Expression of E 6 and E 7 Papillomavirus Oncogenes in the Outer Root Sheath of Hair Follicles Extends the Growth Phase and Bypasses Resting at Telogen 1

Hair follicle growth cycle proceeds through a series of stages in which strict control of cell proliferation, differentiation, and cell death occurs. Transgenic mice expressing human papillomavirus type 16 E6/E7 papillomavirus oncogenes in the outer root sheath (ORS) display a fur phenotype characterized by lower hair density and the ability to regenerate hair much faster than wild-type mice. Regenerating hair follicles of transgenic mice show a longer growth phase (anagen), and although bulb regression (catagen) occurs, rest at telogen was not observed. No abnormalities were detected during the first cycle of hair follicle growth, but by the second cycle, initiation of catagen was delayed, and rest at telogen was again not attained, even in the presence of estradiol, a telogen resting signal. In conclusion, expression of E6/ E7 in the ORS delays entrance to catagen and makes cells of the ORS insensitive to telogen resting signals bearing to a continuous hair follicle cycling in transgenic mice. Introduction Proliferation and death are essential cellular processes during the development of different animal structures. Proliferation is required to produce as many cells as needed to form the tissues of a specific structure, and its inhibition allows terminal differentiation. On the other hand, cell death is essential both to eliminate cells within tissues that have to achieve the right size and form and to eliminate unnecessary tissues in the mature structure. In recent years, many signals that control proliferation, differentiation, and cell death have been characterized, several of which are shared, showing the intimate interactions between these processes (1, 2). Hair follicle development (reviewed in Ref. 3) is an appropriate model to study how proliferation, differentiation, and cell death coordinate to give rise to a complex structure. Hair follicles originate late in embryogenesis from the epidermis by a process involving thickening and down-growth of the epithelial tissue at specific sites; the dermal mesenchyme tissue underneath, whose cells aggregate to form the dermal papilla, determines these sites. The plug of epithelial tissue formed, which is always in intimate contact with the dermal papilla, is initially constituted of what is known as the ORS, which will later differentiate into the cells of the hair matrix at the base of the follicle. Hair follicle growth results from differentiation and migration of cells from the hair matrix, giving rise to the inner root sheath and the shaft. After growth, the base of a hair follicle degenerates up to one-third of its maximum size. The follicle at this stage is composed of nonproliferating cells, which include the putative stem cells responsible for hair regeneration in the adult. The hair growth cycle appears to start from a population of stem cells located in a special part of the ORS, just below the bulge (4, 5). Every cycle is characterized by three phases: (a) anagen, the stage of active cell proliferation during which the follicle is regenerated, and a new hair is produced; (b) catagen, the stage during which hair elongation ceases, and active cell death causes hair follicle regression; and (c) telogen, the stage during which the follicle is at rest, and cell activity is minimal. At this latter stage, the stem cells are ready to start a new cycle. Experimental modifications of hair follicle development may contribute to the understanding of how different regulatory pathways act within the epidermis. To modify the patterns of proliferation, differentiation, and cell death in a variety of renewable stratified epithelia, including the ORS of hair follicles, we have used the K6 promoter to direct the Received 5/18/00; revised 6/13/00; accepted 8/31/00. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 Supported by the Consejo Nacional de Ciencia y Tecnologı́a (Grants 1163 M/9209 and 31730-N), Programa de Naciones Unidas para el Desarrollo (pnud/mex/93/019) and Dirección General de Asuntos del Personal Académico de la Universidad Nacional Autónoma de México (IN 208697). P. G. acknowledges The International Union Against Cancer for a sabbatical fellowship sponsored by Novartis. 2 These authors contributed equally to this work. 3 Present address: Instituto de Fisiologı́a Celular, Universidad Nacional Autónoma de México, Ciudad Universitaria, México, D.F. , México. 4 To whom requests for reprints should be addressed, at Departmento de Genética y Fisiologı́a Molecular, Instituto de Biotecnologı́a, Universidad Nacional Autónoma de México, AP 510-3, Cuernavaca, Mor. 62250, Mexico. Phone: 52-5-622-7636/7631; Fax: 52-73-17-2388; E-mail: covs@ibt.unam. mx. 5 The abbreviations used are: ORS, outer root sheath; HPV, human papillomavirus; K6, keratin 6; bK6, bovine K6; RT-PCR, reverse transcription-PCR; PCNA, proliferating cell nuclear antigen; FGF, fibroblast growth factor; HPRT, hypoxanthine phosphoribosyl transferase; DIG, digoxygenin. 527 Vol. 11, 527–539, October 2000 Cell Growth & Differentiation