Identification, characterization, and cloning of TIP-B1, a novel protein inhibitor of tumor necrosis factor-induced lysis.

Some cancer cells evade elimination by virtue of their insensitivity to agents that induce apoptosis. Conversely, the side effects of anticancer agents could be diminished if normal cells were more resistant. To further elucidate the factors that contribute to the susceptibility of a cell to apoptosis, these investigations were designed to identify proteins isolated from cells exposed to low concentrations of tumor necrosis factor (TNF) that, when incubated with normally TNF-sensitive cells, protect these cells from TNF-induced cytotoxicity. TIP-B1, a novel protein, has been identified, purified, and characterized from cytosolic extracts of TNF-treated human fibroblasts. The approximately 27 kDa pI-4.5 TIP-B1 protein is unique based on both the sequence of three internal peptides (comprising 51 amino acids) and the nucleotide sequence of the corresponding 783-bp cDNA partial clone. Western blot analyses using polyclonal antisera raised against both the purified native TIP-B1 and the approximately 14 kDa product of the cDNA partial TIP-B1 clone, as well as Northern blot analyses using the cDNA insert as a probe, indicate that TIP-B1 may belong to a family of proteins that are expressed in a number of cell lines from diverse tissues. TNF-sensitive cells, when exposed to 4-10 microg/ml concentrations of TIP-B1 prior to the addition of TNF, are completely protected from TNF-induced lysis. Furthermore, TIP-B1 protects cells from apoptotic lysis induced by TNF. Preincubation of TIP-B1 with TNF does not affect the ability of TNF to induce lysis. Moreover, TIP-B1 does not seem to interfere with the interactions between TNF and the TNF receptors, based on a preliminary flow cytometric analysis of the cellular binding of biotinylated TNF. On the basis of these characteristics, TIP-B1 is not a soluble TNF receptor, an anti-TNF antibody, nor a protease that degrades TNF; yet TIP-B1 functions when added exogenously to cells. These characteristics, its novel sequence, and its function when added exogenously to cells indicate that TIP-B1 is unique and is not one of the other proteins reported previously to be involved in resistance to TNF. The ability of TIP-B1 to function after exogenous incubation with target cells makes TIP-B1 a likely candidate for therapeutic manipulation of TNF-induced effects.