Enzyme gives fat cells room to grow
Collagen-clearer is necessary for adipocyte development -- but only in vivo
Without the activity of a proteinase that cuts away extracellular matrix
, fat cells in mice cannot develop into mature adipocytes
, according to a new study
. However, in two-dimensional culture -- the model used by most researchers studying fat cells -- the proteinase is not required, a previously unrecognized difference between adipocyte development in vitro
and in vivo
, the authors say.
Clearing excess collagen matrix ?makes space surrounding the cell, so that they can change cell shape and the cell can move or can expand and proliferate,? said study leader Tae-Hwa Chun
of the University of Michigan in Ann Arbor. These changes are likely necessary for cells to initiate gene expression that turns them into mature adipocytes, Chun said.
While studying mice with a deleted gene for a cell-surface metalloproteinase
called MT1-MMP, Chun and his colleagues noticed that the animals showed poorly developed white adipose tissue. Specifically, the knockouts' adipocytes were up to 10 times smaller in diameter than those of wild-type mice, and contained only small amounts of lipids. Using a microarray, Chun and his co-workers also found that adipose tissue from MT1-MMP null mice contained much lower levels of many gene products associated with lipid metabolism and adipogenesis.
However, when the researchers studied MT1-MMP knockout cells in culture, they found that they were indistinguishable from wild-type cells: When treated with factors that promote adipogenesis, both wild-type and MT1-MMP null cells differentiated, became spherical, incorporated lipid molecules, and expressed adipocyte markers.
?We thought the extracellular matrix environment might be the key? to the difference between two- and three-dimensional adipocyte development, Chun said -- partially because components of extracellular matrix are among the proteins
that MT1-MMP can degrade.
The idea that extracellular matrix may need to move out of the way to allow adipocyte differentiation ?follows really well from some other observations,? including studies
showing that few adipocytes tend to form in regions of dense connective tissue, said Gary Hausman
of the U.S. Department of Agriculture in Athens, Ga., who was not involved in the study.
Chun and his colleagues found that MT1-MMP knockout cells had a much higher density of collagen fibrils in the extracellular matrix than did wild-type adipocytes. When they then cultured cells in a three-dimensional collagen gel instead of on a flat dish, they saw the differences between wild-type and knockout cells: Knockout cells were shrunken, while wild-type adipocytes were spherical and lipid-rich. The wild-type adipocytes were able to organize an actin network and remodel the collagen matrix surrounding them -- allowing the cells to differentiate and expand -- while MT1-MMP knockout cells were unable to cleave collagen and remained trapped in a dense matrix.
Establishing the right cell shape and tension with the surrounding matrix may induce activation of transcription factors key to adipogeneis, Chun told The Scientist
, although ?we are not really sure yet? exactly how mechanical changes may affect gene expression.
Although the researchers don't have direct proof, it makes sense that mechanical cellular tension ?might influence several of those transcription factors, which in turn have control of adipogenic genes,? Hausman said.
Many researchers have not ?fully appreciated the role of this third-dimension in in vivo development of adipose tissue,? said Mitch Lazar
of the University of Pennsylvania in Philadelphia, who was not involved in the study. Most researchers in this field -- including his own lab -- have used models that are "inherently two-dimensional" when teasing out the factors required for making fat cells, Lazar noted.
Three-dimensional context may be key in other types of development, Lazar added, such as when cells stop proliferating when an organ has reached its adult size. Cells may determine this through ?some kind of collaboration between the information contained in the cells and the environment they're sensing in a three-dimensional sense.?
Melissa Lee Phillips
Links within this article
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