Abstract
That environmental factors are predominant in the causation of human cancer is now regarded as a fact. It has been variously estimated (1) that up to 80–90% of neoplasms in humans result from such factors. Diet and smoking probably account for the greatest proportion of these neoplasms, but infectious agents, industrial and occupational exposures, and ultraviolet radiation also play significant roles (2). Yet, with the exception of tobacco abuse as a direct causative factor in the development of bronchogenic and bladder carcinoma, the exact mechanism and proportion of cancers attributable to other agents is not clear. Furthermore, although endogenous hormonal factors play a role in the relationship of parity to breast cancer incidence (3) and possibly to ovarian cancer (4), endogenous factors in the causation of human cancer have only been hinted at. Henderson and his associates (5) have suggested that endogenous estrogens may play a significant role in the development of a number of human cancers. A role of endogenous prolacting in the genesis of mammary cancer in the human has not been proven (6), although this hormone is a significant factor in the genesis of mammary neoplasia in various other species (7). This brief review will consider the contribution of endogenous factors to carcinogenesis both in the human and in experimental animals, with special emphasis on the role of endogenous promoting agents in the development of both spontaneous or fortuitous cancer development, as well as on cancer induced by exogenous agents.
As seen from the above discussion, ample evidence indicates that endogenous promoting agents occur ubiquitously within mammalian organisms and probably within lower forms of life as well. The mechanisms of promotion by these various endogenous agents appear to differ somewhat among the several known classes (Table III).
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