Well, prev written....
">I was wondering why cytokines, and lymphokines for that matter, are not
>usually considered as a functional part of cell-to-cell communication, at
>least not in the textbooks I have used. Is the distinction between
>hormones and -kines merely taxonomic? Or is there a different mechanism
>for -kines, for instance no secondary messangers or specific receptors as
>there are for hormones?
>AB DiBernardo
>University of Pennsylvania
>I don't see any distinction, and they are considered a functional part of
>cell-cell communication, especiially in immunology (What textbooks are you
>reading?). Cytokines ARE hormones; just a sub-category. It's the definition of
>hormone that is more difficult (for discussion see introduction of V.C.
>Medevei's book 'A history of endocrinology' (1982), MTP press. ) The
>distinguishing feature of cytokines is that they maintain local tissue
>homeostasis and function of tissues, rather than systemic homeostsis and
>fnction of the organism, which is the primary role of endocrine (the other
>major category) hormones. (Cytokines are also not produced by speciaised
>organs.) Of course the edges of this categorisation are blurred (e.g. IL-6,
>M-CSF,IGF, neurotrophins?,) since natural selection didn't follow any rules
>and, unfortunately, immunologists, endocrinologists and neuroscientists havn't
>communicated too well until recently, so there is some dispute about
>boundaries, but fits pretty well. (?)"
OK, cytokines all act through a membrane bound receptor. I think that
hormones are membrane soluable and go running in through the outer PM and
get bound to an internal binding protein which can then escort or follow
into the nucleus. That's a difference. Cytokines activate and work through
this membrane bound receptor system and activate a signal transduction
pathway-like PKCs ect. Hormones, when in the nucleus can act
directly-binding and inducing transcription much more primarily. Also I
think that hormones are usually planar-rings-hydrophobic so they go
screaming through the PM, while cytokines are small proteins- many of them
homologous to each other. Certainly, I WOULD call the immune system a
specific organ, but thats me. You must also remember that many of the
immune system's cytokines/lymphokines have been "stolen" from the
developmental biologists, it's just that immunologists may have identified
these molecules first. Example, IL-1 is very important in normal
development, as well as playing a role in the mature individual not
necessarily always as the endogenous pyrogen. Back to the first point, the
main role of cytokines (in their immune system role) is to play a part in
cell-to-cell communication.
Ralph M. Bernstein
Dept of Micro/Immuno
University of Arizona
Ph: 602 626 2585
Fx: 602 626 2100
