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Submitted on June 25, 2004
Accepted on July 20, 2004
IPF PharmaCeuticals GmbH, Feodor-Lynen-Strasse 31, D-30625 Hannover, Germany
* To whom correspondence should be addressed. E-mail: erik.maronde{at}ipf-pharmaceuticals.de.
In the present study, we investigate the coherence of signaling pathways leading to lipolysis in 3T3-L1 adipocytes. We observe two linear signaling pathways: one well-known, acting via cAMP and PKA activation, and a second one induced by PMA treatment involving PKC and MAPK. We demonstrate that both the PKA regulatory subunits RI
and RII
are expressed in 3T3-L1 adipocytes and are responsible for the lipolytic effect mediated via the cAMP/PKA pathway. Inhibition of the PKA pathway by the selective PKA inhibitor Rp-8-CPT-cAMPS does not impair lipolysis induced by PKC activation, and neither PD98059 nor U0126, as known MEK inhibitors, changes the level of glycerol release caused by PKA activation, indicating no cross-talk between these two pathways when only one is activated. However, when both are activated, they act synergistically on glycerol release. Further experiments focusing on this synergy showed no involvement of MAPK phosphorylation and cAMP formation. Phosphorylation of HSL is similar upon stimulation of either pathway, but we demonstrate a difference in the ability of both PKA- and the PKC- pathway activation to phosphorylate perilipin, which in turn may be an explanation for the different maximal lipolytic effect of both pathways.
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