Differentiation Dependent Biphasic Regulation of Adipsin Gene Expression by Insulin and Insulin-Like Growth Factor-1 in 3T3-F442A Adipocytes*

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Abstract
Adipsin is a serine protease with complement factor D activity that is synthesized by adipocytes and secreted into the blood stream. Expression of adipsin is deficient in models of genetic (ob/ob, db/db) and acquired (monosodium glutamate-lesioned) obesity, but the cellular mechanisms responsible for this deficiency are unknown. Because hyperinsulinemia is frequently associated with obesity, we evaluated the effects of this hormone and insulin-like growth factor 1 (IGF-1) on adipsin secretion and adipsin messenger RNA (mRNA) levels in 3T3-F442A adipocytes. In the present study, we report that in fully differentiated adipocytes (after 11 days post confluence), insulin exposure progressively decreases adipsin secretion by 40%, 67%, and 78% after 2, 4, and 6 days of treatment. The inhibition of adipsin secretion by insulin is the result of a corresponding decrease in adipsin mRNA and is specific since two other differentiation-dependent fat cell mRNAs encoding aP2 (a fatty acid binding protein) and glycerophosphate dehydrogenase (GPD), are unaffected. Insulin suppresses adipsin gene expression via high affinity insulin receptors, because physiological levels of insulin produce this effect, and dose-response curves for insulin stimulation of 2-deoxyglucose uptake and glucose utilization are similar to insulin''s effect on adipsin. In contrast, insulin when present during days 1-8 post confluence (during differentiation) markedly increases adipsin secretion and adipsin mRNA levels. This stimulation is due to the ability of insulin to accelerate differentiation as evidenced by corresponding increases in aP2 and GPD mRNAs as well. Insulin and IGF-1 are equipotent in this effect, suggesting that both insulin and IGF-1 receptors can mediate this response. In summary, during the differentiation of 3T3-F442A adipocytes, insulin stimulates adipsin gene expression by accelerating differentiation. As the cells become mature adipocytes, they acquire some differentiation-dependent factor, which couples insulin receptor stimulation to inhibition of adipsin gene expression. This model should aid our search for the molecular links between insulin receptor stimulation and altered gene expression.

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