Oncogenes And Hormonal Functions

In general, the expression of two or more oncogenes is required for a cell to become cancerous. The oncogene products are functional parts of receptors, usually without the ligand-binding domain so that they function constitutively; they may be proteins involved in transduction processes or they are DNA-binding pro teins that could operate at the transcriptional level to cause the overproduction or suppression of specific genes. Thus, in many cancer cells tumor suppressor functions are repressed, and the signal transduction process, often starting with a mitogenic signaling receptor system, is permanently turned on so that the growth of the cell is out of control and is permanently signaled to divide. Moreover, other factors may be turned on, e.g., proteolytic functions that may allow the growing tumor to spread and become metastatic. Some oncogenes may be expressed that interfere with the normal process of cell death and render a cell susceptible to changes leading to the formation of a tumor cell. A new set of genes that stimulate programmed cell death or apoptosis may function as tumor suppressors.

Many of the receptor-related alterations occurring as a result of oncogene expression involve the growth factor receptors. Growth factors with oncogenic potential are listed in Table 20-1. It can be appreciated that this list is still growing. In addition to the growth factors that can form the basis for oncogene activity, the receptors for growth factors themselves can also form

TABLE 20-1 Growth Factors with Oncogenic Potential"

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