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pubmed:abstractText |
Expression of the adenovirus early gene E1A inhibits the nerve growth factor (NGF)-induced differentiation of PC12 pheochromocytoma cells. Expression of the 12S form of E1A, which lacks the transcription activation region, also inhibited PC12 cell differentiation in a manner similar to the wild-type gene. Three cellular proteins--the retinoblastoma susceptibility gene product referred to as 105(Rb)-, 107-, and 300-kDa proteins--stably interacted with the different E1A polypeptides. Analysis of the association of these cellular proteins with mutant E1A polypeptides demonstrated that a functional domain 1, which is minimally involved in the association of the 300-kDa protein with E1A, was sufficient to inhibit neuronal differentiation. Deletion of transformation domain 2, which encodes sequences necessary for the binding of the 105(Rb)- and 107-kDa proteins, did not influence the ability of the mutant E1A polypeptide to inhibit PC12 cell differentiation. E1A was also shown to alter the expression of mRNAs for the early response genes c-fos, c-myc, egr-1, and c-jun and their regulation in response to NGF. In clones expressing either 12S or 13S E1A, NGF stimulation of c-fos and c-myc was repressed. In contrast, basal mRNA levels for c-jun and egr-1 were constitutively elevated and not significantly affected further by challenge with NGF. Simply expressing c-jun by gene transfer, however, did not mimic the action of E1A because constitutively expressing c-jun clones differentiated in response to NGF. Thus, expression of the E1A polypeptide disrupts NGF control of early transcription events that have been shown to be critical for PC12 cell neuronal differentiation.
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