[Show abstract][Hide abstract]ABSTRACT: C21, the C-terminal residue of thrombospondin-4 (TSP-4), was identified as a peptide growth factor during an investigation concerning erythropoietin-dependent, erythroid stimulating factors of endothelial origin. It is active in cultures of several human hematopoietic stem cells, skin fibroblasts and kidney epithelial cells and stimulates red cell formation in anemic mice. A method of affinity chromatography in the presence of high concentrations of Triton X-100, previously developed for identifying proteins associated with the TSP-1 receptor CD47, was utilized for the detection of C21 binding molecules and their detergent-resistant, associated partners. These experiments helped to delineate two different mechanisms of C21 action, which are compatible with its cell proliferating activity. As a cell matrix peptide, C21 binds to the osteopontin receptor CD44 and could act as an osteopontin antagonist, preventing the inhibition of primitive hematopoietic stem cell proliferation. TSP-1, another matrix protein, binds to C21 and could indirectly act as an antagonist, by shunting C21-CD44 interactions. The second mechanism is a direct effect of C21 on cell proliferation. The extremely rapid internalization and nuclear localization of the peptide could be explained by CD44-mediated internalization, followed by a microtubule-mediated transport towards the nucleus, or, eventually, direct membrane insertion. These alternative hypotheses are supported by previously observed membrane insertion of similar synthetic and viral acidic amphipathic peptides, the presence of microtubule-associated protein 1B (MAP1B) and dynactin in the triton-soluble complexes associated with C21 and the presence in such complexes of dual compartment proteins for nuclei and plasma membranes, such as MAP1B, AHNAK and CD44.
[Show abstract][Hide abstract]ABSTRACT: The growth factor progranulin (granulin-epithelin precursor, PC-derived growth factor or acrogranin) regulates proliferation and migration and is implicated in cancer, development, wound repair and neurodegenerative diseases. Under most conditions fibroblasts do not express progranulin in vivo, however its expression is activated following wounding. We hypothesised that progranulin is part of a fibroblast stress response. Fibroblasts in culture were exposed to two physiologically and clinically relevant microenvironmental stresses; hypoxia (1% oxygen) and acidosis, both of which increase progranulin expression. The greatest increases occurred when hypoxia and acidosis were combined. Increased progranulin expression is not a direct response to apoptosis since it occurred under conditions of pH and hypoxia under which cell viability remained high. Low concentrations of progranulin (2 nM) protected fibroblasts from apoptosis induced by extreme acidosis (pH 5.0 and 4.0). We propose that progranulin is part of a fibroblast stress response and is cytoprotective to acidotic stress.
[Show abstract][Hide abstract]ABSTRACT: Progranulin (pgrn; granulin-epithelin precursor, PC-cell-derived growth factor, or acrogranin) is a multifunctional secreted glycoprotein implicated in tumorigenesis, development, inflammation, and repair. It is highly expressed in macrophage and monocyte-derived dendritic cells. Here we investigate its regulation in myeloid cells. All-trans retinoic acid (ATRA) increased pgrn mRNA levels in myelomonocytic cells (CD34(+) progenitors; monoblastic U-937; monocytic THP-1; progranulocytic HL-60; macrophage RAW 264.7) but not in nonmyeloid cells tested. Interleukin-4 impaired basal expression of pgrn in U-937. Differentiation agents DMSO, and, in U-937 only, phorbol ester [phorbol 12-myristate,13-acetate (PMA)] elevated pgrn mRNA expression late in differentiation, suggestive of roles for pgrn in more mature terminally differentiated granulocyte/monocytes rather than during growth or differentiation. The response of pgrn mRNA to ATRA differs in U-937 and HL-60 lineages. In U-937, ATRA and chemical differentiation agents greatly increased pgrn mRNA stability, whereas, in HL-60, ATRA accelerated pgrn mRNA turnover. The initial upregulation of pgrn mRNA after stimulation with ATRA was independent of de novo protein synthesis in U-937 but not HL-60. Chemical blockade of nuclear factor-kappaB (NF-kappaB) activation impaired ATRA-stimulated pgrn expression in HL-60 but not U-937, whereas in U-937 it blocked PMA-induced pgrn mRNA expression, suggestive of cell-specific roles for NF-kappaB in determining pgrn mRNA levels. We propose that: 1) ATRA regulates pgrn mRNA levels in myelomonocytic cells; 2) ATRA acts in a cell-specific manner involving the differential control of mRNA stability and differential requirement for NF-kappaB signaling; and 3) elevated pgrn mRNA expression is characteristic of more mature cells and does not stimulate differentiation.
Article · Jan 2007 · AJP Regulatory Integrative and Comparative Physiology
[Show abstract][Hide abstract]ABSTRACT: Progranulin (pgrn; PC-cell-derived growth factor, epithelin precursor, or acrogranin) has been identified recently as an autocrine regulator of tumorigenesis in several cancer cells including SW-13 adrenal carcinomas and some breast cancers, but how pgrn promotes tumor progression is not well understood. SW-13 cells do not form tumors in nude mice but become highly tumorigenic when their pgrn expression is elevated, and this provides a useful model in which to investigate the role of pgrn in tumorigenesis. Here we show that, in SW-13 cells, the level of pgrn expression is a major determinant of the intrinsic activity of the mitogen-activated protein kinase, phosphatidylinositol 3'-kinase, and focal adhesion kinase signaling pathways. Pgrn stimulates the invasion of SW-13 cells across Matrigel-coated filters, increases the expression of matrix metalloproteinase 13 and 17, protects against anoikis, and overcomes the inhibition of cell growth imposed on SW-13 cells by interstitial type-I collagen. Inhibition of the mitogen-activated protein kinase and phosphatidylinositol 3'-kinase signaling pathways impairs each of the pgrn-dependent biological responses tested, but to different extents. The ability of pgrn to stimulate cell division, invasion, and survival demonstrates that pgrn regulates multiple steps in carcinomal progression, and suggests that the pgrn system may be a possible future therapeutic target.