TUMOR NECROSIS FACTOR-ALPHA-INDUCED APOPTOSIS IN A HUMAN KERATINOCYTECELL-LINE (HACAT) IS COUNTERACTED BY TRANSFORMING GROWTH-FACTOR-ALPHA

The integrity of the human epidermis is guaranteed by a regulated bala nce of proliferation, differentiation, and physiologic cell death of i ts main cellular constituent, the epidermal keratinocyte. Physiologic cell death is known as apoptosis and has been recognized as an active regulatory mechanism, complementary to, but functionally opposite of, proliferation. The regulators of the delicate balance between cell dea th and proliferation are only partially understood in human keratinocy tes. Transforming growth factor-alpha (TGF-alpha) has been identified as a positive regulator of proliferation and growth, while tumor necro sis factor-alpha (TNF-alpha) induces apoptosis. Both mediators are tho ught to influence epidermal keratinocytes under various physiological and pathophysiological conditions. In the current study we have begun to investigate potential regulatory interactions between these two med iators in the human keratinocyte cell line HaCaT. We have found that, when the HaCaT cells were sensitized by the translation inhibitor cycl oheximide, TNF-alpha induced apoptosis, as evidenced by nuclear disint egration, DNA fragmentation (''DNA laddering''), and the appearance of soluble DNA/histone complexes. Moreover, we found that the induction of apoptosis was reduced by preincubation of the cells with TGF-alpha. The protective effect of TGF-alpha was abrogated by translation inhib ition, indicating that it depended on de novo protein synthesis. Moreo ver, the protective effect was not accompanied by a reduced surface ex pression of TNF receptor molecules. We postulate that TNF-alpha-induce d apoptosis in HaCaT cells is counteracted by constitutively produced suppressors of apoptosis, the synthesis of which can be downregulated by inhibition of translation and upregulated by the cytokine TGF-alpha . (C) 1996 Academic Press, Inc.