The previous paragraphs have shown that many proteins that were first identified in the nervous system later turned out to be expressed in various other tissues. Hence, it is important to be aware that the term 'neural' in these cases is only a historical one - a fact that will most likely apply to more and more antigens in the future.
Looking at the function of the proteins mentioned, they fall into (overlapping) categories like (i) maintenance and repair, (ii) directing migration, (iii) secretion and (iv) cell adhesion - fundamental processes of the developing nervous system and vasculature which are switched on again in malign tumors, or as Liotta and Clair  commented: '.. .cancer invasion in general may be a deregulated form of a physiological invasion process required for neuronal wiring in the embryo, tissue remodeling of blood vessels, and healing'.
The expression of 'neuronal' proteins in the tumor may stimulate axonal outgrowth of free nerve endings, thereby enabling the tumor host to 'feel' the cancer as in NGF expression in pancreatic cancer or skeletal cancer which are frequently associated with chronic pain. Expression of 'neuronal' proteins in the nervous tissue on the other hand seems to play an important role in 'unwillingly' guiding tumor growth along nerve routes and may additionally stimulate tumor growth via paracrine mechanisms.
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Prof. Dr. Christian Hagel
Institute of Neuropathology, University Medical Center, Hamburg-Eppendorf
DE-20246 Hamburg (Germany)
Tel. +49 40 42803 2223, Fax +49 40 42803 4929, E-Mail [email protected]
Zänker KS, Entschladen F (eds): Neuronal Activity in Tumor Tissue. Prog Exp Tumor Res. Basel, Karger, 2007, vol 39, pp 78-90
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