Thapsigargin Induces Rapid, Transient Growth Inhibition and c-fos Expression Followed by Sustained Growth Stimulation in Mouse Keratinocyte Cultures

Although the sesquiterpene lactone thapsigargin has been shown to possess hyperplastic and tumor-promoting activities when applied topically to mouse skin in vivo , the cellular mechanism(s) which underlie these effects are unclear. We show here that thapsigargin treatment of primary mouse epidermal keratinocytes increased intracellular free Ca 2+ concentration (Ca i ) in a concentration-dependent manner. Thapsigargin induced a rapid, transient elevation in keratinocyte Ca i , in part due to the release of Ca 2+ from intracellular stores. This response was followed by a sustained elevation in Ca 2+ , resulting entirely from calcium influx. Thapsigargin elicited a biphasic effect on keratinocyte DNA synthesis: a rapid inhibitory effect (50–60% inhibition at 4–8 h), followed by a very marked and sustained elevation. Prolonged treatment of keratinocytes with thapsigargin at relatively high concentrations resulted in cytotoxicity (inhibition of neutral red uptake). The rapid antiproliferative effect of thapsigargin was not associated with cytotoxicity, as determined by either neutral red uptake or by trypan blue exclusion, and was not blocked by pretreatment with Ro 31–7549, a selective inhibitor of protein kinase C. The rapid antiproliferative effect of thapsigargin was associated with rapid, transient activation of keratinocyte c-fos expression and rapid inhibition of total protein synthesis. Taken together, these findings raise the possibility that the hyperplastic and tumor-promoting activities of thapsigargin on epidermis in vivo result from direct keratinocyte growth stimulation as a consequence of a prolonged elevation in levels of Ca i .