Ovarian expression of cellular Ki-ras p21 varies with physiological status.

Abstract

To elucidate the potential role of the ras protooncogene proteins in a specific tissue, the present study determined the levels of individual c-ras-encoded p21 proteins in the rat ovary during various stages of physiological function. p21 protein was extracted from ovaries taken from immature normal female rats, mature nonpregnant animals in the metestrus stage of the estrus cycle, rats at various stages of pregnancy, and actively lactating animals. Levels of individual p2ls were evaluated by immunoblot analysis with specific antibodies to the p21 proteins encoded by the Kirsten, Harvey, and neuroblastoma c-ras protooncogenes, c-Ki-ras, c-Ha-ras, and N-ras. Results showed that c-Ki-ras p21 is at its lowest level in the immature ovary and increases with development of the corpora lutea to its highest levels at day 16 of pregnancy, after which levels decline and then rise again during lactation. This pattern, which mimics that of circulating progesterone levels, suggests that ovarian c-Ki-ras p21 levels are regulated and that c-Ki-ras p21 plays a role in the differentiated function of the rat ovary, likely the luteal compartment. In contrast, levels of c-N-ras p21 did not appear to vary with changes in the physiological function of the ovary but appeared to be constitutive. A preferential role for the c-Ki-ras p21 may be due to the documented unique differences in the structure of the carboxyl terminus of this particular c-ras p21.Results of recent studies show the importance of a variety of cellular oncogene products not only in the control of proliferation but also in differentiated function of normal eukaryotic cells (1). Of particular interest is the cellular ras (c-ras) gene family, a group of very highly conserved genes present in yeasts to primates (2, 3). The c-ras genes encode markedly similar 21-kDa proteins (p21) that are localized to the inner face of the plasma membrane, bind guanine nucleotides, and have significant structural homology to other GTP-binding proteins (G proteins), including the adenylate cyclaseassociated G proteins (2,3). Unlike their role in yeasts, however, in mammalian cells c-ras G proteins do not function as part of the cyclase pathway (4). Although various data demonstrate that mammalian c-ras p21 proteins influence the efficacy of membrane receptor signal-transduction pathways (5,6), the specific function of the c-ras p21s in any mammalian cell type has not been defined. In efforts to more clearly define their specific role, examination of total c-ras p21 expression in various normal human and rat tissues has been performed (7-9). Yet these studies have been limited to evaluation of total p21 content rather than determination of the individual p21 proteins and have not defined whether changes in differentiated function of a tissue are associated with variations in p21 protein levels. Since changes in differentiated function of specific compartments in the mammalian ovary are well documented by the reflected patterns in circulating levels of steroid hormones, the ovary i...

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