Clifford Robert Stevens

Xanthine oxidoreductase in human mammary epithelial cells: activation in response to inflammatory cytokines.

Xanthine oxidoreductase (XOR) in human mammary epithelial cells was shown to have low true specific activity, similar to that in breast milk. Enzymic activity was increased in response to inflammatory cytokines; increases of 2-2.5-fold being seen with TNF-alpha and IL-1beta and of approximately 8-fold with IFN-gamma. No significant increase was seen with IL-6. A combination of IFN-gamma and TNF-alpha, or of these two cytokines plus IL-1beta, led to responses representing the sum of those obtained by using the individual cytokines. The 8-fold increase in enzymic activity, stimulated by IFN-gamma, corresponded to only a 2-3-fold increase in specific mRNA, suggesting the possibility of post-translational activation; a possibility strongly supported by the corresponding 2-3-fold rise in XOR protein, as determined by ELISA. In no case was cytokine-induced activation accompanied by changes in the oxidase-dehydrogenase ratio of XOR. These data strongly support a role for XOR in the inflammatory response of the human mammary epithelial cell, and provide further evidence of post-translational activation of a low activity form of human XOR, similar to that previously observed in vivo for the breast milk enzyme.

Antibacterial properties of xanthine oxidase in human milk

Formula-fed babies contract gastroenteritis more than breast-fed babies, which is of concern to mothers who cannot breastfeed or, as with HIV-infected mothers, are discouraged from breastfeeding. The ability of endogenous breastmilk xanthine oxidase to generate the antimicrobial radical nitric oxide has been measured and its influence on the growth of Escherichia coli and Salmonella enteritides examined. Breastmilk, but not formula feed, generated nitric oxide. Xanthine oxidase activity substantially inhibited the growth of both bacteria. An important natural antibiotic system is missing in formula feeds; the addition of xanthine oxidase may improve formula for use when breastfeeding is not a safe option.

CELLULAR BIOLOGY OF BONE RESORPTION

Past knowledge and the recent developments on the formation, activation and mode of action of osteoclasts, with particular reference to the regulation of each individual step, have been reviewed. The following conclusions of consensus have emerged.

Osteoclast function and its control

Bone resorption appears to be dependent on a range of processes. It requires an adequate number of osteoclasts to access bone mineral. These osteoclasts must be activated by a mechanism which is dependent upon prior osteoblastic stimulation. A range of factors then contribute to the formation of a functionally effective resorptive hemivacuole. These entail osteoclast adhesion to the bone surface leading to the formation of a sealing zone. Only then can subsequent processes such as H+ ion transport, enzyme secretion and matrix digestion become effective. Thus, any one process is potentially limiting to resorption and is a potential target for regulation. Long‐range regulation takes place through the action of hormones, of which the mode of action of calcitonin has been the subject of recent investigations in isolated osteoclasts. Such studies have shown a possible involvement of distinguishable receptor subtypes, the occupancy of which may activate at least two types of triggering mechanism. It is likely that an eventual influence on motility properties through G protein mediation accounts for the actions of this hormone and of related peptides such as amylin and CGRP at the cellular level. Similar pathways may contribute to shorter range modulation of osteoclast activity by increases in ambient Ca2+. Finally, there is recent evidence for a contribution of endothelial cell‐derived product to osteoclast regulation.