Biomedical subjects

G Steven Martin

Publications and source records attributed to G Steven Martin.

10 recordsLinked to original sources

PubMed2006

Dual role of SnoN in mammalian tumorigenesis.

SnoN is an important negative regulator of transforming growth factor beta signaling through its ability to interact with and repress the activity of Smad proteins. It was originally identified as an oncoprotein based on its ability to induce anchorage-independent growth in chicken embryo fibroblasts. However, the roles of SnoN in mammalian epithelial carcinogenesis have not been well defined. Here we show for the first time that SnoN plays an important but complex role in human cancer. SnoN expression is highly elevated in many human cancer cell lines, and this high level of SnoN promotes mitogenic transformation of breast and lung cancer cell lines in vitro and tumor growth in vivo, consistent with its proposed pro-oncogenic role. However, this high level of SnoN expression also inhibits epithelial-to-mesenchymal transdifferentiation. Breast and lung cancer cells expressing the shRNA for SnoN exhibited an increase in cell motility, actin stress fiber formation, metalloprotease activity, and extracellular matrix production as well as a reduction in adherens junction proteins. Supporting this observation, in an in vivo breast cancer metastasis model, reducing SnoN expression was found to moderately enhance metastasis of human breast cancer cells to bone and lung. Thus, SnoN plays both pro-tumorigenic and antitumorigenic roles at different stages of mammalian malignant progression. The growth-promoting activity of SnoN appears to require its ability to bind to and repress the Smad proteins, while the antitumorigenic activity can be mediated by both Smad-dependent and Smad-independent pathways and requires the activity of small GTPase RhoA. Our study has established the importance of SnoN in mammalian epithelial carcinogenesis and revealed a novel aspect of SnoN function in malignant progression.

Animals↗

PubMed2006

Activated Src abrogates the Myc requirement for the G0/G1 transition but not for the G1/S transition.

The transcription factor Myc plays a central role in the control of cellular proliferation. Myc expression is induced by growth factors in a pathway mediated by cellular Src (c-Src), but it is not clear whether Myc induction or activity is required for malignant transformation by activated Src. We introduced v-Src into a c-myc(-/-) derivative of Rat-1 fibroblasts and into 3T9 mouse fibroblasts harboring a conditionally excisable c-myc allele. Expression of activated viral Src in Myc-deficient cells led to loss of actin stress fibers and surface fibronectin, indicating that Myc is dispensable for v-Src-induced morphological transformation. However, v-Src failed to rescue the proliferative defect resulting from the loss of Myc. In Myc-deficient cells, despite its inability to overcome this proliferation block, v-Src was able to regulate the expression of certain Myc transcriptional targets and induce the expression of active cyclin D/Cdk4 and Cdk6 complexes; it also induced the phosphorylation of Rb, albeit at reduced levels. In contrast, however, in the absence of Myc, the level of Cdk2 kinase activity was drastically reduced. This reduction in Cdk2 activity was associated with a decrease in the expression of Cdk7, Cdc25A, and cyclin A. Coexpression of Cdk2 plus cyclin E and/or cyclin A rescued the G1/S block and allowed the cells to enter mitosis. These results indicate that in the absence of Myc, v-Src can activate early G1 cell cycle regulators but fails to activate regulators of the late G1/S transition.

Animals↗

PubMed2006

Transcription of the Schizosaccharomyces pombe gene cdc18+: roles of MCB elements and the DSC1 complex.

In Schizosaccharomyces pombe, commitment to a round of DNA synthesis and entry into the cell cycle are dependent on the function of genes that are transcribed periodically during the cell cycle. Activation of these genes prior to S phase is primarily controlled through cis-acting elements known as MluI Cell-cycle Boxes, or MCBs, and by a family of transcription factors, including Cdc10, Res1, Res2 and Rep2. These transcription factors are also known to be present in a complex, DSC1, that binds to the promoters of pre-S genes. We have demonstrated that within the promoter of cdc18+, a representative pre-S gene, the orientation and spacing of MCBs are crucial for activation and cell-cycle dependence. To our surprise, electrophoretic mobility shift assays showed a highly active mutant form of the promoter, which alters the spacing of the MCB elements, does not bind DSC1 but does bind a higher mobility complex. The binding of this second complex is not dependent on Cdc10 or the Res/Rep proteins. We conclude that, DSC1 binding does not correlate with cell-cycle dependent transcriptional activation, and the higher mobility species may represent a novel transcriptional activation complex that is also likely to function in pre-S transcription.

Cell Cycle Proteins↗

PubMed2006

Fly Src: the Yin and Yang of tumor invasion and tumor suppression.

The non-receptor tyrosine kinase Src is inactivated by the C-terminal Src kinase Csk. In a recent paper in Developmental Cell, Vidal et al. show that loss of Drosophila Csk (dCsk) in a large field of cells results in cell proliferation and disorganization of tissue architecture. In contrast, local inactivation of dCsk in a small field of cells results in loss of cells that are adjacent to normal tissue. This loss occurs by basal migration and death by apoptosis. These findings may shed light on mechanisms that restrain tumor initiation.

Animals↗

PubMed2004

The road to Src.

More than a quarter of a century has elapsed since the identification of the c-src proto-oncogene. During that period, we have learned that cancer arises as the result of mutations in proto-oncogenes and tumor suppressor genes, and we are now seeing the first fruits of these discoveries, in the form of targeted therapies directed against activated tyrosine kinases such as Bcr-Abl, c-Kit and the EGF receptor. But the discovery of the c-src proto-oncogene was in turn based on decades of study on an avian RNA tumor virus, Rous sarcoma virus (RSV). Here I review the work that led up to the identification of the RSV transforming gene and its protein product, and how this information in turn led to the discovery of cellular Src.

Animals↗

PubMed2004

Active Rho is localized to podosomes induced by oncogenic Src and is required for their assembly and function.

Transformation of fibroblasts by oncogenic Src causes disruption of actin stress fibers and formation of invasive adhesions called podosomes. Because the small GTPase Rho stimulates stress fiber formation, Rho inactivation by Src has been thought to be necessary for stress fiber disruption. However, we show here that Rho[GTP] levels do not decrease after transformation by activated Src. Inactivation of Rho in Src-transformed fibroblasts by dominant negative RhoA or the Rho-specific inhibitor C3 exoenzyme disrupted podosome structure as judged by localization of podosome components F-actin, cortactin, and Fish. Inhibition of Rho strongly inhibited Src-induced proteolytic degradation of the extracellular matrix. Furthermore, development of an in situ Rho[GTP] affinity assay allowed us to detect endogenous Rho[GTP] at podosomes, where it colocalized with F-actin, cortactin, and Fish. Therefore, Rho is not globally inactivated in Src-transformed fibroblasts, but is necessary for the assembly and function of structures implicated in tumor cell invasion.

Actins↗

PubMed2003

The transforming activity of Ski and SnoN is dependent on their ability to repress the activity of Smad proteins.

The regulation of cell growth and differentiation by transforming growth factor-beta (TGF-beta) is mediated by the Smad proteins. In the nucleus, the Smad proteins are negatively regulated by two closely related nuclear proto-oncoproteins, Ski and SnoN. When overexpressed, Ski and SnoN induce oncogenic transformation of chicken embryo fibroblasts. However, the mechanism of transformation by Ski and SnoN has not been defined. We have previously reported that Ski and SnoN interact directly with Smad2, Smad3, and Smad4 and repress their ability to activate TGF-beta target genes through multiple mechanisms. Because Smad proteins are tumor suppressors, we hypothesized that the ability of Ski and SnoN to inactivate Smad function may be responsible for their transforming activity. Here, we show that the receptor regulated Smad proteins (Smad2 and Smad3) and common mediator Smad (Smad4) bind to different regions in Ski and SnoN. Mutation of both regions, but not each region alone, markedly impaired the ability of Ski and SnoN to repress TGF-beta-induced transcriptional activation and cell cycle arrest. Moreover, when expressed in chicken embryo fibroblasts, mutant Ski or SnoN defective in binding to the Smad proteins failed to induce oncogenic transformation. These results suggest that the ability of Ski and SnoN to repress the growth inhibitory function of the Smad proteins is required for their transforming activity. This may account for the resistance to TGF-beta-induced growth arrest in some human cancer cell lines that express high levels of Ski or SnoN.

Animals↗

PubMed2003

Cell signaling and cancer.

During the course of tumor progression, cancer cells acquire a number of characteristic alterations. These include the capacities to proliferate independently of exogenous growth-promoting or growth-inhibitory signals, to invade surrounding tissues and metastasize to distant sites, to elicit an angiogenic response, and to evade mechanisms that limit cell proliferation, such as apoptosis and replicative senescence. These properties reflect alterations in the cellular signaling pathways that in normal cells control cell proliferation, motility, and survival. Many of the proteins currently under investigation as possible targets for cancer therapy are signaling proteins that are components of these pathways. The nature of these signaling pathways and their roles in tumorigenesis were the subject of a recent Beatson International Cancer Conference.

Animals↗

PubMed2003

A reporter system for reverse transfection cell arrays.

The incredible speed of gene cloning and sequencing brought about by the genomic revolution has begun to outpace conventional gene discovery approaches in the pharmaceutical industry. High-throughput approaches for studying gene function in vivo are greatly needed. One potential answer to this challenge is reverse transfection, a high-throughput gene expression method for examining the function of hundreds to thousands of genes in parallel. One limitation of reverse transfection technology is the need for posttransfection processing of the arrays to analyze the activity of the expressed proteins. The authors have investigated the use of a reporter construct cotransfected with other genes of interest to monitor and screen gene function on reverse transfection microarrays. They developed a serum response element (SRE) reporter linked to the green fluorescent protein (GFP) that is cotransfected with target genes on reverse transfection arrays for monitoring mitogen-activated protein (MAP) kinase signaling by multiple targets in parallel. The authors show that this reporter system is able to detect inhibition of upstream MAP kinase signaling proteins by the MEK inhibitor U0126. The ability to monitor the activity of multiple signaling proteins in a multiwell format suggests the utility of reverse transfection reporter arrays for high-throughput screening applications.

Butadienes↗

PubMed2000

A human homolog of the C. elegans polarity determinant Par-6 links Rac and Cdc42 to PKCzeta signaling and cell transformation.

BACKGROUND: Rac and Cdc42 are members of the Rho family of small GTPases. They modulate cell growth and polarity, and contribute to oncogenic transformation by Ras. The molecular mechanisms underlying these functions remain elusive, however. RESULTS: We have identified a novel effector of Rac and Cdc42, hPar-6, which is the human homolog of a cell-polarity determinant in Caenorhabditis elegans. hPar-6 contains a PDZ domain and a Cdc42/Rac interactive binding (CRIB) motif, and interacts with Rac1 and Cdc42 in a GTP-dependent manner. hPar-6 also binds directly to an atypical protein kinase C isoform, PKCzeta, and forms a stable ternary complex with Rac1 or Cdc42 and PKCzeta. This association results in stimulation of PKCzeta kinase activity. Moreover, hPar-6 potentiates cell transformation by Rac1/Cdc42 and its interaction with Rac1/Cdc42 is essential for this effect. Cell transformation by hPar-6 involves a PKCzeta-dependent pathway distinct from the pathway mediated by Raf. CONCLUSIONS: These findings indicate that Rac/Cdc42 can regulate cell growth through Par-6 and PKCzeta, and suggest that deregulation of cell-polarity signaling can lead to cell transformation.

Amino Acid Sequence↗