Richard K. Assoian, PhD
Normal cells have a series of molecular controls
that prevents them from proliferating in the absence of essential environmental
cues. These environmental cues include the presence of mitogenic growth
factors and a suitable extracellular matrix (ECM). Cancer cells have lost
these controls and thus can divide in the absence of these extracellular
signals. We and others have examined how mitogenic growth factors and the
ECM cooperate to regulate a cell’s decision to proliferate and which of
these controls are typically lost in cancer cells. The results of these
studies have also provided insights into the functions of oncogenes and
transforming cytokines.
Our initial approach was to examine the effects of
growth factors and the ECM on the class of nuclear enzymes called G1
phase cyclin-dependent kinases (cdks) because these enzymes are now believed
to be responsible for mediating progression through the growth-regulatory
portion of the cell cycle. We found that each of the cell cycle events
required for cell cycle progression through G1 phase (eg, induction
of cyclin D1/PRAD1, phosphorylation of the retinoblastoma protein [pRB],
and induction of cyclin A) requires coordinated signaling by growth factors
and the ECM. Overall, these studies have allowed us to develop a model
showing how the extracellular environment controls proliferation in normal
cells.1-3 We then asked if this information might help to explain
some long-standing questions regarding the mechanisms by which certain
cytokines (eg, transforming growth factor [TGF]-beta) or oncogenes (eg,
ras) deregulate cell growth.
TGF-beta was originally identified and purified by
its ability to induce a transformed phenotype (anchorage-independent growth)
in normal cells. However, subsequent studies showed that this original
effect was restricted to a few isolated cell lines (eg, NRK cells) and
that TGF-beta is typically an inhibitor of cell growth. We have hypothesized
that the transforming effect of
TGF-beta in these isolated cell lines might be providing an important
clue about "multistep progression" to cancer. Specifically, we reasoned
that cells transformed by TGF-beta must have undergone a selective mutation
that, while nontransforming in itself, renders them susceptible to transformation
by TGF-beta. We therefore looked at the regulation of the G1
cyclins in NRK cells and found that these cells had lost their ECM requirement
for induction of cyclin D1. In fact, loss of anchorage-dependent cyclin
D1 expression is causal for transformation by TGF-beta because when we
force cyclin D1 expression in nontransformed cells, they become sensitive
to transformation by TGF-beta.
These studies have potential implications for cell
transformation by the oncogene ras. One of the critical targets
of activated ras is a cytoplasmic kinase called mitogen-activated
protein (MAP) kinase. When ras becomes activated (either in response
to growth factors and the ECM in normal cells or by point mutation in cancer
cells), MAP kinase becomes activated and induces the expression of cyclin
D1.4 If cyclin D1 is abnormally expressed, then pRB can be abnormally
phosphorylated and G1 phase becomes deregulated. Indeed, we
and others have shown that constitutive activation of MAP kinase or constitutive
expression of cyclin D1 induces, at least in part, the phenotype seen in
ras-transformed cells.
In summary, normal cells use growth factors and the
ECM to control the activation of ras and MAP kinase and hence the
expression of cyclin D1. Mutations that result in oncogenic ras
contribute to cell transformation, in part because they deregulate the
activation of MAP kinase and expression of cyclin D1. Full-cell transformation
requires the action of a second oncogene (eg, myc) or cytokine (eg,
TGF-beta) in order to complement the cyclin D1 signal and allow for maximal
pRB phosphorylation and cell cycle progression into S phase. Presumably,
this complementary signal will ultimately affect the cyclin-dependent kinase
machinery.
References
1. Zhu X, Ohtsubo M, Bohmer RM, et al. Adhesion-dependent cell cycle
progression linked to the expression of cyclin D1, activation of cyclin
E-cdk2, and phosphorylation of the retinoblastoma protein. J Cell Biol.
1996;133:391-403.
2. Guadagno TM, Ohtsubo M, Roberts JM, et al. A link between cyclin
A expression and adhesion-dependent cell cycle progression. Science.
1993;262:1572-1575.
3. Assoian RK. Anchorage-dependent cell cycle progression. J Cell
Biol. 1997;13:1-4.
4. Bottazzi ME, Assoian RK. The extracellular matrix and mitogenic growth
factors control G1 phase cyclins and cyclin-dependent kinase inhibitors.
Trends Cell Biol. 1997;7:348-352.