We have isolated and characterized complete cDNAs for two isoforms (HSPDE4D4 and HSPDE4A5) encoded by the human PDE4D gene, one of four genes that encode cAMP-specific rolipram-inhibited 3h,5h-cyclic nucleotide phosphodiesterases (type IV PDEs ; PDE4 family). The HSPDE4D4 and HSPDE4D5 cDNAs encode proteins of 810 and 746 amino acids respectively. A comparison of the nucleotide sequences of these two cDNAs with those encoding the three other human PDE4D proteins (HSPDE4D1, HSPDE4D2 and HSPDE4D3) demonstrates that each corresponding mRNA transcript has a unique region of sequence at or near its 5h-end, consistent with alternative mRNA splicing. Transient expression of the five cDNAs in monkey COS-7 cells produced proteins of apparent molecular mass under denaturing conditions of 68, 68, 95, 119 and 105 kDa for isoforms HSPDE4D1-5 respectively. Immunoblotting of human cell lines

Ser-13 and Ser-54 were shown to provide the sole sites for the protein kinase A (PKA)-mediated phosphorylation of the human cAMP-specific phosphodiesterase isoform HSPDE4D3. The ability of PKA to phosphorylate and activate HSPDE4D3 was mimicked by replacing Ser-54 with either of the negatively charged amino acids, aspartate or glutamate, within the consensus motif of RRES54. The PDE4 selective inhibitor rolipram ¿4-[3-(cyclopentoxy)-4-methoxyphenyl]-2-pyrrolidone¿ inhibited both PKA-phosphorylated HSPDE4D3 and the Ser-54-->Asp mutant, with an IC50 value that was approximately 8-fold lower than that seen for the non-PKA-phosphorylated enzyme. Lower IC50 values for inhibition by rolipram were seen for a wide range of non-activated residue 54 mutants, except for those which had side-chains able to serve as hydrogen-bond donors, namely the Ser-54-->Thr, Ser-54-->Tyr and Ser-54-->Cys mutants. The Glu-53-->Ala mutant exhibited an activity comparable with that of the PKA phosphorylated native enzyme and the Ser-54-->Asp mutant but, in contrast to the native enzyme, was insensitive to activation by PKA, despite being more rapidly phosphorylated by this protein kinase. The activated Glu-53-->Ala mutant exhibited a sensitivity to inhibition by rolipram which was unchanged from that of the native enzyme. The double mutant, Arg-51-->Ala/Arg-52-->Ala, showed no change in either enzyme activity or rolipram inhibition from the native enzyme and was incapable of providing a substrate for PKA phosphorylation at Ser-54. No difference in inhibition by dipyridamole was seen for the native enzyme and the Ser-54-->Asp and Ser-54-->Ala mutants. A model is proposed which envisages that phosphorylation by PKA triggers at least two distinct conformational changes in HSPDE4D3; one of these gives rise to enzyme activation and another enhances sensitivity to inhibition by rolipram. Activation of HSPDE4D3 by PKA-mediated phosphorylation is suggested to involve disruption of an ion-pair interaction involving the negatively charged Glu-53. The increase in susceptibility to inhibition by rolipram upon PKA-mediated phosphorylation is suggested to involve the disruption of a hydrogen-bond involving the side-chain hydroxy group of Ser-54.

Pegvisomant is the pegylated form of mutant growth hormone (B2036). B2036 has increased affinity in one binding site and lowered affinity in its second binding site, it has been shown that this molecule still enables dimerisation of the growth hormone receptor at the cell surface but does not allow the necessary conformational changes for signalling. Pegylation decreases the antagonistic activity of B2036, however the rate of clearance of the pegylated B2036 is greatly reduced compared to the unpegylated form. Even though the antagonistic activity of pegvisomant is lower than B2036, the reduced rate of clearance makes it an effective clinical drug for the treatment of conditions such as acromegaly.

Cytokine hormones have a short plasma half-life and require frequent administration. For example, growth hormone replacement involves daily injections. In common with other cytokines, the extracellular domain of the growth hormone receptor circulates as a binding protein, which naturally prolongs the biological half-life of growth hormone. Here we have studied the biological actions of a ligand-receptor fusion of growth hormone and the extracellular domain of its receptor. The genetically engineered ligand-receptor fusion protein was purified from mammalian cell culture. In rats, the ligand-receptor fusion had a 300-times reduced clearance as compared to native growth hormone, and a single injection promoted growth for 10 d, far exceeding the growth seen after administration of native growth hormone. The ligand-receptor fusion forms a reciprocal, head-to-tail dimer that provides a reservoir of inactive hormone similar to the natural reservoir of growth hormone and its binding protein. In conclusion, a ligand-receptor fusion of cytokine to its extracellular receptor generates a potent, long-acting agonist with exceptionally slow absorption and elimination. This approach could be easily applied to other cytokines.