Adenovirus type 12 early region 1A proteins repress class I HLA expression in transformed human cells

An adenovirus type 5 E1A protein with a single amino acid substitution blocks wild-type E1A transactivation

The E1A gene of adenovirus type 5 encodes a 289-amino-acid (289R) protein that transactivates early adenovirus promoters. We showed that the 289R protein of the E1A missense mutant gene hr5 is novel in that it inhibits the wild-type (wt) E1A protein from stimulating transcription from each of the early viral promoters E2, E3, and E4. Since both the hr5 and wt genes produced similar levels of E1A proteins, the ability of hr5 E1A to block transactivation was attributed to the replacement of serine by asparagine as position 185. We confirmed that this single amino acid substitution was responsible for blocking transactivation by showing equal inhibition with an hr5-wt hybrid E1A gene containing this missense mutation as the only alteration. The smaller 243R E1A protein of hr5 was not necessary for inhibition. Transcriptional activity from each early promoter was inhibited by at least 50% when the hr5 and wt E1A genes were present in equimolar amounts; complete inhibition occurred with a fivefold molar excess of the hr5 gene. Two other E1A missense mutant genes (hr3 and hr4) with amino acid substitutions in close proximity to that of hr5 failed to block wt E1A-induced transcription when similarly tested. Also, the hr5 E1A gene failed to impede the pseudorabies immediate early gene from transactivating the adenovirus E3 promoter, demonstrating that hr5 E1A inhibits wt E1A activation at the transcriptional, rather than the posttranscriptional, level. Although several possibilities were considered to account for this inhibition, the most likely is that the nonfunctional hr5 E1A protein competes with the wt 289R protein for a cellular transcription factor required for transactivation.

An adenovirus type 5 E1A protein with a single amino acid substitution blocks wild-type E1A transactivation

The E1A gene of adenovirus type 5 encodes a 289-amino-acid (289R) protein that transactivates early adenovirus promoters. We showed that the 289R protein of the E1A missense mutant gene hr5 is novel in that it inhibits the wild-type (wt) E1A protein from stimulating transcription from each of the early viral promoters E2, E3, and E4. Since both the hr5 and wt genes produced similar levels of E1A proteins, the ability of hr5 E1A to block transactivation was attributed to the replacement of serine by asparagine as position 185. We confirmed that this single amino acid substitution was responsible for blocking transactivation by showing equal inhibition with an hr5-wt hybrid E1A gene containing this missense mutation as the only alteration. The smaller 243R E1A protein of hr5 was not necessary for inhibition. Transcriptional activity from each early promoter was inhibited by at least 50% when the hr5 and wt E1A genes were present in equimolar amounts; complete inhibition occurred with a fivefold molar excess of the hr5 gene. Two other E1A missense mutant genes (hr3 and hr4) with amino acid substitutions in close proximity to that of hr5 failed to block wt E1A-induced transcription when similarly tested. Also, the hr5 E1A gene failed to impede the pseudorabies immediate early gene from transactivating the adenovirus E3 promoter, demonstrating that hr5 E1A inhibits wt E1A activation at the transcriptional, rather than the posttranscriptional, level. Although several possibilities were considered to account for this inhibition, the most likely is that the nonfunctional hr5 E1A protein competes with the wt 289R protein for a cellular transcription factor required for transactivation.

Tumorigenicity of hamster and mouse cells transformed by adenovirus types 2 and 5 is not influenced by the level of class I major histocompatibility antigens expressed on the cells

Inbred hamster and mouse cells transformed by the nononcogenic adenovirus (Ad) serotypes, Ad2 and Ad5, are nontumorigenic in syngeneic adult animals, while cells from these species transformed by the highly oncogenic Ad12 are tumorigenic in such rodents. By immunoprecipitation and flow cytometry, cells from four of six Ad2- and Ad5-transformed hamster and mouse lines expressed high levels of cell-surface class I major histocompatibility complex (MHC) antigens, while cells from two of these six lines expressed low levels of cell-surface class I MHC antigens. The levels of class I MHC proteins expressed by cells from these latter two lines were comparable to the levels of cell-surface class I MHC proteins expressed by cells from Ad12-transformed hamster and mouse lines. Moreover, an Ad2-transformed line that had become highly oncogenic after in vivo adaptation showed the same high level of MHC expression as the nononcogenic parent. The amounts of class I mRNA, analyzed by RNA blotting, were, in general, consistent with the levels of class I antigens expressed on the surfaces of these cells. These results indicate that there is no correlation between the tumorigenicity in immunocompetent syngeneic adult rodents of Ad2- and Ad5-transformed hamster and mouse cells and the level of class I MHC antigens expressed on the surfaces of these cells. Thus, the expression of different levels of class I MHC proteins does not seem to explain the differences in the oncogenicity between nononcogenic and highly oncogenic human Ad serotypes.