Juan A. Martínez

Potentiation of angiogenic switch in capillary endothelial cells by cAMP : A cross-talk between up-regulated LLO biosynthesis and the HSP-70 expression

During tumor growth and invasion, the endothelial cells from a relatively quiescent endothelium start proliferating. The exact mechanism of switching to a new angiogenic phenotype is currently unknown. We have examined the role of intracellular cAMP in this process. When a non-transformed capillary endothelial cell line was treated with 2 mM 8Br-cAMP, cell proliferation was enhanced by ∼70%. Cellular morphology indicated enhanced mitosis after 32–40 h with almost one-half of the cell population in the S phase. Bcl-2 expression and caspase-3, -8, and -9 activity remained unaffected. A significant increase in the Glc3Man9GlcNAc2-PP-Dol biosynthesis and turnover, Factor VIIIC N-glycosylation, and cell surface expression of N-glycans was observed in cells treated with 8Br-cAMP. Dol-P-Man synthase activity in the endoplasmic reticulum membranes also increased. A 1.4–1.6-fold increase in HSP-70 and HSP-90 expression was also observed in 8Br-cAMP treated cells. On the other hand, the expression of GRP-78/Bip was 2.3-fold higher compared to that of GRP-94 in control cells, but after 8Br-cAMP treatment for 32 h, it was reduced by 3-fold. GRP-78/Bip expression in untreated cells was 1.2–1.5-fold higher when compared with HSP-70 and HSP-90, whereas that of the GRP-94 was 1.5–1.8-fold lower. After 8Br-cAMP treatment, GRP-78/Bip expression was reduced 4.5–4.8-fold, but the GRP-94 was reduced by 1.5–1.6-fold only. Upon comparison, a 2.9-fold down-regulation of GRP-78/Bip was observed compared to GRP-94. We, therefore, conclude that a high level of Glc3Man9GlcNAc2-PP-Dol, resulting from 8Br-cAMP stimulation up-regulated HSP-70 expression and down-regulated that of the GRP-78/Bip, maintained adequate protein folding, and reduced endoplasmic reticulum stress. As a result capillary endothelial cell proliferation was induced.

Tunicamycin Inhibits Capillary Endothelial Cell Proliferation by Inducing Apoptosis

Bovine adrenal medulla microvascular endothelial cells used in this study undergo cellular proliferation and differentiation upon culturing in vitro as observed both by light and scanning electron microscopy. Cells also respond to the growth promoting activity of serum and basic fibroblast growth factor (FGF2). Flow cytometric analysis of a synchronized culture established that cells take 68 hours to complete one cell cycle spending 36 hours in the G1 phase, 8 hours in the S phase, and 24 hours in the G2 + M phase when cultured in EMEM containing 2% heat-inactivated fetal bovine serum (FBS). At 10% serum, or in the presence of FGF2 (10 ng/ml-100 ng/ml) length of the cell cycle is reduced to 56 hours due to shortening of the G1 phase by 12 hours. Tunicamycin (a glucosamine-containing pyrimidine nucleotide), and an inhibitor of glucosaminyl-1-phosphate (GlcNAc 1-P) transferase, the first step of Glc3Man9GlcNAc2-PP-Dol (OSL) biosynthesis is found to inhibit the endothelial cells proliferation by inducing apoptosis as observed by flow cytometry and DNA laddering. Cell shrinkage, compaction of nuclei, membrane fragmentation, etc., typical of apoptotic response are frequently seen by light microscopy in the presence of tunicamycin. Scanning electron microscopy also exhibited a considerable amount of cell surface blebbing. Accumulation of an immunopositive cell specific asparagine-linked (N-linked) glycoprotein, Factor VIII:C in the absence of Glc3Man9GlcNAc2-PP-Dol in tunicamycin treated cells has been proposed as an apoptotic triggering mechanism under the current experimental conditions.

N-Acetylglucosaminyl 1-Phosphate Transferase: An Excellent Target for Developing New Generation Breast Cancer Therapeutic

Studies from our laboratory have explained that breast tumor progression can be attenuated by targeting the N-linked glycoproteins of the tumor microvasculature and that of tumor cells alike with a protein N-glycosylation inhibitor, tunicamycin. Absence of N-glycosylation leads to an accumulation of un- or mis-folded proteins in the ER and the cell develops “ER stress”. The result is cell cycle arrest, and induction of apoptosis mediated by unfolded protein response (upr) signaling. Tunicamycin inhibited in vitro and in vivo (Matrigel™ implants in athymic nude mice) angiogenesis in a dose dependent manner. The action is irreversible and survived under tumor microenvironment, i.e., in the presence of FGF-2 or VEGF or higher serum concentration. Importantly, tunicamycin prevented the progression of double negative (ER−/PR−/Her2+) and triple negative (ER−/PR−/Her2−) breast tumors by ~55 to 65 % in 3 weeks in athymic nude mice [Balb/c(nu/nu)]. Analyses of paraffin sections exhibited “ER stress” in both microvasculature and in tumor tissue.

Importance of a Factor VIIIc-Like Glycoprotein Expressed in Capillary Endothelial Cells (eFactor VIIIc) in Angiogenesis

Factor VIII is a large, 2,332-residue plasma glycoprotein that acts as a regulatory cofactor in the process of blood coagulation [1–3]. It binds to activated factor IX (factor IXa) in the presence of calcium and negatively charged phospholipids at the surface of activated platelets to form a membrane-associated, proteolytically active complex. Upon complex formation, the V max of factor IXa is increased by approximately 200,000-fold, promoting the rapid activation of its substrate, the serine protease factor X. The proteolytic conversion of factor X to its active form, factor Xa, is a central control point in the coagulation cascade, leading to activation of thrombin, formation of a fibrin mesh, and establishment of a stable blood clot. The binding of factor VIIIc and other activated proteins to these membrane surfaces allows for localization of the procoagulation process to sites of vascular damage.