Treatment of mouse F9 embryonal carcinoma cells with all-trans retinoic acid (T-RA) induces differentiation into primitive endodermal type cells. Differentiation requires the action of the receptors for all trans, and 9cis-retinoic acid (RAR and RXR, respectively) and is accompanied by growth inhibition, changes in cell morphology, increased apoptosis, proteolytic degradation of the RARγ2 receptor, and induction of target genes.
We show that the RNA polymerase II transcription factor TFIID subunits TBP and TAFII135 are selectively depleted in extracts from differentiated F9 cells. In contrast, TBP and TAFII135 are readily detected in extracts from differentiated F9 cells treated with proteasome inhibitors showing that their disappearance is due to targeted proteolysis. This regulatory pathway is not limited to F9 cells as it is also seen when C2C12 myoblasts differentiate into myotubes. Targeting of TBP and TAFII135 for proteolysis in F9 cells takes place coordinately with that previously reported for the RARγ2 receptor and is delayed or does not take place in RAR mutant F9 cells where differentiation is known to be impaired or abolished. Moreover, ectopic expression of TAFII135 delays proteolysis of the RARγ2 receptor and impairs primitive endoderm differentiation at an early stage as evidenced by cell morphology, induction of marker genes and apoptotic response. In addition, enhanced TAFII135 expression induces a novel differentiation pathway characterised by the appearance of cells with an atypical elongated morphology which are cAMP resistant.
These observations indicate that appropriately timed proteolysis of TBP and TAFII135 is required for normal F9 cell differentiation. Hence, in addition to transactivators, targeted proteolysis of basal transcription factors also plays an important role in gene regulation in response to physiological stimuli.