Adenosine-mediated inhibition of casein production by mouse mammary glands in culture

Abstract

The present study was carried out to examine whether activation of adenosine receptors by adenosine analogues will affect casein production by mouse mammary epithelial cells. The morphogenesis and functions of epithelial tissue in the mammary gland are influenced by their surrounding adipocytes. Adipocytes are known to release adenosine into the extracellular fluid which can modulate cyclic-AMP levels in surrounding cells through binding to their adenosine receptors. To examine a possible paracrine effect of adenosine, the modulation of casein production in mammary explant culture and mammary epithelial cell (MEC) culture by adenosine receptor agonists has been investigated. We have observed that activation of the A1-adenosine receptor subtype in mammary tissue by an adenosine analogue (-)N6-(R-phenyl-isopropyl)-adenosine (PIA) raised cAMP levels. PIA and another adenosine receptor agonist, isobutylmethylxanthine (IBMX), inhibited casein accumulation both in explants and in MEC cultures in the presence of lactogenic hormones, which suggests that PIA or adenosine can act directly on the epithelial cells. This inhibition does not appear to be caused by elevation of cAMP levels or phosphodiesterase activity. The inhibition of intracellular casein accumulation by PIA and IBMX in explant cultures can be reversed via treatment of pertussis toxin which is known to ADP-ribosylate GTP-binding G alpha i-proteins, indicating that a Gi-protein-dependent pathway may be involved in this inhibition. The results also suggest that local accumulation of adenosine in the extracellular fluids of mammary glands is likely to inhibit the lactogenic response of mammary epithelial cells.

Effects of adenosine 5'-triphosphate and growth hormone on cellular H+ transport and calcium ion concentrations in cloned bovine mammary epithelial cells

“…This subclass of adenosine receptors is known to be responsive to ATP (Burnstock 1990). However, the action of purinergic receptors was probably not caused by cellular cyclic AMP because PIA (N 6phenylisopropyladenosine), an anti-lipolytic agonist, increased (Hom et al 1996), while adenosine suppressed (Saggerson 1980, Vernon et al 1983, the level of cellular cyclic AMP and this anti-lipolytic action was not mimicked by a cyclic AMP derivative in adipocytes (Hom et al 1996). This is also likely to be the case in the mammary epithelial cells of the present experiment because of the swiftness of the onsets of cellular responses to ATP and GH.…”

“…Additionally, ATP is reported to be secreted at 1 µmol/l in response to the mechanical stimulation of repeated pipetting of mammary tumor cells, and to cause increased cellular [Ca 2+ ] c in tumor and normal mouse MEC (Enomoto et al 1991, Furuya et al 1993, Enomoto et al 1994 and myoepithelial cells (Nakano et al 1997). Long-term or chronic exposure of mouse mammary explants and MEC to adenosine analogues has been shown to suppress casein synthesis through A 1 -receptors (Hom et al 1996). This subclass of adenosine receptors is known to be responsive to ATP (Burnstock 1990).…”

Serotonin: A Local Regulator in the Mammary Gland Epithelium

“…Two classes of mechanosensory signaling pathways that could participate in monitoring tissue and cellular stretch are transient receptor potential channels (TRPs; e.g., TRPC and TRPV) and purinergic receptors (e.g., P2X, P2Y) (51,(65)(66)(67). Both of these signaling classes enable tissue-level responses to a variety of relatively nonspecific physiological state changes, including stretch.…”

“…However, the few studies that have been conducted suggest that they may play an important physiological role. Purinergic signaling has been shown to inhibit milk protein gene expression (67) and increase myoepithelial responsiveness to OT (68,69).…”

Gene expression and hormonal regulation of adiponectin and its receptors in bovine mammary gland and mammary epithelial cells

“…The growth and development of the mammary gland, which consists of parenchyma and stroma, are drastically changed by pregnancy and lactation stages. The adipocytes that exist in stroma secrete various cytokines (adipocytokines) and regulate the mammary gland (Hom et al . 1996; Yonezawa et al .…”