Bruce R. Zetter

Factors involved in the modulation of cell proliferation in vivo and in vitro: The role of fibroblast and epidermal growth factors in the proliferative response of mammalian cells

SummaryThe control of proliferation of mesoderm-derived cells by EGF and FGF has been examined taking, as an example, the vascular endothelium. The mechanisms by which cell proliferation can be brought to a stop in vivo and in vitro have been reviewed.

Differential Endothelial Migration and Proliferation to Basic Fibroblast Growth Factor and Vascular Endothelial Growth Factor

Neovascularization is a feature of a variety of pathological processes. We compared the characteristics of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) on migration and proliferation of human umbilical vein endothelium (HUVEC). Both VEGF and bFGF induced endothelial cell migration at similar concentrations (1/2 max. VEGF = approximately 1.0 ng/ml, bFGF = approximately 5.0 ng/ml). However, VEGF-stimulated migration was two-fold greater than bFGF at 1 and 10 ng/ml (p < 0.05). In contrast, bFGF induced proliferation four-fold more effectively than VEGF (1/2 max. 1 ng/ml and 1.4 ng/ml respectively). Checkerboard migration assays for bFGF showed a predominantly chemokinetic pattern, whereas VEGF was predominantly chemotactic. VEGF and bFGF were not synergistic in monolayer proliferation and migration assays. Three angiogenesis inhibitors, alpha-interferon, TNP-470, and platelet factor-4, inhibited VEGF and bFGF induced cell migration. These results indicate that VEGF and bFGF are chemoattractants that stimulate endothelial migration by different mechanisms and that both can be inhibited by known angiogenesis inhibitors.

Quantitative Cancer Proteomics: Stable Isotope Labeling with Amino Acids in Cell Culture (SILAC) as a Tool for Prostate Cancer Research

Microarrays have been the primary means for large-scale analyses of genes implicated in cancer progression. However, more recently a need has been recognized for investigating cancer development directly at the protein level. In this report, we have applied a comparative proteomic technique to the study of metastatic prostate cancer. This technology, termed stable isotope labeling with amino acids in cell culture (SILAC), has recently gained popularity for its ability to compare the expression levels of hundreds of proteins in a single experiment. SILAC makes use of 12C- and 13C-labeled amino acids added to the growth media of separately cultured cell lines, giving rise to cells containing either “light” or “heavy” proteins, respectively. Upon mixing lysates collected from these cells, proteins can be identified by tandem mass spectrometry. The incorporation of stable isotopes also allows for a quantitative comparison between the two samples. Using this method, we compared the expression levels for more than 440 proteins in the microsomal fractions of prostate cancer cells with varying metastatic potential. Of these, 60 were found elevated greater than 3-fold in the highly metastatic cells, whereas 22 were reduced by equivalent amounts. Western blotting provided further confirmation of the mass spectrometry-based quantification. Our results demonstrate the applicability of this novel approach toward the study of cancer progression using defined cell lines.

Platelet-derived growth factor and fibronectin-stimulated migration are differentially regulated by the Rac and extracellular signal-regulated kinase pathways

Directed cell migration is essential for a variety of important biological processes ranging from development and angiogenesis to metastasis. Ras plays a pivotal role in the signaling cascade that governs chemotaxis of fibroblasts toward platelet-derived growth factor-BB (PDGF-BB). Ras activates multiple downstream pathways, which include the extracellular signal-regulated kinase (ERK), Rac, and Ral signaling cascades. We therefore investigated the role of the Rac and ERK pathways in cell migration. We showed that migration of fibroblasts toward PDGF-BB is inhibited by expression of dominant negative Asn-17 Rac1. Blocking of the ERK pathway by either expression of dominant negative Ala-218/Ala-222-mitogen-activated protein kinase kinase (A218/A222-MEK1) or by a MEK-specific inhibitor did not inhibit migration toward PDGF-BB. In contrast, migration toward soluble fibronectin was suppressed by inhibition of the ERK pathway but not by Asn-17 Rac1 expression. These results indicate that directed cell migration mediated by different receptor classes in response to different ligands differentially utilizes the Rac and ERK pathways and suggest that Rac might play a critical role in pathological processes such as angiogenesis and metastasis.

Motility and invasion are differentially modulated by Rho family GTPases

Cell migration in vivo often requires invasion through tissue matrices. Currently little is known regarding the signaling pathways that regulate cell invasion through three-dimensional matrices. The small GTPases Cdc42, Rac and Rho are key regulators of actin cytoskeletal and adhesive structures. We now show that expression of dominant negative forms of either Cdc42, Rac or Rho inhibited PDGF-BB-stimulated Rat1 fibroblast invasion into 3D collagen matrices, indicating that the activity of each of these GTPases is necessary for cell invasion. In contrast, only Rac activation was required for PDGF-BB-stimulated locomotion across a planar substrate in the Boyden chamber. Interestingly, PDGF-induced invasion was also strongly inhibited by expression of constitutively active forms of Cdc42 or Rho, and to a lesser extent by constitutively active Rac. We also show that constitutively active V12-Rac significantly stimulated basal Rat1 fibroblast invasion, independent of PI-3-kinase activity, and that this effect was suppressed by the effector mutant V12/H40-Rac. These results suggest that cellular invasion may require an optimal level of activation of Cdc42, Rho and Rac, and that migration and invasion are differentially modulated by Rho family GTPases.

Cancer biomarkers: knowing the present and predicting the future

In recent years the discovery of cancer biomarkers has become a major focus of cancer research. The widespread use of prostate-specific antigen in prostate cancer screening has motivated researchers to identify suitable markers for screening different types of cancer. Biomarkers are also useful for diagnosis, monitoring disease progression, predicting disease recurrence and therapeutic treatment efficacy. With the advent of new and improved genomic and proteomic technologies such as DNA and tissue microarray, two-dimensional gel eletrophoresis, mass spectrometry and protein assays coupled with advanced bioinformatic tools, it is possible to develop biomarkers that are able to reliably and accurately predict outcomes during cancer management and treatment. In years to come, a serum or urine test for every phase of cancer may drive clinical decision making, supplementing or replacing currently existing invasive techniques.

Stress-inducible, Murine Protein mSTI1 CHARACTERIZATION OF BINDING DOMAINS FOR HEAT SHOCK PROTEINS AND IN VITRO PHOSPHORYLATION BY DIFFERENT KINASES

We have recently isolated the cDNA for the murine homologue of the stress-inducible phosphoprotein STI1 (also known as IEF SSP 3521 or p60). STI1 was previously shown to be 2-fold up-regulated in MRC-5 fibroblasts upon viral transformation and to exist in a macromolecular complex with heat shock proteins of the HSP 70 and 90 families. By peptide-sequencing we have identified the two heat shock proteins that bind to murine STI1 (mSTI1) as HSC 70 and HSP 84/86. We describe two separate binding regions within mSTI1 for the two heat shock proteins. In the presence of cell extracts, the N-terminal region of mSTI1 binds preferentially to HSC 70, whereas the C-terminal portion of the molecule promotes the binding of HSP 84/86. Heat treatment caused a strong induction of mSTI1 message without affecting the steady-state level of the protein significantly. In addition, heat treatment led to changes in the isoform-composition of mSTI1. pp70s6k, pp90rsk, and mitogen-activated protein kinase-activated protein kinase 2 were tested as possible STI1 kinases in vitro using recombinant mSTI1 as a substrate: only pp90rsk was able to phosphorylate recombinant mSTI1. In vitro kinase assays using casein kinase II suggest serine 189 to be a likely phosphorylation site in mSTI1.

Stimulation of human prostatic carcinoma cell growth by factors present in human bone marrow.

Malignant prostatic carcinoma, a major cause of cancer mortality in males, most often metastasizes to secondary sites in bone. Frequently, the growth rate of the secondary tumor in bone marrow is considerably greater than that of the slowly growing primary prostatic tumor. We now report that two lines of human prostatic carcinoma cells proliferate in response to conditioned media from unstimulated human, rat, or bovine bone marrow. Nonprostatic tumor cell lines showed little or no growth response to the same medium. The proliferative activity found in bone marrow was not duplicated by any of a variety of purified growth factors including epidermal growth factor (EGF), acidic or basic fibroblast growth factor (FGF), platelet-derived growth factor (PDGF), transforming growth factor (TGF) alpha or beta, interleukins 1, 2, 3, 4 or 6, granulocyte (G), macrophage (M) or granulocyte-macrophage (GM) colony stimulating factor (CSF). Whereas a mixture of G-CSF, M-CSF, and IL 3 produced a mitogenic response in the prostatic carcinoma cells, these three factors were not present in our bone marrow samples in sufficient quantities to promote the observed proliferative response. To further identify the cellular source of the proliferative activity present in bone marrow-conditioned medium, we tested conditioned media made from human bone marrow stromal cells. The stromal cell conditioned medium stimulated increased growth of the prostatic carcinoma cells to levels equivalent to those observed with the bone marrow conditioned medium. These results suggest that novel mitogenic factors that are produced by bone marrow stromal cells and remain in the bone marrow cavity may account, in part, for the preferential growth of prostatic metastases in bone.

Angiogenic activity of adipose tissue

Adipose tissue has been used to promote wound healing and to revascularize ischemic myocardium. We explored whether fat from various sources was angiogenic in the cornea. Rabbit subcutaneous and omental fat induced grossly visible neovascularization of all rabbit corneas studied, and at a similar rate and intensity. Neovascularization was not observed in any cornea following control implantation of liver or muscle. Neovascularization was blocked in all rabbits in which indomethacin was administered orally 3 days before implantation of fat and continued following implantation, suggesting that prostaglandins are associated with fat induced angiogenesis.

The Endothelial Cells of Large and Small Blood Vessels

Recent progress on the isolation and culture of capillary endothelial cells has allowed comparison of the biochemical and physiologic properties of endothelial cells from large and small blood vessels. These cells share certain common features including monolayer formation, production of Factor VIII antigen, production of prostacyclin, and presence of Weibel-Palade bodies. Endothelial cells from capillaries, however, differ from the endothelial cells of large arteries and veins in their nutritional requirements and in their responses to growth and migration stimuli. The differences found between endothelial cells from large and small blood vessels underscore the importance of using cells derived from vessels of appropriate size when studying the macro- and microangiopathies associated with diabetes.