CERTAIN CONDITIONS DETERMINING ENHANCED INFECTION WITH THE RABBIT PAPILLOMA VIRUS

Studies of the mechanism of action of the Shope rabbit papilloma virus. I. Concerning the nature of the induction of arginase in the infected cells

These studies make plain that the Shope papilloma virus induces the production of an arginase in rabbit squamous epithelium, and provide evidence that the information for the synthesis of the enzyme is derived from the virus rather than the rabbit. This form of induction is therefore different from that brought about by chemicals such as galactosides (36). Striking differences were shown between the physiochemical properties of Shope virus-induced arginase and other arginases of domestic and Kansas cottontail rabbits. The absence of a requirement for manganese suggests the mechanism of splitting off urea from arginine by papilloma arginase probably differs from that of previously described arginases. These findings (particularly the experiments in which it was demonstrated that papilloma growth could be greatly reduced by giving animals supplemental arginine in the presence of small amounts of the arginase inhibitor, canavanine) provide evidence that suggests the following mechanism of action for the virus: viral DNA introduces into rabbit epithelium the information for the synthesis of an arginase for which the cells have no control mechanism. The arginase depletes cellular arginine and, in turn, the synthesis of arginine-rich nuclear histones (2), thereby freeing the nucleus for greater synthetic activity (4, 35), which results in more rapid growth of the papilloma cells. Domestic rabbit papillomas induced with purified virus derived from wild rabbit papillomas were shown to contain an antigen which reacts immunologically like a wild rabbit antigen. It was distinct from the arginase, the protein coat of the virus, and the Vx-2 antigen. The presence of the wild rabbit antigen seems indicative of integration of the virus DNA in the host genome of the wild rabbit. Other incidental findings include the description of a simple method, accurate to within 1 per cent, for determining the partial specific volume of a protein, and the description of a modification of the Richards and Schachman method (9) enabling the determination of the molecular weight and homogeneity of a protein with as little as 0.25 mg and with an accuracy of 2 per cent.

The primary target cells of the high-risk cottontail rabbit papillomavirus colocalize with hair follicle stem cells

Papillomaviruses are small DNA tumor viruses with a life cycle inseparably linked to the differentiation of the pluristratified epithelium. The infection of epithelial layers of the skin may remain latent or may result in the development of benign tumors. A certain number of distinct papillomavirus types, however, cause lesions which have a high risk of progression into carcinomas, and extensive efforts have been made to understand this process. comparatively little is known about the initial events during the establishment of a persistent infection and papilloma development. Although it is generally accepted that the growth of a papilloma requires the infection of cells in the basal layer of the epithelium, it remains unknown which cells perform this task. We have analyzed by in situ hybridization biopsy samples taken at various time points after infection of domestic rabbits with cottontail rabbit papillomavirus. The positive cells detected at a low frequency in biopsy samples taken after 11 days predominantly expressed high levels of E6 and E7 mRNA and were localized in the outer epithelial root sheath and in the bulbs of hair follicles. A clonal analysis of keratinocytes isolated from different subfragments of individual rabbit hair follicles demonstrated a clear colocalization of cottontail rabbit papillomavirus mRNA-positive cells with clonogenic cells in hair follicles. These data suggest that the cells competent to establish papillomatous growth represent a subpopulation of keratinocytes in hair follicles with properties expected of epithelial stem cells.

Change in the structure of Shope papilloma virus-induced arginase associated with mutation of the virus

The change in the state of the virus-induced enzyme associated with a mutation in the virus provides additional evidence that the enzyme is synthesized from virus rather than rabbit genetic information. This change in structure results in differences in stability of polymerization, degree of optical rotary dispersion (ORD) specific rotation, change in elution characteristics from carboxymethyl cellulose, and a reduction in specific activity of the arginase. Liver arginase differs markedly in ORD characteristics from the virus-induced enzyme. In contrast to the virus-induced enzyme, it showed no negative Cotton effect at 233 nm until it was activated with manganese. Manganese had no influence on the ORD spectrum of virus-induced arginase. In addition, liver arginase is denatured by 4 M urea, while the virus-induced enzyme requires 10 M urea for denaturation.