Prolactin blocks glucocorticoid induced cell death by inhibiting the disruption of the mitochondrial membrane

Abstract 

Prolactin (PRL) has been reported to inhibit dexamethasone (Dex) induced cell death. Nevertheless, the mechanism through which PRL exerts its protective effect is still not unravelled. Here, we analyse the effect of PRL at different stages of the glucocorticoid (GC) apoptotic pathway in PRL dependent cells (Nb₂ cells). PRL blocks completely the GC induced loss of the mitochondrial transmembrane potential (ΔΨ_m) and consequently phosphatidylserine (PS) exposure and loss of DNA content. Although PRL promotes an upregulation of the bcl-2 expression, simultaneous addition of PRL to GC fails to maintain even the normal levels of this anti-apoptotic protein. This finding excludes a critical role for bcl-2 in the PRL protective effect against GC. GC induced ΔΨ_m disruption can be inhibited by the ICE-like inhibitor zVAD-fmk but not by ICE inhibitor tetrapeptide acetyl-Tyr-Val-Ala-Asp.chloromethylketone (YVAD-cmk) nor by caspase-3 inhibitor zDEVD. It can be speculated that PRL blocks ΔΨ_m disruption by inhibiting an unknown caspase activated by GC.

Keywords: Apoptosis, Prolactin, Dexamethasone, Mitochondrial transmembrane potential, bcl-2, ICE-like proteases

Abbreviations: C₂, ceramide, Dex, dexamethasone, GC, glucocorticoid, Nb₂ cell line, prolactin dependent cell line, PI, propidium iodine, PRL, prolactin, PS, phosphatidylserine, YVAD, tetrapeptide acetyl-Tyr-Val-Ala-Asp.chloromethylketone, zDEVD.fmk, Z-Asp-Glu-Val-Asp-CH₂F, z-VAD.fmk, N-benzyloxycarbonyl-Val-Ala-Asp.fluoromethylketone