Cell type-specific replication of simian virus 40 conferred by hormone response elements in the late promoter

Herpes and polyoma family viruses in thyroid cancer

Thyroid cancer is considered the most common malignancy that affects the endocrine system. Generally, thyroid cancer derives from follicular epithelial cells, and thyroid cancer is divided into well-differentiated papillary (80% of cases) and follicular (15% of cases) carcinoma. Follicular thyroid cancer is further divided into the conventional and oncocytic (Hürthle cell) type, poorly differentiated carcinoma and anaplastic carcinoma. Both poorly differentiated and anaplastic carcinoma can arise either de novo, or secondarily from papillary and follicular thyroid cancer. The incidence of thyroid cancer has significantly increased for both males and females of all ages, particularly for females between 55–64 years of age, from 1999 through 2008. The increased rates refer to tumors of all stages, though they were mostly noted in localized disease. Recently, viruses have been implicated in the direct regulation of epithelial-mesenchymal transition (EMT) and the development of metastases. More specifically, Epstein-Barr virus (EBV) proteins may potentially lead to the development of metastasis through the regulation of the metastasis suppressor, Nm23, and the control of Twist expression. The significant enhancement of the metastatic potential, through the induction of angiogenesis and changes to the tumor microenvironment, subsequent to viral infection, has been documented, while EMT also contributes to cancer cell permissiveness to viruses. A number of viruses have been identified to be associated with carcinogenesis, and these include lymphotropic herpesviruses, namely EBV and Kaposi's sarcoma-associated herpesvirus [KSHV, also known as human herpesvirus type 8 (HHV8)]; two hepatitis viruses, hepatitis B virus (HBV) and hepatitis C virus (HCV); human papillomaviruses (HPVs); human T cell lymphoma virus (HTLV); and a new polyomavirus, Merkel cell polyomavirus identified in 2008. In this review, we examined the association between thyroid cancer and two oncogenic virus families, the herpes and polyoma family viruses, and we discuss their potential role as causative agents in thyroid carcinogenesis.

Virus receptors and tropism

Polyomaviruses are small, tumorigenic, nonenveloped viruses that infect several different species. Interaction of these viruses with cell surface receptors represents the initial step during infection of host cells. This interaction can be a major determinant of viral host and tissue tropism. This chapter reviews what is currently known about the cellular receptors for each of five polyomavirus family members: Mouse polyomavirus (PyV), JC virus (JCV), BK virus (BKV), Lymphotropic papovavirus (LPV) and Simian virus 40 (SV40). These polyomaviruses serve to illustrate the enormous diversity of virus-cell surface interactions and allow us to closely evaluate the role of receptors in their life cycles. The contribution of other factors such as transcriptional regulators and signaling pathways are also summarized.

Positive and negative regulation of chicken anemia virus transcription

Chicken anemia virus (CAV) is a small circular single-stranded DNA virus with a single promoter-enhancer region containing four consensus cyclic AMP response element sequences (AGCTCA), which are similar to the estrogen response element (ERE) consensus half-sites (A)GGTCA. These sequences are arranged as direct repeats, an arrangement that can be recognized by members of the nuclear receptor superfamily. Transient-transfection assays which use a short CAV promoter construct that ended at the transcription start site and drive expression of enhanced green fluorescent protein (EGFP) showed high basal activity in DF-1, LMH, LMH/2A, and primary theca and granulosa cells. The estrogen receptor-enhanced cell line, LMH/2A, had significantly greater expression than LMH cells, and this expression was significantly increased with estrogen treatment. A long promoter construct which included GGTCA-like sequences downstream of the first CAV protein translation start site was found to have significantly less EGFP expression in DF-1 cells than the short promoter, which was largely due to decreased RNA transcription. DNA-protein binding assays indicated that proteins recognizing a consensus ERE palindrome also bind GGTCA-like sequences in the CAV promoter. Estrogen receptor and other members of the nuclear receptor superfamily may provide a mechanism to regulate CAV activity in situations of low virus copy number.

Chicken Infectious Anemia Virus: An Example of the Ultimate Host–Parasite Relationship

Chicken infectious anemia virus (CIAV) is a resistant and ubiquitous virus of chickens causing disease in young chickens and immunosuppression in all birds. This paper reviews the current knowledge of CIAV with a focus on new findings indicating that immunosuppressive effects have not been fully appreciated, especially as they relate to the development of antigen-specific cytotoxic T cells. A more complete understanding of the immunosuppressive effects of CIAV emphasizes the need for better vaccines, especially for the broiler industry. In addition, a new model is proposed for the control of viral replication in the reproductive tract of specific-pathogen-free chickens, which may be latently infected. This model suggests that virus transcription is controlled by viral enhancer and repressor elements, which are regulated by different hormones. As a consequence, CIAV has a well-adapted relationship with its host, avoiding immune detection, ensuring passage of virus to the next generation, and eliciting limited pathology to the host.

Reconstitution of retroviral fusion and uncoating in a cell-free system

The molecular events underlying the immediate steps of retroviral uncoating, occurring after membrane fusion and leading to the formation of an active reverse transcription complex, are not known. To better understand these processes, we have developed a cell-free system that recapitulates these early steps of retroviral replication by using avian sarcoma and leukosis virus as a model retrovirus. The substrates used in this system are viral particles that are trapped before completing membrane fusion. These virions are induced to fuse out of endosomes and the viral cores are released into solution where they are amenable to biochemical manipulation. This system revealed that membrane fusion is not sufficient to stimulate the formation of a reverse transcription complex. Instead, ATP hydrolysis and cellular factors >5 kDa in size are required. Furthermore, later steps of avian sarcoma and leukosis virus reverse transcription were stimulated by nuclear factors. The cell-free system should now allow for the definition of retroviral uncoating mechanisms and facilitate the identification and characterization of the cellular factors involved.

Requirement of a 12-base-pair TATT-containing sequence and viral lytic DNA replication in activation of the Kaposi's sarcoma-associated herpesvirus K8.1 late promoter

Kaposi's sarcoma-associated herpesvirus (KSHV) K8.1 late promoter consists of a minimal 24-bp sequence, with a TATA-like, 12-bp promoter core, AATATTAAAGGG, and is active on a reporter only in butyrate-induced KSHV-infected cells. The activity of the K8.1 promoter can be enhanced (>15-fold) by the KSHV left-end lytic origin of DNA replication (oriLyt-L) sequence while providing inefficient replication of plasmid DNA and is inhibited by viral DNA replication inhibitors, suggesting that activation of the K8.1 promoter on the reporter is involved in KSHV lytic DNA replication largely by trans.

Hormone response element in SV40 late promoter directly affects synthesis of early as well as late viral RNAs

We previously demonstrated that the presence of a hormone response element surrounding the transcription initiation site of the SV40 major late promoter (+1 HRE) confers a replication advantage to the virus in a cell-type-specific manner. We determine here the mechanism by which the +1 HRE confers this advantage by analyzing in detail the various stages of the viral life cycle of wild-type versus a +1 HRE mutant in MA-134 cells. We show that the mutant overexpresses late genes at the expense of early genes at early times after infection. This initial underproduction of early RNA leads, subsequently, to an underproduction of large T-antigen, viral DNA, and infectious virions. We conclude that the +1 HRE is necessary for the proper initial regulation of transcription from the early as well as late promoter so the cascade of subsequent events can be executed for the optimal production of virions.