Fibroblast Phenotype Plasticity: Relevance for Understanding Heterogeneity in "Fibroblastic" Tumors
Christie Hospital NHS Trust
In the tumors or tumor-like lesions which we instinctively regard as fibroblastic, there is, as in
other tumor groups, a wide range of cellular differentiation, which we can explain or rationalise in
terms of phenotypic plasticity of the "normal" fibroblast. In this process, differentiation can be
altered by molecules of external origin, which interact with surface receptors, and induce cascades of
molecular interactions: these eventually activate gene expression, lead to translation of mRNA into
proteins, organization of proteins into supramolecular complexes (organelles), and hence ultimately to
different cellular appearances, i.e., phenotypes.
In this paper, the various kinds of fibroblast transformation are discussed, and some insights
provided into the molecular mechanisms driving these transdifferentiation processes. Clearly, comparable
molecular events may be taking place in abnormal or neoplastic fibroblasts to produce the heterogeneous
tumors, which we nevertheless identify as fibroblastic. The objective of this paper, therefore, is to
provide a basis for understanding the diverse phenotypes expressed by fibroblastic tumors or lesions.
The most studied transformation - that of the fibroblast to the myofibroblast - will be emphasized,
although other examples of transdifferentiation of relevance to fibroblastic tumors will be mentioned.
In comparing the differentiation of fibroblasts vis-à-vis their neoplastic counterparts, the following
broad categories come to mind:
| "pure" fibroblastic differentiation ® "pure" fibroblastic tumors|
| myofibroblastic transdifferentiation ® myofibroblastic tumors|
| fibroblasts transforming into histiocytes ® fibrohistiocytic tumors|
| fibroblasts undergoing adipocytic differentiation ® lipogenic tumors.|
Lipogenic and fibrohistiocytic differentiation in relation to fibroblasts
Definition of the fibroblast
Fibroblasts are spindled cells with a
cytoplasm dominated by rough endoplasmic reticulum (rER). In addition, they have subplasmalemmal
, a big Golgi apparatus, and sometimes collagen secretion granules are seen , but no
myofilaments or lamina.
Fibroblasts, lipogenesis and liposarcoma
A number of studies have
suggested that the fibroblast is a stem cell for adipocytes
. This fibroblast itself may derive
from a less differentiated mesenchymal cell. In white adipose tissue, Napolitano  has shown initial
lipid synthesis in spindled fibroblastic cells, which then increase their lipogenic activity, develop a
more rounded cell morphology and elaborate a lamina. Such an origin for the adipocyte would explain why
some spindle-cell lipogenic tumors, such as spindle-cell liposarcoma, retain some morphological
reminiscences of a fibroblastic precursor stem cell and may even resemble fibrosarcoma.
Clues to the molecular mechanisms by which fibroblasts switch to lipogenic activity in the normal and
neoplastic state are beginning to emerge. For example, it has been suggested that human fibroblast
subsets exist, defined by presence or absence of the thy-1 receptor. Those possessing the receptor are
directed towards a myofibroblastic phenotype (discussed below), while thy-1-negative fibroblasts
exclusively develop a lipofibroblastic phenotype . Liposarcoma has been shown to be initiated by a
specific protein domain within a fusion gene product (CHOP) resulting from the most common chromosomal
translocation in myxoid liposarcoma, t(12;16)(q13;p11) . CHOP has also been found to prevent
adipocyte differentiation , an observation which goes at least some way to explaining why so many of
the more spindled cells in myxoid liposarcoma show minimal lipid synthesis.
The fibroblast-histiocyte relationship
The overlapping features of
monocytes or macrophages, on the one hand, and fibroblasts, on the other , and the possibility that
they could transdifferentiate one to another
has been recognized for some time. Examples
|Endometrial fibroblasts producing collagen in the proliferative phase are the same cells which then phagocytose collagen in the premenstrual phase
|The pericryptal fibroblast of the colonic mucosa develops morphological and histochemical features of macrophages as it migrates to subtend the free surface epithelium
|Corneal stromal fibroblasts behave as macrophages when presented with colloidal material and synthesise acid hydrolases for intracellular digestion
|Skin fibroblasts can be converted to tissue histiocytes by Snyder-Theilen feline sarcoma virus
|Blood monocytes have been reported to transform into fibroblasts in vitro
|Macrophages have been observed to transdifferentiate into fibroblasts as a result of Schistosoma mansoni infection
These examples of fibroblasts and macrophages interconverting into one another are reflected in the
day-to-day experience of ultrastructurally orientated pathologists who often see at least modest levels
of phagocytic activity in a wide variety of fibroblastic cells and lesions. These range from ingested
melanin in dermal fibroblasts to the co-expression of lysosomes and rER in the fibrohistiocytic (or
histiofibroblastic ) cells of such tumors as malignant fibrous histiocytoma. The basic mechanism by
which a fibroblast or primitive mesenchymal cell might differentiate towards a macrophage is far from
understood. However, we know of a number of cell surface and other proteins which characterize
macrophages - carboxypeptidase M  and the protein product of the CHI3L1 gene , for example - and
we know of an increasing number of molecules which appear to promote macrophage differentiation from
precursor cells, such as 14-membered macrolide compounds  and Imatinib . These data in normal
cells could form the basis for understanding the development of fibrohistiocytic differentiation in
Definition of the myofibroblast
The main features of the myofibroblast
vimentin and a-smooth-muscle actin (aSMA) immunostaining (as well as desmin in certain lesional
myofibroblasts), and an ultrastructure based largely on prominent rough endoplasmic reticulum, sparse
peripheral bundles of myofilaments with focal densities, and fibronexus junctions
Induction of the myofibroblast phenotype by TGFb
One of the principal
differences between fibroblasts and myofibroblasts is the absence of aSMA and myofilaments in fibroblasts
and their presence in the myofibroblast. Results from a variety of sources suggest that the primary
mechanism for the de novo synthesis ofaSMA requires the combined action of growth factors,
principally transforming growth factor-b (TGFb) and platelet-derived growth factor, matrix molecules such
as cellular fibronectin, and mechanical stress
. TGFb can be produced by malignant cells, and
can target receptors on tumor stromal fibroblasts, which then transdifferentiate into early
myofibroblasts by virtue of aSMA synthesis
. Studies of TGFb in fibroblastic and myofibroblastic
lesions are so far limited , and it remains to be seen from future research whether TGFb, as might
be expected, is present in developing myofibroblastic lesions, and minimally expressed or absent in
purely fibroblastic tumors such as giant-cell fibroblastoma.
Since all biological systems show variation and a spectrum of appearances, we cannot expect there to
be a rigid distinction between fibroblastic and myofibroblastic lesions. This is illustrated by
fibroblasts grown in vitro, in basal cell carcinoma stroma, and in adult fibrosarcoma tumor
cells. Basal cell carcinoma possesses a stroma, which, although exhibiting aSMA-staining , contains
less of the fully differentiated myofibroblasts typically seen in squamous cell carcinoma (Brian Eyden,
unpublished observation). Instead, one may see cells, which are all but classical fibroblasts, with
abundant rER, subplasmalemmal linear densities, a big Golgi apparatus and collagen secretion granules.
Closer scrutiny of some of these cells, though, reveals very modestly developed bundles of
subplasmalemmal actin filaments with small numbers of focal densities. Although these cells are
predominantly fibroblasts, they are nevertheless showing the very earliest signs of actin synthesis and
myofilament elaboration - the earliest indications, in short, of myofibroblastic differentiation. On the
basis of our knowledge of the effects of TGFb, we might perhaps expect this growth factor to be
instrumental in the conversion of stromal fibroblasts to these very early "myofibroblasts", but also
perhaps TGFb to be much more upregulated in the stroma of squamous cell carcinoma, where numerous
fibronexus-bearing myofibroblasts are present
Similarly, in fibrosarcoma, the classical herringbone pattern seen in histological sections and the
poor staining for aSMA, might predict a purely fibroblastic ultrastructure. Fibrosarcoma does, as
expected, have abundant rER, but frequently also there are modest numbers of peripheral myofilaments
even a primitive attempt at fibronexus-formation has been noted . In such tumors, one might
expect low levels of TGFb, indicating a low level of activation of the myofibroblastic phenotype. Future
work is needed to confirm these ideas.
The development of aSMA or myofilaments as seen in the myofibroblast is a widespread subcellular
reaction in pathology being found in a very wide range of normal, reactive and neoplastic cells in
vitro and in vivo . This de novo synthesis of aSMA or myofilaments appears to be a
common expression of a pathological state, produced by external trauma (such as wounding in vivo
or cell cultivation in vitro) or inherent abnormality such as malignant transformation. This
widespread distribution could be explained by the equally widespread presence of TGFb. It provides
something of an explanation as to why we so often see modestly developed myofilaments or low levels of
aSMA in the course of our routine diagnostic work, including, for example, otherwise unambiguously
non-smooth-muscle tumors such as chondromyxoid fibroma
, chondroblastoma , osteosarcoma
and rhabdomyosarcoma .
This paper describes some of the principal forms of differentiation exhibited by
"pure" and transforming fibroblasts as a means of explaining the phenotypic variation found in
fibroblastic tumors. Some of the molecular mechanisms driving the differentiation processes are
discussed. While we are beginning to identify some of the characteristic molecules on the surfaces of
fibroblasts and their transformed variants, we are far from understanding a number of features which bear
on the subject of the diverse phenotypes encountered in fibroblastic tumors. We know hardly anything of
the mechanism by which cellular growth patterns are generated, such as the herringbone or the storiform
growth pattern in fibrosarcoma and fibrohistiocytic tumors respectively; or, of the mechanism directing
and controlling nuclear shape, which might explain why a cell produces a spindled nucleus characteristic
of a fibroblast or a reniform nucleus typical of a macrophage, still less the characteristic longitudinal
nuclear fissures of, for example, Dermatofibrosarcoma protuberans, all of which features are important to
the tumor pathologist. Investigations into the mechanisms of differentiation in normal fibroblasts could
prove fertile ground for defining comparable differentiation in tumors.
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