9093

DNAJA3

HCA57|TID1|hTID-1;DnaJ (Hsp40) homolog, subfamily A, member 3;DNAJA3;DnaJ (Hsp40) homolog, subfamily A, member 3

dnaJ homolog subfamily A member 3, mitochondrial|dnaJ protein Tid-1|hepatocellular carcinoma-associated antigen 57|tumorous imaginal discs protein Tid56 homolog

16p13.3

protein-coding

The human homologue of the Drosophila tumor suppressor lethal (2) tumorous imaginal discs (l(2)tid) gene, hTID1, encodes two proteins derived from alternate mRNA splicing. The splice variants TidL and TidS were previously reported from protein overexpression and dominant-negative mutant protein studies to exhibit opposing biological activities in response to exogenous cytotoxic stimuli. TidL was found to promote apoptosis while TidS suppressed it. To elucidate the physiological function of hTID1, we depleted hTID1 proteins using the technique of RNA interference (RNAi). Here, we show that cells essentially lacking expression of hTID1 proteins are protected from cell death in response to multiple stimuli, including cisplatin, tumor necrosis factor alpha/cycloheximide and mitomycin C. We also generated stable cell populations depleted of hTID1 proteins by RNAi using DNA vectors. In addition to apoptosis resistance, stable hTID1 knockdown cells exhibited an enhanced ability for anchorage-independent growth, as measured by an increase in soft-agar colony formation. These results suggest that hTID1 functions as an important cell death regulator and raise the interesting possibility that hTID1 could exert tumor suppressor activity.

Tid1 (DNAJA3), a DnaJ cochaperone, may promote degradation of oncogenic kinases. Tid1 has 2 isoforms, Tid1-L and Tid1-S, that may function differently. In this study, we investigated the role of the Tid1 isoforms in regulating EGF receptor (EGFR) signaling and lung cancer progression. We found that both Tid1-L and Tid1-S expressions were reduced in patients with non-small cell lung cancer compared with normal counterparts. Tid1-L expression correlated inversely with EGFR expression. Low Tid1-L/high EGFR expression predicted poor overall survival in patients with lung adenocarcinoma. Tid1-L overexpression in lung cancer cells attenuated EGFR signaling and inhibited cell proliferation, colony formation, and tumor growth in subcutaneous and orthotropic xenograft models. Conversely, depletion of Tid1 restored EGFR signaling and increased cell proliferation and colony formation. Tid1-L, but not Tid1-S, interacted with EGFR/HSP70/HSP90 through the DnaJ domain, counteracting the EGFR regulatory function of HSP90 by causing EGFR ubiquitinylation and proteasomal degradation. Tid1-L inhibited EGFR signaling even more than the HSP90 inhibitor 17-allylamino-demethoxy geldanamycin. We concluded that Tid1-L acted as a tumor suppressor by inhibiting EGFR signaling through interaction with EGFR/HSP70/HSP90 and enhancing EGFR ubiquitinylation and degradation.