Effect of endocrine and paracrine factors on protein synthesis and cell proliferation in bovine hoof tissue culture

Laminitis is a major cause of lameness in dairy cattle, and is widely attri buted to a defect in the horny tissue that gives the hoof its mechanical st rength. Defective horn is associated with, and may be preceded by, impaired keratin deposition in the hoof epidermis. The cause of abnormal keratin de position is not easily identified but, like epidermal keratinization in oth er tissues, is likely to be controlled by hormones and the paracrine action of locally produced growth factors. The hormonal regulation of keratin syn thesis and cell proliferation in the bovine hoof was studied using tissue e xplants in organ culture. As the highest incidence of laminitis is in early lactation, the study focused on insulin, cortisol and prolactin, three hor mones implicated in lactogenesis and galactopoiesis. Incubation of tissue e xplants for 24 h in medium containing insulin (10-5000 ng/ml) stimulated pr otein synthesis measured by incorporation of S-35-labelled amino acids. His tochemical examination showed that insulin binding co-localized with the si te of protein synthesis. Insulin also stimulated DNA synthesis, an index of cell proliferation, which was measured by incorporation of [H-3]methyl thy midine. Cortisol (10-5000 ng/ml) decreased protein synthesis, whereas prola ctin (10-5000 ng/ml) had no significant effect on protein or DNA synthesis. Epidermal growth factor (10-200 ng/ml), a potent inhibitor of keratinizati on in other tissues, stimulated protein synthesis compared with untreated c ontrols. Epidermal growth factor binding was located microscopically to the germinal and differentiating epidermal lavers. SDS-PAGE and fluorography s howed that the population of proteins synthesized in the presence of any ho rmone or growth factor combination did not differ from that in untreated co ntrols and included the keratins involved in horn deposition. The results s how that bovine hoof keratinization is under endocrine and growth factor co ntrol, and suggest that systemic changes in lactogenic hormones may act to inhibit keratin deposition.