Intercellular adhesion molecule-1 (ICAM-1; CD54) expression in human hepatocytic cells depends on protein kinase C

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.

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.

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.

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.