Biology Reference
In-Depth Information
An upregulation of PDE4 was first described by Torphy in a monocytic cell line
U937 (Torphy et al.
1992a
). Incubation of these cells with salbutamol and rolipram
for 3 h was shown (1) to increase intracellular cAMP, (2) to activate PKA and (3) to
enhance PDE4 activity two- to threefold. After withdrawal of the stimulating
conditions, PDE4 activity returned to the basal levels within a 3 h period. Thus, a
feedback mechanism reacting to a sustained increase of cAMP was discovered.
Similar experiments were performed with the human keratinocyte cell line
HACAT, where a 6 h exposure to salbutamol raised PDE4 activity about threefold
(Tenor et al.
1995a
). The cAMP increase in these “induced cells” in response to
incubation with ß-receptor agonists was now considerably reduced, which might be
of relevance for the situation in airway diseases where there is continuous exposure
to long-acting ß-mimetics.
In primary human MCs, a two- to threefold induction of PDE4 activity was
demonstrated by Verghese and the upregulation of the splice variants PDE4A5,
4B2 and 4D3 was shown on the level of protein analysis (Verghese et al.
1995
).
Analogously, in peripheral T cells, the fenoterol-induced upregulation of PDE4A4,
4D1, 4D2 and 4D3 was shown by expression data and western blot analysis
(Seybold et al.
1998
). In view of these results, a continuous exposure of inflamma-
tory cells in a patient's lung to long-acting ß-mimetics should increase the reactivity
to irritants, allergens, stimulators and inflammatory mediators. In conclusion, the
finding that (i) ß mimetics drive inflammatory cells to an enhanced hyperresponsive
state and that (ii) PDE4 inhibitors reduce this up-regulated activity may provide a
further rationale for the therapeutic use of PDE4 inhibitors.
4.5 Correlation Diagrams: How Do Biochemical Data
Correspond to Pharmacological Results?
T cells and smooth muscles contain a complex mixture of PDE isoenzymes and the
actual activity of these different isoenzymes can only be roughly approached by
biochemical methods. In order to develop and optimise selective and efficacious
PDE inhibitors it was essential to know the participation of a certain isoenzyme in
the regulation of defined cell responses. One possibility used to analyse the
involvement of a PDE isoenzyme in cellular activity is the correlation of the two
parameters that reflect action of PDE inhibitors, i.e. (1) EC50 of the cellular
response to treatment with a given inhibitor and (2) IC50 of inhibition of the
isolated enzyme by that compound. If the correlation coefficient for a variety of
different substances equals 1, then it can be assumed that this very PDE isoenzyme
participates in regulation of this cellular function. For platelet aggregation and
tracheal relaxation, a correlation with PDE3 inhibition by various weakly selective
substances was already mentioned in Sects.
3.3.6
and
4.1
showing that PDE3 was
causally and quantitatively involved in the inhibition of functional responses
(Schudt et al.
1991c
; Harris et al.
1989
). Therefore, the inhibitor potency measured
