By catalyzing hydrolysis of cAMP and cGMP, cyclic nucleotide phosphodiesterases (PDEs) are critical regulators of intracellular concentrations and biological effects of cyclic nucleotides. Understanding cellular expression and regulation of PDE isoforms [which belong to eleven gene families (PDEs 1-11)] will be of increasing importance for targeting specific PDEs in treating various diseases, including pulmonary disorders. PDE3B appears (or greatly increases in activity) during differentiation of cultured human adipocytes, murine 3T3-L1 adipocytes, or human monocyte-derived macrophages. Our results suggest that activation of CREB proteins plays a crucial role in regulation of PDE3B expression during differentiation of 3T3-L1 adipocytes. In differentiated human adipocytes or 3T3-L1 adipocytes, TNFalpha increases lipolysis, at least in part, by downregulation of PDE3B expression, leading to increases in cAMP and activation of hormone-sensitive lipase. TNFalpha also inhibits insulin-induced activation of PDE3B. It is not known if effects of TNFalpha on PDE3B are involved in induction of insulin-resistance by TNFalpha. Our results also indicate that insulin-induced activation of PDE3B is mediated by PI3-K- and PKB-dependent signals, and that PDE3B is a substrate of PKB. Serine 273 in murine PDE3B seems to be critical for activation of PDE3B by insulin and PKB in intact cells and in vitro, respectively. In addition a portion of the intracellular pool of PKB is found in association with intracellular membranes, co-elutes with membrane-associated PDE3B during gel filtration chromatography of solubilized 3T3-L1 microsomal membranes, and co-immunoprecipitates with PDE3B. The structural determinants for this latter interaction reside in the N-terminal regulatory region of PDE3B; proline-rich peptide sequences from the N-terminal region near serine 273 seem to inhibit the interaction between PDE3B and PKB. To further examine functional roles of PDE3 isoforms, PDE3A and PDE3B null mice have been generated. Female PDE3A mice are sterile, most likely because the absence of functional PDE3A in oocytes leads to a cAMP-induced block in meiotic progression and oocyte maturation (marked by germinal vesicle breakdown (GVBD)), and failure of fertilization. PDE3B KO mice exhibit signs of disruption of insulin homeostatic mechanisms and insulin resistance. Administration of a Beta-3 agonist to intact mice results in a much larger increase in serum insulin but less glucose disposal in PDE3B KO mice, which also demonstrate aberrant i.p. insulin tolerance tests with respect to reduction in blood glucose and serum free fatty acids. Studies are ongoing to understand the roles of PDE3A and PDE3B in these and other phenotypic changes.
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