Insulin-stimulated high affinity cyclic AMP phosphodiesterase in rat mammary acini

Cyclic adenosine monophosphate (cAMP)-specific phosphodiesterase is functional in bovine mammary gland

Previous studies have shown that using nonspecific phosphodiesterase (PDE) inhibitors such as caffeine improved milk production, supporting the premise that modulation of intracellular concentration of cyclic nucleotides (cyclic AMP, cyclic guanosine 3'-5'-monophosphate) is involved. Intracellular cyclic nucleotides are degraded by the PDE enzyme family. The contribution of type IV PDE (PDE4) in the secretion of casein has been reported in rat mammary gland. The objective of this study was to demonstrate the functional presence of the PDE4 family in the bovine mammary gland. To understand the enzymatic expression pattern in the mammary gland, tissue samples were taken randomly from udders obtained from a local slaughterhouse. Reverse transcription PCR revealed that the PDE4D transcript was amplified, and the expected size fragment was obtained in a 1% agarose gel. Sequence analysis of the amplicon resulted in 99% homology to PDE4D. Moreover, Western blotting using a specific PDE4D antibody has confirmed that the protein of the isoenzyme PDE4D1 is present. A clear immunoreactive signal was also observed within the acini where epithelial cells are located. Assaying cyclic AMP PDE activity reported a total activity of 38.71 +/- 3.22 fmol/min per microg of total protein. Rolipram, a specific PDE4 inhibitor, showed a sensitive activity of 8.48 +/- 1.28 fmol/min per microg of total protein, indicating that PDE4 is responsible for one-fifth of the total enzymatic activity of PDE in the mammary gland. To further validate the presence of PDE4D in the bovine mammary epithelial cells, protein extracts from bovine mammary epithelial cells were separated on SDS-PAGE gels, and PDE4D protein was detected. The PDE assays reported a total activity of 30.16 +/- 4.82 fmol/min per microg of total protein. Rolipram showed a sensible activity of 11.91 +/- 5.93 fmol/min per microg of total protein. In conclusion, these results not only demonstrate the presence of PDE4D transcript and protein, but also show an active enzyme, suggesting a functional role of PDE4D in bovine mammary gland.

Type IV phosphodiesterase activity specifically regulates cAMP-stimulated casein secretion in the rat mammary gland

This study investigates the regulation of cAMP-stimulated casein secretion in rat mammary explants by cAMP phosphodiesterase (cAMP-PDE) activity. cAMP-PDE activity of the lactating rat mammary gland is shown to be provided by three families, types II, III and IV. In mammary explants, general inhibition of the cAMP-PDE activity significantly increased the rate of cAMP-stimulated casein secretion. This effect could be mimicked using the type-IV specific inhibitor rolipram but not by the specific, or combined, inhibition of the type II and type III activity. Only type IV activity significantly affected intracellular accumulation of cAMP whereas all three cAMP-PDE activities were shown to influence the PKA activation ratio in cells. RtPCR analysis showed that the mammary gland apparently expresses just three type IV isozymes, RNPDE4A5, RNPDE4A8 and RNPDE4D3. A specific role for type IV cAMP-PDE activity in the regulation of casein secretion is suggested and possible mechanisms for the effects of PDEIV activity discussed.

Modulation of intracellular cyclic AMP content and rate of lipogenesis in mammary acini in vitro

Relationships between the cyclic AMP content, the rate of lipogenesis and the activity of acetyl-CoA carboxylase in acini prepared from lactating rat mammary tissue were investigated by exposing them to agents that increase their cyclic AMP content in the presence or absence of insulin. The dose-dependent inhibition of lipogenesis by theophylline in acini isolated from fed rats was highly correlated with the induced increases in acinar cyclic AMP content. Cyclic AMP of acini from 24 h-starved lactating rats was more sensitive in its response to theophylline than that in acini from fed animals. Neither forskolin nor a mixture of isoprenaline and Ro 7-2956 were able significantly to change either the rate of lipogenesis or the activity of acetyl-CoA carboxylase in acini from fed rats when added to incubations in vitro, in spite of the large increases in cyclic AMP concentration produced by these agents. Insulin was without effect on the activity of acetyl-CoA carboxylase and on either the basal or isoprenaline-stimulated cyclic AMP content of acini. These results are discussed in terms of the possibility that the rate of lipogenesis and the cyclic AMP content in mammary acini can vary independently of one another and of the activity of acetyl-CoA carboxylase.

Acute change in the cyclic AMP content of rat mammary acini in vitro. Influence of physiological and pharmacological agents

The cyclic AMP content of acini, freshly prepared from mammary tissue of lactating rats, was measured during incubation in vitro. Neither adrenergic agonists nor cyclic AMP phosphodiesterase inhibitors alone caused a change of more than 2-fold in the basal cyclic AMP content of acini. Together, however, these agents provoked increases of around 20-fold in acini cyclic AMP content. Forskolin caused similar effects. The relative potency of adrenergic agonists in increasing cyclic AMP in acini, together with the ability of selective antagonists to oppose such rises, indicated that beta 2-adrenergic receptors were involved in mediating the effects. Receptor-binding experiments using [3H]dihydroalprenolol and selective beta-antagonists confirmed the predominant presence of beta 2-adrenergic receptors on acini membranes and on membranes prepared from purified mammary secretory epithelial cells. These results elucidate some previous findings [Robson, Clegg & Zammit (1984) Biochem. J. 217, 743-749; Williamson, Munday, Jones, Roberts & Ramsey (1983) Adv. Enzyme Regul. 21, 135-145], questioning the role of cyclic AMP in the regulation of lipogenesis in mammary acini.

The relationship between adenosine diphosphate-ribosylation and mammary gland differentiation

Poly(adenosine diphosphate [ADP]-ribosyl)ation, although associated with differentiation in many systems, exhibited a reciprocal relationship with mammary gland differentiation, and both the synthetic and degradatory pathways complemented each other in this regard. Poly(ADP-ribosyl)synthetase activity declined during pregnancy and lactation, while poly(ADP-ribose) degradatory activity rose late in pregnancy and peaked during lactation. In explant cultures, similar changes occurred and appeared to be under separate hormonal control; prolactin suppressed the synthetase activity, whereas insulin stimulated the poly(ADP-ribosyl)glycohydrolase activity. This latter effect may be mediated by a decline in cAMP levels for the following reasons: the glycohydrolase is known to be inhibited by cAMp, insulin decreased cAMP concentrations in mammary explants by 70%, and cholera toxin blocked the effects of insulin on poly(ADP-ribose) degradation. This reciprocal relationship between poly(ADP-ribosyl)ation and mammary gland differentiation is further supported by pharmacological studies: in the presence of insulin, cortisol, and prolactin, an inhibitor of the synthetase stimulated alpha-lactalbumin three-fold over hormone stimulation alone. However, this inhibitor was unable to induce differentiation in the absence of prolactin. Therefore, although there is a close association between a decline in enzyme activity and mammary differentiation, the data are insufficient to support a causal relationship.