Can 4-nonylphenol induce vitellogenin-like proteins in the clam Tapes philippinarum?

Vitellogenins (Vg) are precursors of egg-yolk proteins (vitellins) constituting energy reserves for embryos. In oviparous females, Vg are produced in the liver (or equivalent organs) under the control of endogenous estrogens, released into the bloodstream (plasma or hemolymph), and stored in developing oocytes. Although Vg levels are generally undetectable in male plasma, they may significantly increase in (xeno-)estrogen-exposed males. The aim of the present study was to evaluate estrogenic effects of 4-nonylphenol (NP) in sexually differentiated males and females of the Manila clam Tapes philippinarum. Clams were exposed for 7 days to various sublethal NP concentrations (0, 0+acetone, 0.025, 0.05, 0.1, and 0.2 mg NP/L) and Vg-like protein levels were evaluated in both hemolymph and digestive glands of mollusks by the alkali-labile phosphate assay. This method, based on the determination of phosphates released by Vg, has previously been used to detect Vg concentrations in both fish plasma and bivalve hemolymph. Although Vg levels were generally higher in both hemolymph and digestive gland of T. philippinarum females, males appeared to be more sensitive than females to the estrogenic effects of NP. In males exposed to 0.1 and 0.2 mg NP/L, Vg levels increased significantly in both hemolymph (P < 0.05 and P < 0.01, respectively) and digestive gland (p < 0.05), whereas no changes were observed in those of females. Although the mechanisms by which NP exerts its in vivo endocrine-modulating effects in bivalve mollusks remain unclear, the results of this study demonstrate that NP may induce Vg-like proteins in T. philippinarum, particularly males.