INDUCTION OF APOPTOSIS IN HUMAN HACAT KERATINOCYTES

Although cell death by apoptosis has been recognized as an important c ontrol mechanism in the maintenance of tissue homeostasis and in the e limination of cells with damaged DNA, information on the induction and characteristics of apoptosis in keratinocytes is rather scarce. Apopt otic mechanisms may play an important role in normal and disturbed hom eostasis of the skin, In the present study, we therefore investigated tile effects of several potential inducers of apoptosis in the human k eratinocyte cell line HaCaT. Apoptosis was assessed with respect to mo rphological changes by light and electron microscopic examinations and to DNA integrity by a specific ELISA. UVB irradiation induced the mor phology and internucleosomal DNA fragmentation characteristic of apopt osis in a dose- and time-dependent manner. Interferon-gamma ceased DNA cleavage at the linker regions without producing morphological featur es consistent with apoptotic cell death. In contrast, treatment with d ithranol and NP-40 resulted in necrotic alterations in the keratinocyt es. Treatment with the calcium ionophore A23187 caused morphological c hanges which were similar to the characteristics of 'nonapoptotic prog rammed cell death'. Dexamethasone, tumor necrosis factor-alpha, transf orming growth factor beta, TPA, retinoic acid, the podophyllin derivat ive etoposide, the thromboxane A(2) analogue U46619, cycloheximide, an d the nitric oxide donors sodium nitroprusside and S-nitroso-glutathio ne, which are all known to induce apoptosis in other cell types, did n ot affect HaCaT keratinocytes. These results demonstrate that apoptosi s can be induced in keratinocytes in vitro but the apoptosis differs f rom that in other cell types, such as haematopoietic cells, with regar d to the type of inducer and/or the sensitivity of the target cells. S ince keratinocytes are affected by numerous external and internal stim uli, they might posses several protective mechanisms to prevent apopto sis and to ensure the structural integrity of the outer most barrier o f the body.