TRANSFORMATION
OF CELLS
It’s the change in the
phenotype of a cell due to a new genetic material. As regards the culture
cells, transformation involves spontaneous or induced permanent phenotypic
alteration as a result of heritable changes in DNA and consequently genes
expression.
Transformation
of cells may occur due to any one of the following causes result in a changed
genetic material:
1.
Spontaneous
2.
Infection with
transforming virus
3.
From gene transfection
4.
Exposure to chemical
carcinogen’s
5.
Exposure to ionizing
radiations
Transformation is associated
with genetic instability, immortalization, aberrant growth controle and
malignancy
Genetic instability: In general, the cell line in culture is prone to genetic instability. A majority of normal finite cell lines are usually genetically unstable, and can get easily transformed. The continuous cell lines derived from tumors of all species are unstable. The normal occurring genetic variation in the cultured cells are due to the following causes:
1.
High rate of
spontaneous mutation in the invitro condition, possibly due to high rate of
cell proliferation.
2.
The continued presence
of mutant cells in the culture, as they are not normally eliminated.
Immortalization: The finite life span of cultured cells is regulated by about 10 senescence genes. Those dominantly acting genes synthesize products which inhibit the cell cycle progression. Its strongly believed that immortalization occurs due to inactivation of some of the cell cycle regulatory genes. E,g, Retinoblastoma, p53 genes.
Immortalization of cells
by viral genes: several viral genes can be used to immortalize cells. Some of
these genes are listed below:
·
SV40LT
·
HPV16E6/E7
·
hTRT
·
EBV
Among the above viral
genes, SV40LT is most commonly used to induce immortalization. The product of
this gene (T antigen) bind to senescence gene such as Rb and p53. This binding
restricts surveillance activity of senescence genes. The result is an increased
genomic instability and activity, leading to further mutations favoring
immortalization.
For the process of immortalization,
the cells are infected with retrovirus containing immortalizing gene before
they enter senescence. By this way, the life span of the cells can be extended
by 20-30 population doublings. Thereafter the cell cease to proliferate, and
enter a crisis phase that may lost for several months. At the end of the crisis
phase, a small portion of cells can be growing, and eventually become
immortalized.
Immortalization of cells
by telomerase induction: The most cause of finite life span of cells is due to
telomeric shortening, followed by cell death. If the cells are transfected with
telomerase gene htrt, the life span of the cells can be extended, and a small
portion of these cells become immortal.
Aberrant growth control: the transformed cells and the cells from tumors grown in culture show many aberration with respect to growth and its control. The growth characteristics of these cells are listed as:
Genetic characters’
|
Structural characters’
|
Growth characters’
|
Neoplastic characters’
|
Aneuploid
|
Altered cytoskeleton
|
Immortalized cells
|
Tumorigenic
|
Heteroploid
|
Changed extracellular
matrix
|
Loss of contact
inhibition
|
Invasive
|
High spontaneous
mutation rate.
|
Modified expression of
cells adhesion molecules
|
Anchorage independent
|
Increased protease
secretion
|
Over expressed
oncogenes
|
Disrupted cell polarity
|
Density limitation of
growth reduced
|
|
Mutated or deleted
suppressor genes
|
Growth factor independent
|
||
Low serum requirement
|
|||
Shorter population doubling
time
|
Anchorage independence
There occur several
changes on the cell surface of transformed cells. These include alteration in
the cell surface glycoprotein and integrins, and loss of fibronectin. Some of
the transformed cells may totally lack cell adhesion molecules. The modification
on the surface of transformed calls leads to a decrease in call-cell and
cell-substrate adhesion. The net result is that there is a reduced requirement
for attachment and spreading of the cells to proliferate. This phenomenon is
reffered to as “anchorage independence”
Anchorage independent
cells grow in a disorganized fashion. These calls may be comparable with the
tumor cell detached from the native tissue which can grow in foreign tissues,
e.g the spread of cancer from one part of body to another.
Contact inhibition:
The transformed cells are
characterized by loss of contact inhibition. This can be observed by the
morphological changes in the disoriented and disorganized monolayer cells. This
result in a reduced density limitation of growth, consequently leading to
higher saturation density compared to normal cells.
Low serum requirement:
In general, transformed
cells or tumor cells have lower serum dependence than the normal cells. This is
mostly due to the secretion of autocrine growth factors by the transformed
cells. Some growth factors produced by tumor cells are:
·
Colony stimulating
factor (CSF)
·
Transforming growth
factor (TGFa)
·
Interleukin I, II, and
III
·
Vasoactive intestinal
peptide (VIP)
·
Gastrin release
peptide.
Tumorigenicity:
Cell transformation is a
complex process that often results in the formation of neoplastic cells. The cell
lines obtained from malignant tumors are already transformed. Such cells may
undergo further transformation in the invitro culture due to
·
Increased growth rate
·
Immortalization
·
Reduced anchorage
dependence.
