Differences in type II, IV, V and VI adenylyl cyclase isoform expression between rat preadipocytes and adipocytes.

Adenylyl cyclase catalytic activity is low in preadipocyte membranes when compared to adipocytes. Under conditions promoting inhibition of adipocyte adenylyl cyclase activity by Gpp(NH)p, a stable GTP analog, a paradoxical increase in preadipocyte adenylyl cyclase activity was obtained. In order to explain this contradiction, expression of types II, IV, V and VI adenylyl cyclase isoforms was compared in adipocytes and undifferentiated preadipocytes both by western blots and by a semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) assay. Type II, IV, V and VI mRNAs and proteins were present in both adipocytes and preadipocytes. However, in undifferentiated preadipocytes, expression of type II mRNA and protein were significantly higher whereas expression of type IV, V and VI adenylyl cyclase mRNAs and proteins were significantly weaker than in adipocytes. In late differentiated preadipocytes, the adenylyl cyclase subtype mRNA expression pattern was intermediary between the undifferentiated and the full differentiation states except for type IV which remained weakly expressed. Moreover, one of the representative regulators of G-protein signaling (RGS protein), RGS4, was less expressed in undifferentiated preadipocyte membranes and cytosol extracts, which contrasts with adipocytes where RGS4 is clearly expressed. Thus, the preferential expression of type II adenylyl cyclase (G(betagamma) subunit-stimulated) in preadipocytes might explain why Gpp(NH)p elicits stimulation of adenylyl cyclase under conditions designed to promote inhibition. Conversely, the preferential expression of type V and VI adenylyl cyclases and the slightly higher expression of type IV adenylyl cyclase in adipocytes could contribute to explain the elevated total catalytic activity observed in mature fat cells compared to their precursor cells.

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