Cell and tissue-type specific expression of Ras-related GTPase RhoB.

Ras-homologous (Rho) GTPases are involved in the regulation of a variety of cellular processes such as the organization of the actin cytoskeleton, malignant transformation and genotoxic stress-induced signaling. Here we show that, among the family of Rho GTPases, specifically rhoB mRNA expression is rapidly induced upon UV-irradiation, whereas the level of rac 1 and cdc42 mRNA is not affected. Increase in rhoB mRNA was accompanied by a approximately 4-fold increase in the amount of membrane-bound RhoB protein. Basal expression of rhoB mRNA appears to be cell-type specific with low amounts in rodent NIH 3T3, V79, H4IIE, CHO and human HaCat cells and comparably high amounts in monkey COS, human HeLa and HepG2 cells. In rabbit tissues, exceptionally high levels of rhoB mRNA and RhoB protein were found in lung whereas its expression was quite low in heart, liver, spleen and kidney. Variations in rhoB mRNA expression level are not due to cell-type specific differences in rhoB mRNA stability as shown by inhibitor experiments. However, transiently transfected rhoB promoter CAT construct was expressed at significantly higher level in HeLa and HepG2 as compared to NIH 3T3 and CHO cells. Thus, cell-type specific differences in the level of rhoB mRNA are likely to be due to variations in the transcriptional activity of the rhoB gene. The data indicate that, among the family of Rho GTPases, only the expression of rhoB is rapidly stimulated by genotoxic stress. Furthermore basal rhoB expression appears to be regulated in a cell and tissue-type specific manner. This may be related to yet unknown tissue-specific physiological function of RhoB.