motility defects of FAK-/- cells to levels found in FAK+/+ or FAK+/+-[ts72v-Src] cells, and, in the case of monolayer wound healing motility, there was an enhancement. Src activation failed to increase the high basal tyrosine phosphorylation of the Crk-associated substrate, CAS, found in FAK-/- MEF, indicating that CAS phosphorylation alone is insufficient to induce motility in the absence of FAK- or v-Src-induced cytoskeletal remodeling. Compared with FAK+/+[ts72v-Src] controls, FAK-/-[ts72v-Src] clones exhibited 7-10-fold higher anchorage-independent proliferation that could not be attributed to variations in either v-Src protein level or stability. Re-expression of FAK diminished the colony-forming activities of FAK-/-[ts72v-Src] without altering ts72v-Src expression levels, suggesting that FAK attenuates Src-induced anchorage independence. Our data also indicate that the enhanced Pyk2 level found in FAK-/- MEF plays no role in v-Src-induced anchorage independence. Overall, our data indicate that FAK, although dispensable, attenuates v-Src-induced oncogenic transformation by modulating distinct signaling and cytoskeletal pathways.

Metastasis suppressor pathways--an evolving paradigm

A greater understanding of the processes of tumor invasion and metastasis, the principal cause of death in cancer patients, is essential to determine newer therapeutic targets. Metastasis suppressor genes, by definition, suppress metastasis without affecting tumorigenicity and, hence, present attractive targets as prognostic or therapeutic markers. This short review focuses on those twelve metastasis suppressor genes for which functional data exist. We also outline newly identified genes that bear promising traits of having metastasis suppressor activity, but for which functional data have not been completed. We also summarize the biochemical mechanism(s) of action (where known), and present a working model assembling potential metastasis suppression pathways.

SSeCKS regulates angiogenesis and tight junction formation in blood-brain barrier

The blood-brain barrier (BBB) is essential for maintaining brain homeostasis and low permeability. BBB maintenance is important in the central nervous system (CNS) because disruption of the BBB may contribute to many brain disorders, including Alzheimer disease and ischemic stroke. The molecular mechanisms of BBB development remain ill-defined, however. Here we report that src-suppressed C-kinase substrate (SSeCKS) decreases the expression of vascular endothelial growth factor (VEGF) through AP-1 reduction and stimulates expression of angiopoietin-1 (Ang-1), an antipermeability factor in astrocytes. Conditioned media from SSeCKS-overexpressing astrocytes (SSeCKS-CM) blocked angiogenesis in vivo and in vitro. Moreover, SSeCKS-CM increased tight junction proteins in endothelial cells, consequently decreasing [3H]sucrose permeability. Furthermore, immunoreactivity to SSeCKS gradually increased during the BBB maturation period, and SSeCKS-expressing astrocytes closely interacted with zonula occludens (ZO)-1-expressing blood vessels in vivo. Collectively, our results suggest that SSeCKS regulates BBB differentiation by modulating both brain angiogenesis and tight junction formation.

Insulin-like growth factor I triggers nuclear accumulation of cyclin D1 in MCF-7S breast cancer cells

Stimulation of the breast cancer-derived MCF-7S cell line with insulin-like growth factor I (IGF-I; 20 ng/ml) leads to enhanced expression of cyclin D1, hyperphosphorylation of pRb, DNA synthesis, and cell division. 17beta-Estradiol (E(2); 10(-9) m) is not able to stimulate proliferation of MCF-7S cells, although addition of E(2) to serum-starved cells does result in induction of cyclin D1. However, in combination with submitogenic amounts of IGF-I (2 ng/ml), E(2) induces cell proliferation. We have previously shown that the synergistic action of E(2) and IGF-I emanates from the ability of both hormones to induce cyclin D1 expression and that IGF-I action is required to induce activity of the cyclin D1-CDK4 complex, which triggers cell cycle progression. Here, we show that IGF-I (but not E(2)) is able to induce nuclear accumulation of cyclin D1 by a phosphatidylinositol 3-kinase-dependent mechanism. Nuclear accumulation of cyclin D1 and cell cycle progression were also observed when LiCl, a known inhibitor of GSK3beta, was added to E(2)-stimulated cells. Thus, inhibition of GSK3beta activity appears to trigger nuclear accumulation of cyclin D1 and cell cycle progression. This notion was confirmed by overexpression of constitutively active GSK3beta, which blocks IGF-I-induced nuclear accumulation of cyclin D1 as well as S phase transition.

Ligand-specific control of src-suppressed C kinase substrate gene expression

The src-suppressed C-kinase substrate, SSeCKS, is now recognized as a key regulator of cell signaling and cytoskeletal dynamics. However, few ligands that control SSeCKS expression have been identified. We report that platelet-derived growth factor-BB (PDGF-BB), lysophosphatidic acid (LPA), and eicosapentaenoic acid (EPA) potently modulate SSeCKS gene expression in cultured smooth muscle (RASM) cells relative to other bioactive ligands tested. In addition, EPA-dependent regulation of SSeCKS expression correlates with distinct changes in cell morphology and adhesion in RASM cells. Independent evidence that ligand-specific control of SSeCKS expression links to the regulation of cell adhesion and morphology was obtained using ras-transformed fibroblasts, KNRK. Sodium butyrate (NaB) upregulates SSeCKS mRNA and protein expression corresponding to increased cell-spreading and adhesion. In addition, ectopic expression of recombinant SSeCKS recapitulates attributes of NaB-induced morphogenesis in KNRK cells. The data provide novel evidence that SSeCKS functions in PDGF-BB-, LPA-, EPA-, and NaB-mediated cell signaling.

Control of pre-BCR signaling by Pax5-dependent activation of the BLNK gene

The developmental progression from pro-B to pre-B cells is controlled by pre-B cell receptor (pre-BCR) signaling which depends on BLNK (SLP-65) for coupling the Syk kinase to its downstream effector pathways. Here we identified BLNK as a direct target of the transcription factor Pax5 (BSAP). Restoration of BLNK expression in Ig(mu) transgenic Pax5(-/-) pro-B cells resulted in constitutive pre-BCR signaling and increased cell proliferation without inducing progression to the pre-B cell stage. Ig(mu)(+) Pax5(-/-) pro-B cells expressing a BLNK-estrogen receptor fusion protein initiated signaling immediately upon hormone addition, which facilitated analysis of pre-BCR-induced gene expression changes. The pre-BCR was shown to execute its checkpoint function by regulating genes involved in cell proliferation, intracellular signaling, growth factor responsiveness, and V(D)J recombination.

Mitogen-induced, FAK-dependent tyrosine phosphorylation of the SSeCKS scaffolding protein

The ability of mitogens to rapidly induce tyrosine phosphorylation of cellular proteins has been taken as evidence of participation in subsequent signaling pathways. SSeCKS, a major protein kinase C (PKC) substrate with protein scaffolding and tumor suppressive properties, becomes tyrosine phosphorylated in NIH3T3 and rodent embryo fibroblasts after short-term treatment with epidermal growth factor (EGF), platelet-derived growth factor (PDGF), or fetal calf serum in the presence of pervanadate, but not by treatment with insulin or insulin-like growth factor-1. The relative phosphotyrosine level on SSeCKS was higher in actively dividing cells than in confluent cultures. Tyrosine phosphorylation of SSeCKS was apparent in cells deficient in Src, Fyn, Yes, or Abl tyrosine kinases or in NIH3T3 cells expressing a temperature-sensitive v-Src allele, but not in FAK-deficient embryo fibroblasts. Purified FAK or Src enzyme failed to directly phosphorylate SSeCKS in vitro. EGF failed to induce SSeCKS tyrosine phosphorylation in FAK-/- fibroblasts, indicating that the EGF receptor is probably not the direct kinase of SSeCKS. Phosphorylation under these conditions was rescued by the transient reexpression of wt-FAK but not FAK mutated at Y397, a major autophosphorylation and SH2-based docking site. Adhesion of FAK+/+ cells to fibronectin failed to significantly induce SSeCKS tyrosine phosphorylation although FAK was activated, suggesting that SSeCKS phosphorylation is mediated through a growth factor receptor-FAK rather than an integrin-FAK pathway. Moreover, PDGF could induce SSeCKS tyrosine phosphorylation in the absence of FAK activation, suggesting a role for FAK SH2-based docking rather than kinase activity. Immunofluorescence analysis showed that in FAK-/- cells, SSeCKS costains along F-actin stress fibers, in contrast to FAK+/+ cells, where most SSeCKS stains at the cell edge and along a cortical cytoskeletal matrix. This correlated with increased coprecipitation of SSeCKS with biotin-phalloidin-bound F-actin from FAK-/- compared to FAK+/+ cell lysates. Similarly, bacterially expressed, unphosphorylated SSeCKS cosedimented with F-actin in ultracentrifugation assays. These data suggest that mitogen-induced, FAK-dependent tyrosine phosphorylation of SSeCKS modulates its binding to the actin-based cytoskeleton, suggesting a role for SSeCKS in mitogen-induced cytoskeletal reorganization.

Global analysis of gene expression in renal ischemia-reperfusion in the mouse

Ischemia-induced acute renal failure (ARF) is a relatively common disorder with major morbidity and mortality. To study global gene expression during ARF, 6-week-old C57BL/6 male mice underwent 30 min of bilateral renal ischemia followed by reperfusion [I/R] or sham operation. Oligonucleotide microarrays [Affymetrix] with approximately 10,000 genes, 6,643 of which were present in mouse kidney, were used to analyze mRNA expression for up to 4 days following I/R. Fifty-two genes at day 1 and 40 at day 4 were up-regulated more than 4-fold [400%]. Seventy genes at day 1 and 30 genes at day 4 were down-regulated to under 0.25-fold from baseline [25%]. Real-time quantitative RT-PCR confirmed changes in expression for 8 genes of interest. Most of the induced transcripts are involved in cell structure, extracellular matrix, intracellular calcium binding, and cell division/differentiation. Our data identified several novel genes that may be important in renal repair after ischemia.

Discovery of osteoblast-associated genes using cDNA microarrays

Osteoblast maturation is a complex process and involves distinct genotypic changes that are accompanied by specific phenotypic alterations. To identify new bone-related genes in osteoblasts we utilized the high-density mouse GEM1 microarray gene chip from IncyteGenomics, Inc. (St. Louis, Mo). We examined the expression profiles of over 8700 genes during the proliferation (day 3) and the mineralization (day 34) phases of MC3T3-E1 development. More than 8600 genes provided measurable signals. Of these genes, 252 were found to be differentially expressed on days 3 and 34. A large number of these genes have never been previously recognized in the context of osteoblast development. Approximately, 60% of the genes with expressions that were dominant in proliferating osteoblasts consisted of growth-related genes such as TACC3 and Pr22. The expressions of TIS21/BTG2, and a novel gene EST350, were found to peak during the differentiation phase (day 12), suggesting that they may play important roles in osteoblast differentiation. The majority of the genes with expressions that were dominant during the mineralization phase consisted of signal transduction genes and extracellular matrix (ECM) proteins such as lumican and cystatin-C. It is significant that lumican expression could not be detected on day 3, which indicates that this gene may serve as an important marker of postmitotic osteoblasts. The establishment of the expression profiles of these and other genes with various phases of MC3T3-E1 osteoblast development will allow us to distinguish the molecular events at different phases of osteoblast biology.

Proliferation of Schwann cells and regulation of cyclin D1 expression in an animal model of Charcot-Marie-Tooth disease type 1A

Overexpression of PMP22 is responsible for the most common form of inherited neuropathy, Charcot-Marie-Tooth disease (CMT) type 1A. The PMP22-transgenic rat (CMT rat) is an animal model of CMT1A, and its peripheral nerves show the characteristic features of ongoing demyelination and remyelination that is also seen in CMT1A patients. Since Schwann cell proliferation is a prominent feature of peripheral nerves in inherited peripheral neuropathies, we examined proliferation and the expression of cyclin D1 in CMT rats. D-type cyclins are required for the initial steps in cell division and nuclear import is crucial for the function of cyclin D1 in promoting cell proliferation. Like normal myelinating Schwann cells in wild-type rats, remyelinating Schwann cells in CMT rats show perinuclear cyclin D1 expression. Schwann cells with nuclear cyclin D1 expression, as well as proliferating Schwann cells, were both associated with demyelinated axonal segments. Supernumerary onion bulb Schwann cells, however, do not express cyclin D1 and were not proliferating. Thus, cyclin D1 expression and its subcellular localization correlate directly with distinct physiological states of Schwann cells in this animal model of CMT1A.

Calmodulin and cyclin D anchoring sites on the Src-suppressed C kinase substrate, SSeCKS

SSeCKS and its human orthologue, Gravin, are large scaffolding proteins that are thought to facilitate mitogenic control by anchoring key signal mediators such as protein kinase (PK) C, PKA, the plasma membrane associated isoform of alpha-1,4-galactosyltransferase (GalTase), beta2-adrenergic receptor, and cyclins. SSeCKS is also a major PKC substrate and phosphatidylserine-dependent PKC binding protein whose phosphorylation sites shares homology with a site in the MARCKS protein that encodes phosphorylation-sensitive calmodulin (CaM) binding activity. In the present study, we mapped the in vitro binding sites for CaM and cyclins on SSeCKS. Four CaM binding sites were identified by binding assays that conform to the so-called 1-5-10 motif. Notably, CaM binding was antagonized by prephosphorylation of SSeCKS by PKC. We also identified two major cyclin binding (CY) sites that overlap a major PKC phosphorylation site in SSeCKS (Ser(507/515)), and showed that cyclin D binding is attenuated if SSeCKS is prephosphorylated by PKC. These data suggest that the scaffolding activities of SSeCKS are modulated by mitogenically stimulated kinases such as PKC.

Emerging targets: molecular mechanisms of cell contact-mediated growth control

Contact inhibition of cell proliferation evokes a unique cellular program of growth arrest compared with stress, age, or other physical constraints. The last decade of research on genes activated by cell-cell contact has uncovered features of transmembrane signaling, cytoskeletal reorganization, and transcriptional control that initiate and maintain a quiescent phenotype. This review will focus on mechanisms controlling contact inhibition of cell proliferation, highlighting specific gene expression responses that are activated by cell-cell contact. Although a temporal framework for imposition of these mechanisms has not yet been well described, contact inhibition of cell proliferation clearly requires their coordinated function. Novel targets for intervention in proliferative disorders are emerging from these studies.

Differential cyclin D1 requirements of proliferating Schwann cells during development and after injury

Neurons regulate Schwann cell proliferation, but little is known about the molecular basis of this interaction. We have examined the possibility that cyclin D1 is a key regulator of the cell cycle in Schwann cells. Myelinating Schwann cells express cyclin D1 in the perinuclear region, but after axons are severed, cyclin D1 is strongly upregulated in parallel with Schwann cell proliferation and translocates into Schwann cell nuclei. During development, cyclin D1 expression is confined to the perinuclear region of proliferating Schwann cells and the analysis of cyclin D1-null mice indicates that cyclin D1 is not required for this type of Schwann cell proliferation. As in the adult, injury to immature peripheral nerves leads to translocation of cyclin D1 to Schwann cell nuclei and injury-induced proliferation is impaired in both immature and mature cyclin D1-deficient Schwann cells. Thus, our data indicate that the molecular mechanisms regulating proliferation of Schwann cells during development or activated by axonal damage are fundamentally different.

Cellular delivery of impermeable effector molecules in the form of conjugates with peptides capable of mediating membrane translocation

Most molecules that are not actively imported by living cells are impermeable to cell membranes, including practically all macromolecules and even many small molecules whose physicochemical properties prevent passive membrane diffusion. The use of peptide vectors capable of transporting such molecules into cells in the form of covalent conjugates has become an increasingly attractive solution to this problem. Not only has this technology permitted the study of modulating intracellular target proteins, but it has also gained importance as an alternative to conventional cellular transfection with oligonucleotides. Peptide vectors derived from viral, bacterial, insect, and mammalian proteins endowed with membrane translocation properties have now been proposed as delivery vectors. These are discussed comprehensively and critically in terms of relative utility, applications to compound classes and specific molecules, and relevant conjugation chemistry. Although in most cases the mechanisms of membrane translocation are still unclear, physicochemical studies have been carried out with a number of peptide delivery vectors. Unifying and distinguishing mechanistic features of the various vectors are discussed. Until a few years ago speculations that it might be possible to deliver peptides, proteins, oligonucleotides, and impermeable small molecules with the aid of cellular delivery peptides not only to target cells in vitro, but in vivo, was received with scepticism. However, the first studies showing pharmacological applications of conjugates between macromolecules and peptide delivery vectors are now being reported, and therapies based on such conjugates are beginning to appear feasible.

Mutational analysis of the Cy motif from p21 reveals sequence degeneracy and specificity for different cyclin-dependent kinases

Inhibitors, activators, and substrates of cyclin-dependent kinases (cdks) utilize a cyclin-binding sequence, known as a Cy or RXL motif, to bind directly to the cyclin subunit. Alanine scanning mutagenesis of the Cy motif of the cdk inhibitor p21 revealed that the conserved arginine or leucine (constituting the conserved RXL sequence) was important for p21's ability to inhibit cyclin E-cdk2 activity. Further analysis of mutant Cy motifs showed, however, that RXL was neither necessary nor sufficient for a functional cyclin-binding motif. Replacement of either of these two residues with small hydrophobic residues such as valine preserved p21's inhibitory activity on cyclin E-cdk2, while mutations in either polar or charged residues dramatically impaired p21's inhibitory activity. Expressing p21N with non-RXL Cy sequences inhibited growth of mammalian cells, providing in vivo confirmation that RXL was not necessary for a functional Cy motif. We also show that the variant Cy motifs identified in this study can effectively target substrates to cyclin-cdk complexes for phosphorylation, providing additional evidence that these non-RXL motifs are functional. Finally, binding studies using p21 Cy mutants demonstrated that the Cy motif was essential for the association of p21 with cyclin E-cdk2 but not with cyclin A-cdk2. Taking advantage of this differential specificity toward cyclin E versus cyclin A, we demonstrate that cell growth inhibition was absolutely dependent on the ability of a p21 derivative to inhibit cyclin E-cdk2.

Gene trap insertion reveals two open reading frames in the mouse SSeCKS gene: the form predominantly detected in the nervous system is suppressed by the insertion while the other, specific of the testis, remains expressed

Scaffold proteins play an important role in regulating signal transduction by targeting kinases and phosphatases in close proximity to their relevant substrates. SSeCKS protein has been described as a protein kinase C and A (PKC/PKA) anchoring protein as well as a PKC substrate with a tumor suppressor activity. In this study, we report the generation, via gene trapping in embryonic stem cells of mice carrying an insertion in the mouse SSeCKS gene. Through the molecular analysis of the insertion site, we show that SSeCKS contains two alternative promoters directing the synthesis of mRNAs (P1- and P2-mRNA), encoding two different proteins, one of which would be a truncated form of the other. Interestingly, these RNAs are differentially expressed, P2 being found exclusively in the male germ line, while P1 exhibits a dynamic and wider pattern of expression during embryonic development and in the adult; its expression is predominant in the nervous system. Finally, we show that P1- but not P2-mRNA expression is abolished by the insertion and furthermore that mice homozygous for the mutation lack SSeCKS in all tissues except the male germ cells. Nevertheless and surprisingly, these mice do not exhibit any obvious phenotype. The functional implications of these observations are discussed.

Functional interaction between the SSeCKS scaffolding protein and the cytoplasmic domain of β1,4-galactosyltransferase

The β1,4-galactosyltransferase family contains at least seven unique gene products, of which β1,4-galactosyltransferase I (GalT) is the most exhaustively studied. GalT exists in the Golgi complex, similar to many other glycosyltransferases, as well as on the cell surface, where it functions as a signaling receptor for extracellular glycoside ligands. When expressed on the surface, GalT associates with the cytoskeleton and, upon ligand-induced aggregation, induces cell-type specific intracellular signal cascades. In an effort to define the mechanisms by which surface GalT exerts these intracellular effects, we used the yeast two-hybrid system to identify proteins that specifically interact with the GalT cytoplasmic domain.

The yeast two-hybrid screen identified two distinct clones (1.12 and 2.52) that showed identity to portions of SSeCKS (Src Suppressed CKinase Substrate). SSeCKS is a previously defined kinase and cytoskeleton scaffolding protein whose subcellular distribution and functions are remarkably similar to those attributed to GalT. Both SSeCKS and GalT have been localized to the perinuclear/Golgi region as well as to filopodia/lamellipodia. SSeCKS and GalT have been implicated in regulating cell growth, actin filament dynamics, and cell spreading. Interestingly, 1.12 and 2.52-GFP constructs were localized to subcellular domains that correlated with the two purported subcellular distributions for GalT; 2.52 being confined to the Golgi, whereas 1.12 localized primarily to filopodia. Coimmunoprecipitation assays demonstrate stable binding between the GalT cytoplasmic domain and the 1.12 and 2.52 domains of SSeCKS in appropriately transfected cells. Similar assays demonstrate binding between the endogenous GalT and SSeCKS proteins also. Coimmunoprecipitation assays were performed in both directions and produced similar results (i.e. using either anti-GalT domain or anti-SSeCKS domain antibodies as the precipitating reagent). A functional interaction between the GalT cytoplasmic domain and SSeCKS was illustrated by the ability of either the 1.12 or 2.52 SSeCKS domain to restore a normal adhesive phenotype in cells overexpressing the TL-GFP dominant negative construct. TL-GFP is composed of the GalT cytoplasmic and transmembrane domains fused to GFP, and leads to a loss of cell adhesion on laminin by displacing the endogenous GalT from its cytoskeleton binding sites. This is the first reported interaction between a glycosyltransferase and a scaffolding protein, and suggests that SSeCKS serve to integrate the various functions ascribed to the GalT cytoplasmic domain.

Jun, the oncoprotein

Cellular Jun (c-Jun) and viral Jun (v-Jun) can induce oncogenic transformation. For this activity, c-Jun requires an upstream signal, delivered by the Jun N-terminal kinase (JNK). v-Jun does not interact with JNK; it is autonomous and constitutively active. v-Jun and c-Jun address overlapping but not identical sets of genes. Whether all genes essential for transformation reside within the overlap of the v-Jun and c-Jun target spectra remains to be determined. The search for transformation-relevant targets of Jun is moving into a new stage with the application of DNA microarrays technology. Genetic screens and functional tests remain a necessity for the identification of genes that control the oncogenic phenotype.

A novel retinoid-response gene set in vascular smooth muscle cells

A modified suppression subtractive hybridization assay was performed to uncover genes induced by all-trans retinoic acid in cultured smooth muscle cells (SMC). Northern blotting studies confirmed the induction of 14 genes, many of which have heretofore been unrecognized as retinoid-inducible. Temporal expression and cycloheximide studies allowed us to categorize these genes as either immediate-early (LOX-1, endolyn, Stoned B/TFIIA alpha/beta-like factor, Src Suppressed C Kinase Substrate, and tissue transglutaminase) or delayed (cathepsin-L, ceruloplasmin, epithelin, importin alpha, alpha(8)-integrin, lactate dehydrogenase B, retinol dehydrogenase, spermidine/spermine N(1)-acetyltransferase, and VCAM-1) retinoid-response genes. A survey of rat tissues showed two of the genes (tissue transglutaminase and alpha(8)-integrin) to be highly restricted to vascular tissue. In situ hybridization verified expression of both tissue transglutaminase and alpha(8)-integrin to SMC in balloon-injured rat carotid artery. These findings unveil a new retinoid-response gene set that should be exploited to define molecular pathways involved in the antagonistic effects of retinoids on SMC growth and neointimal formation.