Investigating cell signaling

Philadelphia - Health and illness are rooted in biological processes of construction and deconstruction.

Cell signaling molecules in the Src family of tyrosine kinases, which are important regulators of epithelial cell growth and differentiation, are an example of the complexity of the picture.

Dermatopathologist John T. Seykora, M.D., Ph.D., has focused his research at the University of Pennsylvania, Philadelphia, on the Src family of tyrosine kinases. Disrupting the normal function of Src family kinases is associated with a majority of human cancers.

Dr. Seykora's laboratory discovered a novel regulator of Src family tyrosine kinases called Srcasm (Src-activating and signaling molecule).

His laboratory has subsequently studied Srcasm and shown the levels of Srcasm are decreased in cutaneous squamous cell carcinoma (SCC).

"In tumors of the skin, the Src kinase levels are up, the Srcasm levels are down," Dr. Seykora tells Dermatology Times.

Using transgenic mice to manipulate levels of the Src family member Fyn, Dr. Seykora found that increasing the expression of Fyn leads to "very thick and scaly skin with increased proliferation of keratinocytes. The skin of this mouse mimics hyperproliferative diseases like squamous cell carcinoma in situ.

"When we made a mouse that expressed increased levels of Srcasm in the skin, we corrected that phenotype. However, if we crossed the Fyn mouse with a mouse that expressed increased levels of a non-functional Srcasm, which cannot be phosphorylated by Fyn or associate with it, then the phenotype persisted."

Pushing deeper, he discovered that phosphorylated Srcasm targets the Fyn kinase for protein degradation. Introducing more Srcasm into cells can lead to reduced levels of Fyn.

"But if there was too much Srcasm around, Fyn levels actually rebound a little."

Cooperative molecules?

Dr. Seykora believes that this observation suggests that Srcasm does not singularly regulate Fyn but probably acts in consort with other regulatory molecules.

A series of experiments involving the GAT domain of Srcasm - essential for binding a protein called Tollip or monoubiquitinated proteins - prompted researchers to conclude that both phosphorylation and the GAT domain are required for downregulation of Fyn.

Skin connection

"We also showed that Fyn can activate additional key signaling pathways in the mouse keratinocytes, including STAT3 and PDK-1," Dr. Seykora explains.

The former is important in skin carcinogenesis and psoriasis, while PDK-1 regulates AKT, an important protein in inhibiting apoptosis.

Dr. Seykora says, "If you want to be a tumor or a hyperproliferative cell, you need activation pathways that inhibit apoptosis. We can control the activation of these important activation pathways through a combination of Fyn and Srcasm."

He and his research team have formulated what they call the Srcasm modulation hypothesis.

"What that means in practical terms is, if you want to become a tumor, you need to increase Src kinase activity, and that is usually achieved by disrupting a regulatory mechanism that holds the Src kinases in check."

Dr. Seykora has also looked at the levels of these proteins in psoriasis. Unlike cancer, where Srcasm levels are down and Src kinase levels are up, in psoriasis tissue samples, researchers have found that Src kinase levels are up, but so too are Srcasm levels, compared with normal skin on that same patient.

Both psoriasis and squamous cell carcinoma are hyperproliferative disorders of keratinocytes, "but squamous cell carcinoma keeps going, while psoriasis, if you treat it effectively, goes away and is not an autonomous clonal growth. So the mechanism that holds cell growth in check is still there. We think Srcasm may be part of that growth-regulating mechanism," he says.

Divergence, implications, research

Dr. Seykora believes that Srcasm levels may represent a key difference between the inflammatory-induced hyperproliferative processes such as psoriasis and the neoplastic hyperproliferative processes such as carcinoma.