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Introduction
Sesquiterpene lactones are a large and structurally diverse group of plant metabolites (Heinrich et al., 1998). Several biological activities of sesquiterpene lactones have been reported, including anti-tumour (Robles et al., 1995), anti-migraine (Beekman et al., 1997) and gastric cytoprotector effects (Giordano et al., 1992; Penissi et al., 1998).
These activities are mediated principally by the alpha-methylene-gamma-lactone function. This chemical group is a powerful alkylating agent due to a Michael-type addition to suitable nucleophiles, e.g. nucleophilic attachment to cysteine sulfhydryl groups through the alpha,beta-unsaturated carbonyl group. However, this alkylating activity is non-specific, leading to the inhibition of a large number of enzymes or factors involved in key biological processes (Heinrich et al., 1998).
Dehydroleucodine (DhL), a sesquiterpenic lactone, was isolated and purified from aerial parts of Artemisia douglasiana Besser, a medicinal herb used in Argentina (Giordano et al., 1990). DhL is an alpha-methylenebutyro-gamma-lactone ring connected to a seven-membered ring fused to an exocyclic alpha,beta-unsaturated cyclopentenone ring.
It has been previously shown that DhL selectively induces a dose-dependent transient arrest in G2 of both meristematic cells (López et al., 2002) and vascular smooth muscle cells (Cruzado et al., 2005). Treatment with DhL of Bufo arenarum fully grown oocytes arrested at G2, at the beginning of meiosis I, induces an inhibition of spontaneous and progesterone-induced maturation in a dose-dependent manner (Sánchez Toranzo et al., 2007). However, the nature of the mechanisms involved in the process is still unknown.
In amphibians, meiotic arrest is released under the stimulus of progesterone, a process termed oocyte maturation (Fortune et al., 1975; Schuetz, 1985). Although progesterone is the established maturation inducer in Bufo arenarum, meiosis resumption also occurs spontaneously when follicular cells are removed and oocytes are cultured in vitro under suitable conditions. In this species oocytes competent or incompetent to undergo spontaneous maturation can be obtained according to the seasonal period in which animals were captured (Zelarayán et al., 1995). However, fully grown Bufo arenarum oocytes always respond to progesterone, independently of the time of year in which they are obtained (Zelarayán et al., 1996).
Meiotic maturation, which represents the transition from G2 to the...