INITIATOR

The term "initiator," when used in the field of carcinogenesis, refers to a chemical or physical agent or a condition, that is capable of beginning the process that leads to cancer. Early work in chemical and radiation carcinogenesis recognized that is was a multistep process. These steps were usually called initiation, promotion, and progression. Studies in mouse skin, as well as other organs and species, demonstrated that a single application of certain chemical carcinogens was capable of causing a skin tumor much later in the lifespan of the animal. These initiating chemicals had the common characteristics of being genotoxic and mutagenic.

Further, there were other chemicals, known as promoters, that if subsequently applied to the same area of the skin could lessen the time needed for the tumor to appear or decrease the dose of the initiator that was needed. Modern molecular biology has expanded the knowledge of carcinogenesis into recognition of the multiple steps that under-lie most human cancers, including mutations in the systems that control cell growth—so-called oncogenes and repressor genes. However, the basic concept remains that certain chemicals are capable of initiating the cancer-causing process.

Bernard D. Goldstein

(see also: Cancer; Carcinogen; Carcinogen Assessment Groups; Carcinogenesis; One-Hit Model )

Bibliography

Higginson, J. (1993). "Environmental Carcinogenesis." Cancer 72:971–977.

Weinberg, R. A. (1989). "Oncogenes, Antioncogenes, and the Molecular Bases of Multistep Carcinogenesis." Cancer Research 49:3713–3721.

initiator Transfer ribonucleic acid (t-RNA), that in eukaryotes carries methionine and in prokaryotes N-formylmethionine, which binds to the small unit of a ribosome bearing messenger ribonucleic acid (m-RNA), thus forming an initiation complex. This, in the presence of three protein initiation factors and GTP which is hydrolysed, enables the large ribosomal subunit to associate with the complex and peptide-chain synthesis to proceed.

initiator The transfer ribonucleic acid (tRNA) that in eukaryotes carries methionine and in prokaryotes N-formylmethionine, that binds to the small unit of a ribosome-bearing messenger ribonucleic acid (mRNA), thus forming an initiation complex. This, in the presence of 3 protein initiation factors and GTP, which is hydrolysed, enables the larger ribosomal subunit to associate with the complex and peptide-chain synthesis to proceed.