Why do epithelial cells arrest in response to serum?

Question

Primary epithelial cells, for example human mammary epithelium, fail to proliferate (arrest) in serum-containing medium. Therefore, a common growth medium for epithelium contains pituitary extract instead of serum (Hammond et al, 1984). This may be related to the fact that epithelium is normally not in contact with serum in the body.

On the other hand, many epithelium-like cell lines grow well in serum, and most tumor-derived cell lines are cultured this way. But it can be difficult to establish such lines from epithelial cancers: often, only fibroblasts grow out of tumor explants in serum-containing medium, while the (epithelial) cancer cells do not. In contrast to epithelium, fibroblasts increase their proliferation when exposed to serum, and this process is well studied (Iyer et al, 1999).

1. Why does epithelium arrest in response to serum? Are any mechanisms known? Is there any study of epithelium gene expression response to serum? Why is the behavior of epithelium so different from that of fibroblasts --- is there a physiological explanation, perhaps related to wound healing?

2. Is this true for all types of (human) epithelium?

3. Should we consider epithelial-like cells growing in serum to be adapted / selected? Have such cell lines then lost some part of the epithelial phenotype? Is this a serious artefact?

Any pointers to literature would be appreciated!

Answer 1

TGF-beta would be a good candidate. To cite: "TGF-β inhibits G1/S progression in a variety of eukaryotic cell types. Among these, untransformed epithelial cells are particularly sensitive to the growth inhibition by TGF-β." http://genesdev.cshlp.org/content/14/24/3093.full

Fetal bovine serum (FBS) contains a high level of latent TGF-β. Human serum as well has high level of TGF-beta (20-50ng/ml)

It is also possible that growth arrest is part of the induction of terminal differentiation by serum factor(s) such as TGFb. "Type beta transforming growth factor is the primary differentiation-inducing serum factor for normal human bronchial epithelial cells." http://www.ncbi.nlm.nih.gov/pubmed/2871553

Edit: got more reputation, so now I can post more than two links. Here is a review on TGFb signalling: http://genesdev.cshlp.org/content/14/6/627.full

Also a link to serum levels of TGFb: http://www.ncbi.nlm.nih.gov/pubmed/16845225

Answer 2

I don't think that there is a one-size-fits-all answer to this question. There are different types of arrest and there might be different triggers depending on the epithelial cell type.

Even in the cited Hammond et al paper, note that the effect of serum is not an immediate arrest. Human mammary epithelial cells (HMEC) could already go through 3 to 4 passages in serum (at 1:10 splits), according to the Introduction to the paper; the improvement with the serum-free medium was to allow 10-20 passages. And even in that paper, near the end of the Results you will see a difference between HMEC and human keratinocytes in terms of response to calcium; calcium makes keratinocytes differentiate and lose proliferative capacity, while HMEC do best at high calcium.

Recent work has developed ways that can allow many types of epithelial cells (both normal and tumor cells) to proliferate indefinitely in culture, in the presence of 5% serum. Inhibition of the Rho-associated kinase ROCK is critical, as is the production of some yet-undefined factor by feeder fibroblasts. So even if some component of serum tends to lead to differentiation or senescence and growth arrest, that effect can be avoided.

Note, however, that the authors of this newer work do call the cells treated this way "reprogrammed," recognizing that they are not the same as the original epithelial cells in some important ways. What that "reprogramming" entails is under active investigation, and your concerns about selection, adaptation, and potential artifacts of culture are very important. But even if this is considered an artifact, understanding the mechanisms at work should produce some important advances in understanding epithelial biology.