Ultraviolet irradiation and c-jun over-expression regulates replication of polyoma sequences in WOP cells through a PEBP2 binding site

Identification of TAZ as a binding partner of the polyomavirus T antigens

A polyomavirus mutant isolated by the tumor host range selection procedure (19) has a three-amino-acid deletion (Delta2-4) in the common N terminus of the T antigens. To search for a cellular protein bound by wild-type but not the mutant T antigen(s), a yeast two-hybrid screen of a mouse embryo cDNA library was carried out with a bait of wild-type small T antigen (sT) fused N terminally to the DNA-binding domain of Gal4. TAZ, a transcriptional coactivator with a WW domain and PDZ-binding motif (17), was identified as a binding partner. TAZ bound in vivo to all three T antigens with different apparent affinities estimated as 1:7:100 (large T antigen [lT]:middle T antigen [mT]:sT). The Delta2-4 mutant T antigens showed no detectable binding. The sT and mT of the host range transformation-defective (hr-t) mutant NG59 with an alteration in the common sT/mT region (179 D-->NI) and a normal N terminus also failed to bind TAZ, while the unaltered lT bound but with reduced affinity compared to that seen in a wild-type virus infection. The WW domain but not the PDZ-binding motif of TAZ was essential for T antigen binding. The Delta2-4 mutant was defective in viral DNA replication. Forced overexpression of TAZ blocked wild-type DNA replication in a manner dependent on the binding site for the polyomavirus enhancer-binding protein 2alpha. Wild-type polyomavirus T antigens effectively block transactivation by TAZ. The functional significance of TAZ interactions with polyomavirus T antigens is discussed.

Natural biology of polyomavirus middle T antigen

"It has been commented by someone that 'polyoma' is an adjective composed of a prefix and suffix, with no root between--a meatless linguistic sandwich" (C. J. Dawe). The very name "polyomavirus" is a vague mantel: a name given before our understanding of these viral agents was clear but implying a clear tumor life-style, as noted by the late C. J. Dawe. However, polyomavirus are not by nature tumor-inducing agents. Since it is the purpose of this review to consider the natural function of middle T antigen (MT), encoded by one of the seemingly crucial transforming genes of polyomavirus, we will reconsider and redefine the virus and its MT gene in the context of its natural biology and function. This review was motivated by our recent in vivo analysis of MT function. Using intranasal inoculation of adult SCID mice, we have shown that polyomavirus can replicate with an MT lacking all functions associated with transformation to similar levels to wild-type virus. These observations, along with an almost indistinguishable replication of all MT mutants with respect to wild-type viruses in adult competent mice, illustrate that MT can have a play subtle role in acute replication and persistence. The most notable effect of MT mutants was in infections of newborns, indicating that polyomavirus may be highly adapted to replication in newborn lungs. It is from this context that our current understanding of this well-studied virus and gene is presented.

Elevated binding to URE/PEBP2 during the late stages of NNK and benzo[a]pyrene-induced carcinogenesis in A/J mice

To provide better understanding about alterations in transcription factor activities during the promotion and progression stages of lung carcinogenesis we have utilized the A/J mice lung tumor model in which two potent lung carcinogens, 4-(methyl-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and benzo[a]pyrene (BaP), were co-administered. Nuclear proteins prepared from lung, brain, kidney, liver and colon of the A/J mice, 19 weeks after the last carcinogen administration, as well as from their respective non-treated controls, were tested for their binding to polyoma enhancer binding protein 2 (PEBP2) target sequence and UV-responsive element (URE). PEBP2 represents a newly identified family of transcription factors that was shown to play a role in cellular differentiation and transformation. URE is similar to CRE and AP1 target sequences, to which the members of ATF/AP1 transcriptional factors family bind. We demonstrate here that there is a marked increase in binding to both PEBP2 and URE sequences in lung, liver, kidney and brain of the treated mice. Such binding appears to be dependent on the mode of carcinogen administration as it was better noticed in the intragastric injected group than in the intraperitoneal group. Taken together, our findings suggest that increased binding to the URE and PEBP2 target sequence reflects changes in transcriptional activities which occur at late stages of lung carcinogenesis in a fashion which appear to depend on mode of carcinogen administration.