Background/aims: Intracellular regulation of intercellular adhesion molecule-1 has mainly been studied in lymphoid, endothelial, and epithelial cells. Intercellular adhesion molecule-1 plays a central role in many immune responses, and we have previously studied its regulation in hepatocytes. Here we report how manipulation of intracellular signal systems influenced its expression.
Methods: The constitutive and cytokine-induced expression of intercellular adhesion molecule-1 mRNA and protein was studied in the human hepatocytic cell lines Hep G2 and SK-Hep-1.
Results: When agonists and antagonists of protein kinase C, calmodulin, and protein kinase A were introduced in addition to prostaglandin E2 and a cyclooxygenase inhibitor, only the protein kinase C activator phorbol 12-myristate 13-acetate resulted in a rapid and dose-dependent increase in intercellular adhesion molecule-1 protein and mRNA. Phorbol 12-myristate 13-acetate stimulated sustained high levels of intercellular adhesion molecule-1 protein, whereas the corresponding mRNA response was biphasic, peaking at 3 h. Actinomycin D blocked the stimulatory mRNA phase, suggesting that de novo transcription was induced. Coincubation with phorbol 12-myristate 13-acetate and the protein synthesis inhibitor cycloheximide gave considerably higher mRNA levels than with phorbol 12-myristate 13-acetate alone. Protein kinase C may therefore even stimulate synthesis of proteins that speed up the turnover of intercellular adhesion molecule-1 mRNA. The protein kinase C inhibitor staurosporine abrogated the induction of intercellular adhesion molecule-1 by phorbol 12-myristate 13-acetate, indicating that this effect was indeed exerted by protein kinase C. More original was our observation that staurosporine also completely blocked the stimulatory effects of interferon-gamma, tumour necrosis factor-alpha, and interleukin-1. Recent reports have noted that these cytokines apparently use receptors which activate different intracellular pathways. We also noted that the glucocorticoid dexamethasone partially inhibited the stimulation of intercellular adhesion molecule-1 by these cytokines. This phenomenon could be important for the immunosuppressive effects of corticosteroids in patients with liver disease.
Conclusions: Our data suggest that a certain level of protein kinase C activity is mandatory for liver cells in cytokine-mediated upregulation of intercellular adhesion molecule-1.