Acquisition of an Insulin‐Sensitive Activity of Adenosine‐3′,5′‐Monophosphate Phosphodiesterase during Adipose Conversion of 3T3‐L2 Cells

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Abstract
Mouse 3T3‐L2 fibroblasts differentiate when cultured and express a new phenotype which is characteristic of adipose cells. One aspect of this differentiation is the acquisition of hormone sensitivities typical of adipocytes. The sensitivity of adenosine‐3′,5′‐monophosphate (cAMP) phosphodiesterase to brief insulin treatment in 3T3‐L2 cells at various stages of adipose conversion was examined. The capacity to increase the activity of a low‐Km form of the enzyme in response to acute insulin exposure is a property which is acquired as the cells begin to express the adipose phenotype. Enzyme activity in preadipocytes is not affected by insulin, nor is there an effect in the non‐differentiating 3T3‐C2 line under any of the conditions tested. Since insulin receptors and cAMP phosphodiesterase are both present in the preadipocytes, one result of this cytodifferentiation is to effectively couple receptors to the enzyme activity.Insulin‐sensitive cAMP phosphodiesterase in 3T3‐L2 adipocytes is characterized by a 40–50% increase in enzyme activity during insulin treatment when determined at 0.1 μM cAMP. This activation results from an increase in the apparent V of the enzyme and does not involve a change in enzyme‐substrate affinity. Maximal stimulation is seen within 2–5 min and is sustained for at least 45 min when high levels of insulin (850 nM) are used. Lower concentrations of insulin (1.7 nM) also bring about rapid activation, although the activation is not completely sustained during longer incubations. Stimulated activity falls off to about 60% of peak values by 30 min. Re‐addition of insulin after 30 min raises enzyme activity back to the maximal level. Further, the insulin response is completely reversible in that the insulin‐sensitive cAMP phosphodiesterase activity disappears within 10 min after removal of insulin from the cultures.