Differentiation-dependent expression of carbonic anhydrase II and III in 3T3 adipocytes

Carbonic anhydrase (CA) was examined in two adipocyte cell lines, 3T3-L1 and 3T3-F442A. Both CA III and non-CA III activities, measured by 18O mass spectrometry, were present in 3T3-L1 and 3T3-F442A adipocytes; however, no CA activity was detected in 3T3 preadipocytes of either line. These observations were supported by immunoblot experiments employing CA III and CA II isoform-specific antisera. CA III, a major protein in rodent and murine adipocytes, and CA II, another isoform known to be present in adipose tissue, were observed only in the differentiated 3T3 adipocytes. The differentiation-dependent expression of these isozymes may imply an adipocyte-related role for CA. Compared with cultures maintained in the absence of insulin, 3T3 adipocytes maintained in the presence of insulin exhibited 65-90% lower concentrations of CA III. CA II was unaffected. This negative effect of insulin on CA III may explain the metabolic regulation of adipose CA III observed in vivo. After media changes, 3T3 adipocyte cultures rapidly lower media pH, which in turn lowers the bicarbonate/CO2 of bicarbonate/CO2-buffered media. Cultures maintained at low pH displayed 50-90% lower concentrations of CA II and CA III. Similarly, cultures maintained in a low bicarbonate/CO2 media (GibCO2-I medium containing 1 mM bicarbonate under an atmosphere of 100% humidified air) displayed 30-50% lower CA II and CA III concentrations. Thus CA II and CA III concentrations are influenced by pH and bicarbonate/CO2. Neither effect, the pH or the GibCO2-I media effect, was associated with changes in the concentration of pyruvate carboxylase or ATP citrate lyase (2 markers of adipocyte differentiation). Because the regulation by pH and bicarbonate/CO2 may be relatively selective for CA in adipocytes, a simple method for reducing the concentration/activity of CA in 3T3 adipocytes is described that may be a useful tool for studies on the physiological role of the enzyme.