Ubiquitination of tissue transglutaminase is modulated by interferon alpha in human lung cancer cells.

The addition of 2500 i.u./ml interferon alpha (IFNalpha) for 48 h induced apoptosis, and caused an approx. 4-fold increase in the activity and expression of tissue transglutaminase (tTG), in human lung cancer H1355 cells. However, the increase in mRNA levels for tTG was just 1.6-fold. On the basis of these data, we investigated whether tTG levels may be regulated through regulation of its degradation via ubiquitination. It was found that 2500 i.u./ml IFNalpha induced a time-dependent decrease in tTG ubiquitination. On the other hand, addition of the proteasome inhibitor lactacystin led to accumulation of the ubiquitinated form of the enzyme and to a consequent increase in its expression. Treatment of the cells with the two agents combined antagonized the accumulation of the ubiquitinated isoforms of tTG induced by lactacystin and caused a potentiation of tTG expression. Moreover, the tTG inducer retinoic acid was also able to cause increased expression and ubiquitination of tTG in H1355 cells. The addition of monodansylcadaverine (a tTG inhibitor) to IFNalpha-treated H1355 cells completely antagonized growth inhibition and apoptosis induced by the cytokine. In conclusion, we demonstrate for the first time that tTG is ubiquitinated and degraded by a proteasome-dependent pathway. Moreover, IFNalpha can, at least in part, induce apoptosis through the modulation of this pathway.