March 2006
Volume 5
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March 2006
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Susceptibility for Malignant Conversion Resides in the Target Cells
Figure 1. The high frequency of malignant conversion in FVB/N mice is demonstrated in vivo, in vitro, and in keratinocyte grafts. In vivo: After induction of skin tumors by an initiation-promotion protocol in FVB/N, BALB/c, and C57BL/6 mice, the conversion of papillomas to squamous cell carcinomas was compared in the mouse strains. In vitro: Primary skin keratinocytes isolated from each of the strains were used in a colony assay in cell culture to assess the frequency of carcinogen-induced progression to malignancy. Grafts: Human papillomavirus (HPV) type 16–immortalized keratinocyte cell lines from each mouse strain were grafted to the dorsal skin of athymic nude mice, and the frequency of carcinoma development in grafts was compared. To address this question, we isolated skin keratinocytes from each of the above strains and used them in an in vitro colony assay to assess the frequency of carcinogen-induced progression to malignancy of genetically initiated keratinocytes. FVB/N keratinocytes were 10 times more sensitive to chemically induced malignant conversion than keratinocytes from other strains, consistent with their known sensitivity for premalignant progression in vivo. Furthermore, the high frequency of conversion in FVB/N keratinocytes in vitro was suppressed in keratinocytes that were derived from the offspring of a cross between C57BL/6 and FVB/N mice, indicating that this susceptibility trait is not dominant. Keratinocytes from each strain were infected with retroviruses containing HPV-16 E6 and E7 oncogenes to compare progression to immortal foci. FVB/N keratinocytes could be immortalized by E6 or E7 proteins as easily as keratinocytes from other strains. When immortal cells were grafted to the dorsal skin of nude mice to examine tumor development in a common genetic background, HPV-16–immortalized FVB/N keratinocytes formed carcinomas in the grafts more frequently (50%) than keratinocytes from SENCARA/Pt (14%), BALB/c (1.9%), or C57BL/6 (2.5%). Although tumor stromal cells and inflammatory cells can contribute to the tumor phenotype in several cancer models, normal dermal fibroblasts from either sensitive or resistant strains did not influence the tumor outcome in this model. In addition, the frequency of immortalization of keratinocytes among the strains did not correlate with susceptibility to malignant conversion. Thus, immortalization is not the rate-limiting step in this model. To determine whether the sensitivity of the FVB/N genotype for malignant conversion is a dominant trait, FVB/N females were crossed with SENCARA/Pt males, and skin tumors were induced in the offspring by an initiation-promotion protocol. Enhanced premalignant progression and malignant conversion in the FVB/N strain were suppressed by the SENCARA/Pt genotype, a finding consistent with the suppression detected in the F1 cross between C57BL/6 and FVB/N in the in vitro conversion assay. Thus, the frequency of malignant conversion of papillomas in F1 hybrids of FVB/N and SENCARA/Pt crosses reflects the frequency in the SENCARA/Pt parent. Our results show that keratinocytes from FVB/N mice are significantly more susceptible to malignant progression than keratinocytes from other strains, and that this is not a dominant characteristic. Although our studies have not yet revealed the precise factors responsible for the intrinsic sensitivity of FVB/N keratinocytes to premalignant progression, they do provide some clues and a foundation for further analysis at the cellular level. Using keratinocytes from inbred strains that vary substantially in susceptibility and retain this characteristic in vitro should be helpful in defining the underlying basis for these susceptibility differences. |
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induction of skin tumors on mice from different genetic backgrounds provides
a valuable model for studying the genetics of cancer susceptibility or resistance.
Skin tumors are superficial, multiple, and display reproducible phenotypic markers
as they progress to malignancy. A number of inbred mouse strains display characteristic
high (SENCARA/Pt) or low (BALB/c, C57BL/6) susceptibility to skin tumor induction.
Mice of the FVB/N strain have been widely used for carcinogenesis studies and
for the development of transgenic mice. An important feature is their susceptibility
to carcinogenesis at several organ sites including skin, lung, colon, and mammary
glands. Studies of multistage chemical carcinogenesis in skin have shown that
FVB/N mice are moderately susceptible to induction of benign papillomas, but
the papillomas are unusually prone to undergo malignant conversion to carcinomas.
In addition, transgenic mice that express human papillomavirus (HPV) type 16
E6 and E7 oncogenes in skin rapidly develop squamous carcinomas
in the FVB/N strain, but develop only a few benign tumors when tested in BALB/c,
SENCARA/Pt, and C57BL/6 mice. An important question is whether this unusual
propensity for rapid and high-frequency malignant progression in the FVB/N phenotype
is determined by systemic factors or whether it is an inherent property of the
keratinocyte (