Effect of stypoldione on cell cycle progression, DNA and protein synthesis, and cell division in cultured sea urchin embryos

SJ White and RS Jacobs

We have found that stypoldione, a bright red o-quinone isolated from the brown alga Stypopodium zonale, inhibits the division of sea urchin embryos in a concentration-dependent manner (IC50 approximately 2.5 X 10(-6) M). Although previous studies have shown this marine natural product to inhibit beef brain microtubule assembly in vitro [Fed. Proc. 39:26-29 (1980); Mol. Pharmacol. 24:493-499 (1983)], we have found that stypoldione does not accumulate sea urchin embryos in mitosis and hence does not act like a mitotic spindle poison. We have also shown this marine natural product to inhibit both amino acid and nucleoside uptake. By preloading sea urchin embryos with nucleoside (i.e., [3H]thymidine) in order to dissociate effects on uptake from those on incorporation, we found that stypoldione in fact produces no significant inhibition of the M phase-independent S1 period of DNA synthesis, a result which suggests that stypoldione has no direct effect on DNA synthesis. In contrast, stypoldione did reduce the extent of amino acid incorporation in embryos preloaded with [3H]leucine. An inhibition of incorporation was apparent as early as 20 min after fertilization, and incorporation was reduced to 50% of control by 40 min postfertilization. This result suggests that stypoldione might inhibit cleavage via an inhibition of translation, although the existence of other inhibitory mechanisms cannot yet be ruled out. Cytological examination revealed that sea urchin embryos did not progress beyond-interphase or very early prophase when incubated in the presence of 1.0 X 10(-5) M stypoldione. The nuclear membranes remained intact, and chromatin did not condense into chromosomes in these arrested embryos. These results indicate that embryos exposed to stypoldione early in the cell cycle initiate and complete the M phase- independent S1 period of DNA synthesis, but stop cell cycle progression prior to the start of prophase of mitosis. The period between S phase and mitosis is referred to, by definition, as the "G2" phase of the cell cycle. The result therefore suggest that stypoldione blocks cell cycle progression (and, ultimately, cell division) by inhibiting progression through G2. This compound may represent a new class of G2- accumulating agents.

Volume 24, Issue 3, pp. 500-508, 11/01/1983
Copyright © 1983 by American Society for Pharmacology and Experimental Therapeutics