material of a cell. The order, or sequence, of DNA bases is a
key determinant of what proteins are produced by a cell and
how much of each protein is produced. Many different types
of mutation contribute to cancer initiation and development,
primarily by altering the amount or function of certain
proteins (see sidebar on Genetic Mutations, p. 20).
In addition to genetic mutations, most cancer cells also
have profound epigenetic abnormalities, compared with
normal cells of the same tissue. In many cases, epigenetic
alterations and genetic mutations work together to
promote cancer development. Although genetic mutations
are permanent, some epigenetic abnormalities appear to
be reversible, and harnessing this discovery for therapeutic
purposes is an area of intensive investigation.
Genetic mutations that disrupt the orderly processes
controlling the multiplication and life span of normal cells
are the main cause of cancer initiation and development.
However, interactions between cancer cells and their
environment—known as the tumor microenvironment—
as well as interactions with systemic factors, also have
an important role in cancer development (see sidebar
on Cancer Growth: Local and Global Influences).
In fact, cancer cells often exploit tumor microenvironment
components to promote their multiplication and survival.
CANCER GROWTH: LOCAL AND GLOBAL INFLUENCES
Solid tumors are much more complex than an isolated mass of proliferating cancer cells because cancer
initiation, development, and progression are strongly influenced by interactions among cancer cells and
numerous factors in their environment. Among the components of the tumor microenvironment are normal
parts of the tissue in which the cancer is growing, systemic factors that transiently percolate through the
tissue, and cells that are actively recruited to the tissue.
The matrix of proteins that surrounds the cancer cells can influence
cancer formation, metastasis, and other processes.
Cancer cells can stimulate the growth of blood and
lymphatic vessel networks, which supply the cancer cells
with the nutrients and oxygen required for rapid growth
and survival, and provide a route for cancer cell escape
to distant sites (metastasis).
Systemic factors in the circulation, such as hormones and nutrients,
influence the development and growth of cancer.
The immune system can identify and eliminate cancer cells,
although in many cases this system is suppressed, permitting
the formation and progression of a tumor. In some situations
of chronic inflammation, however, the immune system can
promote cancer development and progression.
Adapted from ( 1)