Expression of cytokeratin 16 in human papillomavirus type 11-infected genital epithelium

ERK2-mediated C-terminal serine phosphorylation of p300 is vital to the regulation of epidermal growth factor-induced keratin 16 gene expression

We previously reported that the epidermal growth factor (EGF) regulates the gene expression of keratin 16 by activating the extracellular signal-regulated kinase 1 and 2 (ERK1/2) signaling which in turn enhances the recruitment of p300 to the keratin 16 promoter. The recruited p300 functionally cooperates with Sp1 and c-Jun to regulate the gene expression of keratin 16. This study investigated in detail the molecular events incurred upon p300 whereby EGF caused an enhanced interaction between p300 and Sp1. EGF apparently induced time- and dose-dependent phosphorylation of p300, both in vitro and in vivo, through the activation of ERK2. The six potential ERK2 phosphorylation sites, including three threonine and three serine residues as revealed by sequential analysis, were first identified in vitro. Confirmation of these six sites in vivo indicated that these three serine residues (Ser-2279, Ser-2315, and Ser-2366) on the C terminus of p300 were the major signaling targets of EGF. Furthermore, the C-terminal serine phosphorylation of p300 stimulated its histone acetyltransferase activity and enhanced its interaction with Sp1. These serine phosphorylation sites on p300 controlled the p300 recruitment to the keratin 16 promoter. When all three serine residues on p300 were replaced by alanine, EGF could no longer induce the gene expression of keratin 16. Taken together, these results strongly suggested that the ERK2-mediated C-terminal serine phosphorylation of p300 was a key event in the regulation of EGF-induced keratin 16 expression. These results also constituted the first report identifying the unique p300 phosphorylation sites induced by ERK2 in vivo.

Distinct changes in gene expression induced by A-Myb, B-Myb and c-Myb proteins

The c-Myb, A-Myb and B-Myb transcription factors have nearly identical DNA-binding domains, activate the same reporter gene constructs in animal cells, but have different biological roles. The Myb proteins are often coexpressed in the same cells, raising questions about whether they activate similar or distinct gene expression profiles, and whether they cooperate or compete in regulating the same promoters. Here, recombinant adenoviruses were used to express each protein in human mammary cells, and then microarray assays were used to assess global changes in gene expression. Each Myb protein induced a unique and specific set of changes, displaying activities far more complex than revealed by standard reporter gene assays. These results have important implications for the roles of various Myb proteins in normal and transformed human cells, for regulatory pathways that might modify their activities and for the importance of acquired mutations that may qualitatively alter their functions in tumors.

Human papillomavirus type 11 alters the transcription and expression of loricrin, the major cell envelope protein

Human papillomavirus (HPV) does not induce lysis of infected keratinocytes, and the exact mechanisms of viral escape are not known. As keratinocytes differentiate, the cornified cell envelope (CCE) develops, providing a protective barrier to the host. We previously showed that the normally durable CCE in HPV 11-infected epithelium is fragile compared to CCEs in healthy epithelium. In this study, we examined uninfected and HPV 11-infected human genital epithelium for expression of loricrin, the major CCE protein in healthy epidermis. In HPV 11-infected human genital epithelium, detection of loricrin was reduced in immunoelectron microscopic and immunoblot assays, suggesting that loricrin incorporation into the CCE was reduced or that loricrin synthesis was reduced. Loricrin detection was reduced in immunohistochemical assays in areas of high viral replication. Mathematical modeling by least squares was performed using the amino acid composition of highly purified CCE fragments, confirming that loricrin was markedly reduced and that the small proline-rich proteins and cytokeratins were increased in those derived from HPV 11-infected epithelium compared to healthy genital epithelium. In RNase protection and RT-PCR assays, loricrin transcripts were markedly reduced in HPV 11-infected epithelium compared to uninfected epithelium. Loricrin transcripts were detectable by RNA in situ analysis in isolated cells of HPV 11-infected epithelium, but were absent in large regions of epithelium. We conclude that HPV 11 infection reduces transcription of the loricrin gene and that this leads to a reduction in loricrin incorporation into the CCE. Further studies will examine the effects of specific HPV gene products on transcription of loricrin and other CCE components, as it is likely that viral egress from the infected keratinocyte depends on these effects.