Over 60 different types of human papillomavirus (HPV) have been identified, and they are classified into high and low risk groups based on the risk for malignant progression of HPV associated lesions. HPVs belonging to a high risk group have been shown to express two major transforming proteins, E6 and E7. With respect to the transforming activity of these proteins, many investigators have reported the location of these proteins in the cell, but their results are still controversial. In the present study, HPV type 16 E6 or E7 open reading frame (ORF) proteins were expressed and localized in human epidermal keratinocytes (RHEK-1) using the vaccinia virus as an expression vector. Immunofluorescence detection using monoclonal antibodies against E6 or E7 ORF proteins revealed that E6 or E7 proteins of HPV type 16 were located in the cytoplasm of RHEK-1 cells. These results suggest that E6 and E7 proteins bind to the tumor suppressor counterparts, thereby preventing transport of these proteins into the nucleus. These antioncogene products that fail to be rapidly transported out of the cytosol may be degraded by certain proteases such as the ubiquitin dependent system. In this way, the precise function of antioncogene products in the regulation of cell growth could be destroyed, and abnormal cell growth could occur.
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