Revival: Biological Effects of Low Level Exposures to Chemical and Radiation (1992)

CHAPTER 6

Cellular Adaptation as an Important Response

During Chemical Carcinogenesis

Emmanuel Farber, Departments of Pathology and of Biochemistry,
University of Toronto, Ontario, Canada

INTRODUCTION

All living forms have evolved in a largely unfriendly, or even hostile,
environment in which protective responses to many different forms of
potential injury or harm have been essential for survival and reproduction.
The acquisition of mechanisms for many different adaptive responses could
be considered to be just as important as the development of the essential
pathways for the basic physiological needs, such as for energy and for the
syntheses of small and large molecules that are the essential components of
the pathways. It is widely recognized that a variety of physiological adaptive
responses to varying altitudes, other environmental influences, and hormo­
nal modulations, as well as aging, are fundamental properties of living
organisms. In addition, the vast array of different xenobiotic chemicals and
organisms as well as radiations to which all living forms have been exposed
since the early forms evolved some 2 to 3 billion years ago would require
highly versatile protective systems to be developed. Such protective or adap­
tive mechanisms are known to be present throughout the whole spectrum of
living forms —from single cell microorganisms to highly differentiated
eukaryotes.
Since disease processes are largely expressions of how living organisms
react and respond to perturbations in the external and internal environ­
ments, adaptive or protective responses and their modulations and mecha­
nisms are of the greatest concern in fundamental studies of disease patho­
genesis. Such considerations are also of the greatest relevance in toxicology,
including how living organisms respond to low levels of single and multiple
xenobiotics and radiations.
In the face of the many hazards, including carcinogenic ones, it seems
appropriate to ask the question — what mechanisms might have evolved to
preserve biological continuity? In species that have survived and endured

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