Molecular Human Reproduction, Vol 4, 259-268, Copyright © 1998 by Oxford University Press
L Bonaccorsi, C Krausz, P Pecchioli, G Forti and E Baldi
Indirect studies suggested that protein kinase C (PKC) has a role in sperm
motility and the acrosome reaction. Physiological inducers of the sperm
acrosome reaction include progesterone, which can increase intracellular
calcium ([Ca2+]i), tyrosine phosphorylation of proteins and chloride efflux
in human spermatozoa. PKC may be involved in progesterone-stimulated
acrosome reaction, although controversial results have been obtained
concerning the effect of PKC inhibition on progesterone-stimulated [Ca2+]i
increase. In the present study, we investigated the direct effect of
progesterone on the activity of PKC, as well as the effect of a panel of
PKC inhibitors on progesterone- stimulated [Ca2+]i increase and tyrosine
phosphorylation of proteins. We found that progesterone stimulates sperm
PKC activity and that PKC inhibition with staurosporine and
bisindolylmaleimide partially reversed the effect of progesterone on
acrosome reaction, indicating an involvement of the enzyme in the effect of
the steroid. We next evaluated the effect of three different PKC inhibitors
(sangivamycin, staurosporine and bisindolylmaleimide) on
progesterone-stimulated [Ca2+]i increase. Neither short-term (15 min) nor
long-term (90 min) preincubation with any of the three compounds had a
substantial effect on the stimulatory effect of progesterone on sperm
[Ca2+]i. Nor was responsiveness to progesterone affected by either
short-term (determining activation of PKC) or long-term (determining down-
regulation of PKC) incubation with the tumour promoter phorbol myristate
acetate (PMA), a known non-physiological stimulator of PKC. These results
indicate that progesterone-stimulated calcium influx is independent of PKC
activation. In addition, we found that preincubation with PKC inhibitors
had a stimulatory effect per se on tyrosine phosphorylation of sperm
proteins. When compared with the appropriate control, the effect of
progesterone on tyrosine phosphorylation was slightly (but not
significantly) reduced by the inhibitors, sangivamycin, staurosporine and
bisindolylmaleimide, but was significantly inhibited by calphostin C. These
results do not permit a final conclusion on the involvement of PKC in
progesterone-stimulated tyrosine phosphorylation of sperm proteins.
However, the lack of effect of PMA on tyrosine phosphorylation indicates
that PKC stimulation is not sufficient to induce this effect. In
conclusion, our results indicate that PKC plays a role in
progesterone-induced acrosome reaction and that progesterone-stimulated PKC
activation is downstream to stimulation of calcium influx by the steroid.
JOURNAL ARTICLE
Progesterone-stimulated intracellular calcium increase in human spermatozoa is protein kinase C-independent
Dipartimento di Fisiopatologia Clinica, Universita di Firenze, Italy.
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