Molecular and Cell Biological Differentiation Capabilities on Human Germ Cell Tumor Cells

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NCR-G2 and G3 cells were the human embryonal carcinoma (EC) cells established from testicular mixed embryonal carcinomas. G3 cells were capable of differentiation towards somatic cell lineage together with trophoectoderm cell lineage when they were exposed to retinoic acid or N,N^1-hexamethylene-bis-acetamide (HMBA). In particular, induction of human chorionic gonadotropin (hCG) production in retinoic acid-treated G3 cells was most characteristic and was closely related to the duration of retinoic acid treatment. Although G2 cells did not show any differentiation with retinoic acid treatment, expression of a variety of cytoskeletal proteins became evident with HMBA treatment at both protein and mRNA levels. These findings suggest that some human EC cells that do not react with retinoic acid contain differentiation antigens that are inducible by other agents such as HMBA. In order to isolate genes responsible for the early stage differentiation of human EC cells by using subtracted hybridization method, we prepared a cDNA library from retinoic acid-treated G3 cells. This cDNA library was screened for genes that exhibit an induction in expression during differentiation of these cells. From 5 X 10^4 clones screened, three independent sequences were isolated. Clone1002 is an unknown sequence at this moment and clone 0734 codes for line-1, a member of the repetitive sequence family. On the other hand, clone 2403 was found to code for a 90-kD b heat shock protein (HSP90). The expression of HSP90 was up-regulated in a retinoic acid exposure time-dependent fashion. When G3 cells were cultivated at 42 C, the expression of HSP90 was up-regulated and began to produce hCG at both mRNA and protein levels. Thus, HSP90 may play an important role in human EC cell differentiation. The molecular mechanism of HSP90 and human EC cell differentiation is now under investigation.