Proteasome mediated degradation of Id-1 is associated with TNF-alpha induced apoptosis in prostate cancer cells

Abstract

Overexpression of the he Helix-loop-helix protein Id-1 has been reported in over 20 types of cancer and recent study suggested that, in addition to its effect on cell growth and differentiation, Id-1 also involves in regulation of apoptosis. Previously, we have reported that suppression of Id-1 expression, which leads to downregulation of NF-KappaB activities, resulted in sensitization of the androgen independent prostate cancer cell to TNF-α induced apoptosis. In this study, we have shown that exposure of the androgen independent prostate cancer cells to TNF-α resulted in rapid and significant downregulation of the Id-1 protein level. The decrease in Id-1 expression is not due to decrease in gene transcription, since Id-1 promoter activities as well as the Id-1 mRNA level was not affected by the TNF-α treatment. Instead, the half-life of the Id-1 protein was decreased in both cell lines in the presence of TNF-α, and the addition of the proteasome inhibitor prior to the TNF-α treatment completely abolished the effect of the TNF-α on Id-1 protein. In addition, introduction of a Flag-tag epitode into the N-terminus of the Id-1, which has been shown to protect the protein from proteasome induced degradation, confer the protein resistance to the TNF-α treatment, suggesting that TNF-α induced the degradation of the Id-1 protein through activation of the proteosome degradation pathway in prostate cancer cells. Interestingly, while the two cell lines were initially resistant to TNF-α, prolong treatment of the cell result in initiation of apoptosis which coincided with disappear of the Id-1 protein, suggesting that Id-1 level correlates with the response of the cell to TNF-α induced apoptosis. This was further support by the finding that Id-1 protein are relatively more stable in the late passage of the LNCaP cells, which was also more resistant to the TNF-α induced apoptosis, than in the early passage. In conclusion, our finding indicated that Id-1 protein is regulated by TNF-α through the proteasome degradation pathway and the stability of the Id-1 protein appears to correlate with the sensitivity of the cells toward TNF-α induced apoptosis.