The regulation of human adrenomedullin (AM) and tumor necrosis factor alpha (TNF-alpha) receptors on human epithelial carcinoma (HeLa) cells. The role of AM secretion in tumor cell sensitivity
Abstract:
The cytostatic cytokine tumor necrosis factor-alpha (TNF-alpha) and proliferative hormone adrenomedullin (AM) are abundantly expressed in human tumors. However, little is known about mechanism (s) through which TNF-alpha and AM exert their regulatory effects, especially in the regulation of proliferative activity in malignant cells. Also the role played by TNF-alpha in pathogenesis and treatment of cancer (targeted cancer therapy) remains less understood. The purpose of this study was therefore to characterize the significance of TNF-alpha induced apoptosis with down-regulation of plasma-membrane TNF-alpha receptors and up-regulation of AM receptors with increased production of human AM mRNA, i.e. mechanisms that subsequently control aberrant cellular proliferation in malignant cells. Cytotoxicity, and the whole cell ligand binding assays for TNF-alpha and AM receptors, and RIA-assays of AM production were accomplished in control experiments using pharmacologically pretreated HeLa cells. AMincreased proliferation of HeLa cells andAMantagonist (Ala6,21)AM(22-52) significantly antagonized this increase. TNF-alpha inhibitor of cell growth actinomycin-D significantly increased cytotoxicity of TNF-alpha in HeLa cells. Hypoxia increased TNF-alpha production and increased surface-membrane [125I]AM binding. Tumor promotor PMA and histamine down-regulated specific binding of [125I]TNF- alpha on HeLa cells. Mitogenic peptide endothelin-1 increased and specific ET-1 antagonist BQ123 and significantly reduced AM binding. Production of AM in HeLa cells markedly increased after exposure to hypoxia >ET-1 >PMA. BAY11-7082 at concentrations that inhibited IęB phosphorylation and thus nuclear translocation and surface membrane TNF-alpha expression increased AM specific binding. Pretreatment of cells inhibitor of HMGCoA reductase inhibitor VULM1457 significantly increased the total number of specific [125I]AM binding sites on HeLa cells. These results suggest relative and contradictory TNF-alpha and AM surface-membrane receptor signaling in HeLa cells and findings reveal a novel proliferative mechanisms that controlAMproduction and thus oncogenic signaling in cells. This implies that several putative inhibitors of TNF-alpha and AM signaling may be considered in oncology for treatment of tumors otherwise nonresponding to cytostatic therapy.