PMC:5608921 / 14626-19845 JSONTXT

{"project":"2_test","denotations":[{"id":"28846079-24018418-79081919","span":{"begin":359,"end":361},"obj":"24018418"},{"id":"28846079-19209183-79081920","span":{"begin":363,"end":365},"obj":"19209183"},{"id":"28846079-8610145-79081921","span":{"begin":508,"end":510},"obj":"8610145"},{"id":"28846079-15699322-79081922","span":{"begin":512,"end":514},"obj":"15699322"},{"id":"28846079-21502497-79081923","span":{"begin":516,"end":518},"obj":"21502497"},{"id":"28846079-19741607-79081924","span":{"begin":646,"end":647},"obj":"19741607"},{"id":"28846079-20804975-79081925","span":{"begin":649,"end":650},"obj":"20804975"},{"id":"28846079-16597322-79081926","span":{"begin":867,"end":869},"obj":"16597322"},{"id":"28846079-16597322-79081927","span":{"begin":948,"end":950},"obj":"16597322"},{"id":"28846079-8610145-79081928","span":{"begin":1739,"end":1741},"obj":"8610145"},{"id":"28846079-15699322-79081929","span":{"begin":1743,"end":1745},"obj":"15699322"},{"id":"28846079-8390671-79081930","span":{"begin":2087,"end":2089},"obj":"8390671"},{"id":"28846079-11063126-79081931","span":{"begin":2229,"end":2231},"obj":"11063126"},{"id":"28846079-9790766-79081932","span":{"begin":2789,"end":2791},"obj":"9790766"},{"id":"28846079-10550545-79081933","span":{"begin":2793,"end":2795},"obj":"10550545"},{"id":"28846079-18548112-79081934","span":{"begin":3043,"end":3045},"obj":"18548112"},{"id":"28846079-7969162-79081935","span":{"begin":3385,"end":3387},"obj":"7969162"},{"id":"28846079-16282489-79081936","span":{"begin":3389,"end":3391},"obj":"16282489"},{"id":"28846079-24983759-79081937","span":{"begin":4169,"end":4171},"obj":"24983759"},{"id":"28846079-11390614-79081938","span":{"begin":4471,"end":4473},"obj":"11390614"},{"id":"28846079-11063126-79081939","span":{"begin":4778,"end":4780},"obj":"11063126"}],"text":"Contribution of suprabasal epithelial cells to carcinogenesis and regeneration\nIn stratified epithelia, the proliferative compartment is restricted to the basal layer. Here, cycling stem cells and progenitors live together and derive into the non-dividing terminally differentiated cells of suprabasal layers which mature up to producing the cornified tissue.13, 23 Commonly, hyperplasia and dysplasia in skin and cervix are characterized by a high number of proliferating cells located in suprabasal layers.19, 20, 24 The recurrent hypothesis to this phenotype is the invasion of basal proliferating cells into upper layers. However, recent data5, 6 open alternative hypotheses that need to be considered to trace the origin of cancer-initiating cells.\nIt has been proposed that a productive cycle of HPV in the cervix starts with the infection of basal layer cells.16 Upon differentiation, suprabasal cells start to express the early viral genes.16 Therefore, the primary cells for E6/E7 action are suprabasal cells which, in response, re-enter into the cell cycle. Tg(K6b-E6/E7) mice reproduced this pattern in the cervix, and accordingly, cell proliferation was promoted in suprabasal layers. This function is active in the growing phases of the estrous cycle when K6b was detected to be highly expressed. Since suprabasal E6/E7 expression did not affect the number of proliferating basal cells, few of these cells contributed to the suprabasal proliferation pattern observed. Therefore, suprabasal cells are relevant candidates to carry the role of cancer-initiating cells in the cervix infected with HPV. In contrast, although Tg(K14-E6/E7) mice can reproduce many aspects of cervical–uterine cancer when chronically treated with E2,19, 20 the compelled premise for the search of cancer-initiating cells in these mice is that they are stem/progenitor cells of the basal layer. Therefore, these mice unlikely model the initiation of cervical carcinogenesis in humans infected with HPV as, possibly, Tg(K6b-E6/E7) mice do.\nIt is interesting that, in contrast with Tg(K14-E6/E7) mice,25 the interfollicular epidermis of Tg(K6b-E6/E7) mice is not affected and skin alterations are restricted to the hair follicle renewal cycle.26 This is expected because K6b is expressed in the companion layer of the inner root sheath of hair follicles and regulated during the renewal cycle. In contrast, upon ear punch injury, a marked increase in K6b and K16 expression was observed that correlates with the increased oncogene expression that, in the ear regenerating epidermal area with well-organized basal layer correlated with the restricted presence of K6b in suprabasal layers. This is similar to the growing cervical epithelium (Figure 1c) and the stimulated growing epidermis without injury.27, 28 The disorganized epidermis commonly seen in the growing area of regenerating ears is possibly due to the high contribution of migratory cells from the wound borders and a delay in the reconstruction of the basal layer at early regeneration stages.21 In agreement with our observations, the epidermis of Tg(K10-E6/E7) mice, with expected constitutive limited oncogene expression in suprabasal layers, showed hyperplasia/hyperkeratosis and dividing cells in suprabasal layers under untreated conditions but, in contrast, a marked number of dividing cells were also increased in basal layers.29, 30 This latter effect might be a long-term consequence of the constant re-entering into the cell cycle of suprabasal cells (see below), since neither hyperplasia in the interfollicular epidermis under untreated conditions nor increased basal cell proliferation were observed in Tg(K6b-E6/E7) mice.\nAn interesting observation was the large area of coincidence of K5 and K6b expression and the location of most dividing cells in the growing epithelial tissue. In particular, we found that the zone of coincidence of K6b and K5 expression in the cervical epithelial tissue correlates with the area where dividing cells in suprabasal layers were present. Accordingly, extension of K5 expression to upper layers has also been observed in organotypic raft cultures transfected with HPV.31 Therefore, K6b+/K5+ cells in the cervix might identify those susceptible for oncogene-induced cell cycle re-entry. Although we detected the highest amount of E7 in upper suprabasal layers, it has been reported that high levels of p21/p27 make differentiated cells refractory to E7 mitogenic effects.32\nIn Tg(K6b-E6/E7) mice, longer duration of the growing phases is the likely cause of the thicker cervical epithelium during proestrus–estrus phase. This is similar to the effect of E6/E7 on hair follicles, which also show higher cycling frequency and a longer growing phase (anagen) than those of WT mice.26 We interpreted this phenomenon as oncogenes preventing from the entry of stem/progenitor cells of the bulge into the G0 cell cycle phase, expected for resting hair follicles (telogen). An alternative interpretation accordingto the present study is that E6/E7 drives the re-entry into the cell cycle of postmitotic K6b+ cells causing hair follicle growth from these cells instead of the regular stem cells that receive the resting signals."}