The Biology of Cancer - Vibrational Spectroscopy for Tissue Analysis - page 39

peanuts and other grains). The carcinogens in tobacco smoke (including
benzo (a) pyrene, dimethylnitrosamine, and nickel compounds) are the major
identified causes of human cancer [7].
Other carcinogens contribute to cancer development by stimulating
cell proliferation, rather than by inducing mutations. Such compounds
are referred to as tumour promoters . The phorbol esters that stimulate cell
proliferation by activating protein kinase C are classic examples. Some
hormones, such as estrogens, can be important tumour promoters in
the development of some human cancers (such as endometrial cancer in
women), as well [7].
In addition to chemicals and radiation, some viruses induce cancer both in
experimental animals and in humans. The common human cancers caused
by viruses include liver cancer and cervical carcinoma. Hepatitis B and C
viruses, papillomaviruses, adenomaviruses, herpesviruses, retroviruses,
and polioviruses are some instances of tumour viruses [7].
Mechanisms in Generation of Tumours
Tumours, as explained, result from disruption of the process that controls
the normal growth and the mortality of cells. This loss of normal control
mechanisms arises from the acquisition of mutations in three broad catego-
ries of cells:
1. Proto-oncogenes, the normal products of which are components of
signaling pathways that regulate proliferation and which in their
mutated form, become dominant oncogenes [39].
2. Tumour suppressor genes, which generally exhibit recessive behav-
iour, the loss of function of which leads to deregulated control of cell
cycle progression and inability to undergo apoptosis.
3. DNA repair genes, mutations in which promote genetic instability.
Oncogenes
The oncogene hypothesis was first proposed by Robert J. Huebner and
George J. Todaro [40]. The term oncogene itself is derived from the Greek
word “oncos,” meaning tumour [18]. Cancer results from alterations in
critical regulatory genes that control cell proliferation, differentiation, and
survival. Studies of tumour viruses revealed that specific genes (called
oncogenes ) are capable of inducing cell transformation. These genes were