Immortalized human skin epithelial cell lines provide useful models to study progressive
stages in human carcinogenesis. Alterations have been examined occurring with immortalization
and malignant progression of the human keratinocyte cell line HaCaT, which developed
spontaneously in a long-term culture from trunk skin keratinocytes. The cell line
has maintained many features of epidermal growth and differentiation in vitro and
has acquired clonogenicity. HaCaT cells exhibited a transformed phenotype (aneuploidy
and clonogenicity in soft agar) but remained non-tumorigenic. On transplantation they
formed normally structured and differentiating epithelia, but did not grow invasively.
Following transfection with the c-Ha-ras oncogene (EJ) randomly selected (G418-resistant)
clones exhibited different stages of tumour progression. They formed either (1) rapidly
regressing cysts, as seen with the parental line, (2) slowly growing benign tumours
or (3) progressively enlarging well differentiated carcinomas. Tumorigenic (benign
and malignant) clones had higher levels of mRNA expression and produced mutated p21.
However, no correlation existed between both parameters and malignant growth. Ras-transfected
clones showed improved morphological differentiation in vitro, in transplants, and
in tumours and expressed differentiation-specific keratins. Tumorigenic HaCaT-ras
clones were clonogenic in serum-free medium but had lost their ability to grow in
soft agar. Thus, c-Ha-ras oncogene expression initiated tumour progression in immortalized
human keratinocytes by altering growth regulation in vitro and in vivo, but per se
was insufficient for malignant transformation.