Inhibition of TG2 transamidation activity decreases both amine incorporation into proteins and the protein cross-link content in the cytosol and the nucleus of maturing NB4 cells. NB4 cells were cultured for 96 hours in the presence of 1 μM ATRA or 1 μM ATRA plus 15 μM MDC. (A) Transglutaminase activity in MDC-treated cells. Total cell lysate (50 μg) was used for assaying TG2 activity. Activity was measured by incorporation of [³H]putrescine into casein. The means of 3 separate experiments performed in duplicate are shown. (B) Western blot analysis of TG2 in NB4 cells. The amount of TG2 in ATRA- and ATRA plus MDC–treated samples was determined by SDS-PAGE following immunoblotting with monoclonal anti-TG2 antibody. The arrow points to the TG2 bands. (C) Cytosolic and nuclear N^ϵ-(γ-glutamyl)-lysine cross-link content in the absence or presence of TG2 inhibitor on day 4 of differentiation. Results are expressed as the mean ± SD of 3 independent experiments.