Inhibition of retroviral mRNA expression in the murine macrophage cell line GG2EE by biologic response modifiers

We immortalized the GG2EE macrophage (M phi) cell line by infection of freshly isolated bone marrow cells with the recombinant J2 retrovirus carrying v-raf and v-myc oncogenes. We investigated the expression of J2 virus mRNA in relationship with the proliferative ability and tumoricidal activity of GG2EE cells exposed to biologic response modifiers (BRM). Calcium ionophore (Ca2+I), picolinic acid (PA), or IFN-gamma were employed to activate GG2EE cells. Each BRM was due to inhibit the proliferation of GG2EE cells in a dose-dependent manner, whereas only Ca2+I or the combined treatment with PA plus IFN-gamma induced tumoricidal GG2EE cells. J2 virus mRNA expression was not affected by PA or IFN-gamma, but it was dramatically decreased by Ca2+I or PA plus IFN-gamma. These results indicated that the expression of J2 mRNA can be inhibited in GG2EE cells by appropriate BRM such as Ca2+I or IFN-gamma plus PA. In contrast, the expression of 2'5'-oligoadenylate synthetase mRNA was augmented to similar levels by treatment of the GG2EE cells with IFN-gamma alone or in combination with PA. The down-regulation of J2 mRNA expression was not associated with the antiproliferative activity of the BRM but rather with their ability to induce tumoricidal activity. These results suggest that the process of activation of tumoricidal macrophages also triggers a mechanism(s) of resistance to viral mRNA expression. Moreover, the finding that IFN-gamma or PA inhibit cell proliferation but not J2 mRNA expression indicates that the intracellular targets of these BRM are intact, independent from and unaffected by J2 virus expression.     

Extracellular proteolytic cleavage by urokinase is required for activation of hepatocyte growth factor/scatter factor.

The extracellular protease urokinase is known to be crucially involved in morphogenesis, tissue repair and tumor invasion by mediating matrix degradation and cell migration. Hepatocyte growth factor/scatter factor (HGF/SF) is a secretory product of stromal fibroblasts, sharing structural motifs with enzymes of the blood clotting cascade, including a zymogen cleavage site. HGF/SF promotes motility, invasion and growth of epithelial and endothelial cells. Here we show that HGF/SF is secreted as a single-chain biologically inactive precursor (pro-HGF/SF), mostly found in a matrix-associated form. Maturation of the precursor into the active alpha beta heterodimer takes place in the extracellular environment and results from a serum-dependent proteolytic cleavage. In vitro, pro-HGF/SF was cleaved at a single site by nanomolar concentrations of pure urokinase, generating the active mature HGF/SF heterodimer. This cleavage was prevented by specific urokinase inhibitors, such as plasminogen activator inhibitor type-1 and protease nexin-1, and by antibodies directed against the urokinase catalytic domain. Addition of these inhibitors to HGF/SF responsive cells prevented activation of the HGF/SF precursor. These data show that urokinase acts as a pro-HGF/SF convertase, and suggest that some of the growth and invasive cellular responses mediated by this enzyme may involve activation of HGF/SF.     

The receptor for urokinase-plasminogen activator

Many human cells and cell lines possess a specific receptor that binds urokinase plasminogen activator (uPA) with an affinity of about 10(-10) M. Bound enzyme is not internalized, is slowly dissociated, and retains its enzymatic activity. The amino acid sequence of uPA responsible for receptor binding is located within the first 35 aminoterminal residues, ie, in the growth factor domain. Binding, however, is not competed for by other proteins that contain the growth factor domain (including epidermal growth factor). Cells that produce uPA secrete the pro-uPA form, which subsequently binds to the receptor. A431 cells, in fact, have their receptors completely saturated with pro-uPA. It is proposed that uPA:uPA-receptor interaction plays a direct role in physiological and pathological processes that require cell migration.     

Picolinic acid, a catabolite of tryptophan, as the second signal in the activation of IFN-gamma-primed macrophages.

We have studied the effects of picolinic acid, a product of tryptophan degradation, on the activation of mouse peritoneal macrophages (M phi). Picolinic acid acts synergistically with IFN-gamma in activating M phi from C57BL/6 mice. Moreover, M phi from C3H/HeJ mice and C3H/HeN that do not become cytotoxic in response to IFN-gamma alone could be fully activated by exposure to picolinate plus IFN-gamma. These results indicate that picolinic acid is a potent costimulator of M phi activation that functions as a second signal. Inasmuch as we have previously demonstrated that the activation of cytotoxic M phi correlates with specific changes in ribosomal RNA (rRNA), we investigated whether picolinic acid could modify M phi RNA metabolism. Picolinic acid inhibited the synthesis of total M phi RNA, the accumulation of newly synthesized 28S rRNA, and augmented the steady state levels of rRNA precursors (pre-rRNA). These changes in RNA metabolism were similar to those previously described in murine M phi activated in vitro or in vivo to express tumoricidal activity. These results demonstrate that picolinic acid is a potent, biologic M phi second signal, suggest that the changes in rRNA are causally connected with the expression of tumoricidal activity, and suggest the existance of an autocrine effect mediated by picolinic acid.