Lewis J. Kleinsmith

Summary In order to test the hypothesis that embryonal carcinoma cells are multipotential stem cells of a teratocarcinoma, an in vivo cloning technic was designed. Small embryoid bodies containing mostly embryonal carcinoma were obtained from ascitic conversion of a murine teratocarcinoma and were dissociated with trypsin to form a suspension of single cells; the single cells were picked up in small capillary tubes and transplanted directly into mice. From over 1700 single cell grafts, 44 clonal lines were obtained; 43 of these lines were teratocarcinomas composed of as many as fourteen well differentiated somatic tissues in addition to embryonal carcinoma. These 43 lines varied in their degree of differentiation, capacity to produce embryoid bodies, and in growth rate. The remaining clonal line showed limited potential for differentiation, producing only yolk sac, trophoblast, and embryonal carcinoma. The results demonstrated the multipotentiality of single embryonal carcinoma cells, as well as the heterogeneity of the embryonal carcinoma of a teratocarcinoma. The capacity of single embryonal carcinoma cells to differentiate into benign tissues supports neither the dogma of the irreversibility of the malignant transformation nor the somatic cell mutation theory of cancer. These findings were interpreted as giving strong support to the stem cell theory of cancer.

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Evidence from several model systems suggests that nonhistone chromosomal proteins may regulate gene expression in eukaryotic cells. The data indicate that the synthesis of new species of nonhistone chromosomal proteins as well as modifications of preexisting nonhistone chromosomal proteins are involved in the control of transcription. However, from the vast number of proteins included in this class, it is apparent that, in addition to regulating the transcription of defined genome loci, the nonhistone chromosomal proteins include enzymes that have a general function, proteins that are involved in determining the structure of chromatin, as well as proteins that serve as recognition sites for binding of regulatory macromolecules. The presence of a nucleoplasmic pool of nonhistone chromosomal proteins which may exchange with the chromatin has also been reported (89). While it is clear that the nonhistone chromosomal proteins play a key role in the regulation of gene expression, the exact manner in which they interact with the genome to initiate, modify, or augment the transcription of specific RNA molecules remains to be resolved.

Human lymphocytes treated with phytohemagglutinin undergo extensive gene activation, as evidenced by augmented synthesis of ribonucleic acids. This activation is preceded by an early stimulation in the rate of phosphorylation and dephosphorylation of nuclear proteins. This finding is consistent with a hypothesized role of phosphoproteins in the modification of chromatin structure and in modulation of the template activity of DNA in vivo.

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