Mechanisms of α‐Sialosyl Cholesterol Action to Suppress Both Cyclic AMP Production and DNA Synthesis of Rat Glial Cells

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Abstract
α‐Sialosyl cholesterol (α‐SC) that elicited morphological differentiation of rat astrocytes not only lowered intracellular cyclic AMP (cAMP) levels but also inhibited cAMP production induced by either α‐isoproterenol, cholera toxin, or forskolin. The targets of α‐SC in the cAMP production system of rat astrocytes were investigated to understand the mechanism of the a‐SC action on cAMP production. cAMP production evoked by a‐isoproterenol (1 μM) was entirely canceled by β blockers such as propranolol and timolol (1 μM), but not by α‐SC. Concentrations of α‐SC greater than 15 μM were required for 50% inhibition of the activation by a β agonist. Although α‐SC inhibited in a dose‐dependent manner the activities of membrane‐associated adenylate cyclase that had been stimulated by either GTPγS or forskolin, α‐SC inhibited neither GTP‐binding activities nor GTPase activities of the membrane‐associated G proteins. These findings suggest that α‐SC suppresses adenylate cyclase directly, but not β receptors or G proteins, and that it promotes the morphological differentiation of rat astrocytes through a mechanism regulating directly the cytoskeletal organization, regardless of intracellular cAMP level. α‐SC (30 μM) suppressed 40% of DNA synthesis in the cell‐free system, which contained the cytosolic extracts and the nucleus fraction prepared from rat astrocytoma C6 cells. Approximately 25% of α‐SC incorporated in the astrocyte cytoplasm was transferred to the nuclei by 10 min after the addition. Thus, it is likely that α‐SC that had been incorporated in the cytosol suppressed adenylate cyclase by acting from the cytosolic side of the plasma membrane, and separately suppressed nuclear DNA synthesis.