Anchorage dependence of growth

Anchorage dependence of growth - the role of the cytoskeleton

Page updated 29/1/03

Most metazoan cell type require a surface on which to flatten out and divide, even if the final stage (cytokinesis) is to all but loose contact with it. This is anchorage dependence of growth, a control to cell division that many transformed cells loose. The ability to grow on "soft agar" is a routine test taken as an indication that cells with this ability are anchorage independent (see below). Anchorage-independence correlates strongly with tumourogenicity and invasiveness in several cell types, such as small-cell lung carcinoma (Carney et al, 1980). Many types of normal cells are programmed to undergo apoptosis if they are prevented from contacting other cells. Anchorage dependence is at least a two part phenomenon.

One early hypothesis to explain the phenomenon was based on the suggestion that growth factor receptors are exposed maximally on the cell surface when the surface is flat (A), this can only take place on the surface of flattened cells. (B). Cells in suspension produce hundreds of villi and other folds so that the membrane area can be accommodated. Many of the growth factor receptors are hidden within these folds and so may not have the opportunity to bind the growth factors. In agreement with this suggestion, some anchorage independent cell lines expressed increased numbers of growth factor receptors, while others express growth factor receptors that are mutated so that they seem bound by growth factors constantly. All in line with a role of growth factor receptors in anchorage dependence. Another possible or partial explanation is the signalling that is known to occur between the cell surface and the nucleus namely the Wnt pathway.

Asbestos fibres that gain entry into the lung surface offer a surface onto which cells may attach. Cells can then grow on these fibres until the surface is covered perhaps with some over hang. This situation encourages the growth of cells that are selected for their ability to grow in a more rounded shape as there may be no neighbouring cells to adhere to. Eventually, cells may grow that no longer are limited by a requirement for a surface on which to grow. Having cells around to make adhesions to is an oncogenic influence.

Some light is beginning to be shed on the signalling pathways involved in anchorage dependence and it seems that PKA, PAK and the MAP kinases play important roles (Howe & Juliano, 2003).

Protein Name	Affect on Anchorage Dependence	References

EPLIN		Song et al, 2002
Tropomyosin		Boyd et al, 1996

Table 1. Cytoskeletal Proteins that Affect Anchorage Dependence

Soft Agar Assay for Anchorage independence

0.3% agarose in media is layered over a firmer base consisting of 0.5% agarose in media. Cells that can grow and form colonies on the soft-agarose are generally held to exhibit anchorage independence since the soft-agarose afford only moderate opportunity to adhere and spread.

References:-

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Boyd, J., Risinger, J. L., Wiseman, R. W., Merrick, B. A., Selkirk, J. K. & Barrett, J. C. (1996) Regulation of microfilament organization and anchorage-independent growth by tropomyosin. PNAS. 92, 11534-11538.

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Carney, D. N., Gazdar, A. F. & Minna, J. D. (1980) Positive correlation between histogical tumor involvement and generation of tumour cell colonies in agarose in speciments taken directly from patients with small-cell carcinoma of the lung. Cancer Res. 40, 1820-1823.

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