No abstract

UV radiation from the sun activates both the membrane death receptor and the intrinsic or mitochondrial apoptotic signaling pathways in epidermal keratinocytes, triggering apoptosis and affording protection against skin cancer formation. We have investigated the involvement of caspase-9 in the UV death effector pathway in human keratinocytes, since this is the initiating caspase in the mitochondrial pathway required for UVinduced apoptosis in some, but not all, cell types. UV radiation triggered activation of caspase-3, caspase-9, and caspase-8 with similar kinetics, although the rank order of activation was caspase-3 > caspase-9 > caspase-8. Inhibition of caspase-9 with either the peptide inhibitor benzyloxycarbonyl-Leu-Glu(OCH 3 )-HisAsp(OCH 3 )-fluoromethyl ketone, or expression of a catalytically inactive caspase-9 by retroviral transduction, protected normal keratinocytes from UV-induced apoptosis. HaCaT keratinocytes harboring mutant p53 alleles were also protected from UV-induced apoptosis by the dominant negative caspase-9. The dominant negative caspase-9 blocked UV-induced activation of caspase-3, caspase-9, and caspase-8, and also protected cells from the loss of mitochondrial membrane potential. In contrast, the dominant negative caspase-9 did not protect from anti-Fas-induced apoptosis or caspase activation. These results identify caspase-9 as the critical upstream caspase initiating apoptosis by UV radiation in human keratinocytes, the relevant cell type for this important environmental carcinogen.

By deciphering the dysregulation of apoptosis in melanoma cells, new treatment approaches exploiting aberrant control mechanisms regulating cell death can be envisioned. Among the Bcl-2 family, a BH3-only member, NOXA, functions in a specific mitochondrial-based cell death pathway when melanoma cells are exposed to a proteasome inhibitor (e.g., bortezomib). Some therapeutic agents, such as bortezomib, not only induce proapoptotic Bcl-2 family members and active conformational changes in Bak and Bax but also are associated with undesirable effects, including accumulation of antiapoptotic proteins, such as Mcl-1. To enhance the bortezomib-mediated killing of melanoma cells, the apoptotic pathway involving NOXA was further investigated, leading to identification of an important target (i.e., the labile Bcl-2 homologue Mcl-1 but not other survival proteins). To reduce Mcl-1 levels, melanoma cells were pretreated with several different agents, including Mcl-1 small interfering RNA (siRNA), UV light, or the purine nucleoside analogue fludarabine. By simultaneously triggering production of NOXA (using bortezomib) as well as reducing Mcl-1 levels (using siRNA, UV light, or fludarabine), significantly enhanced killing of melanoma cells was achieved. These results show binding interactions between distinct Bcl-2 family members, such as NOXA and Mcl-1, in melanoma cells, paving the way for novel and rational therapeutic combination strategies, which target guardians of the proapoptotic Bak-and Baxmediated pathways, against this highly aggressive and often fatal malignancy.

The tissue-specific expression pattern and the intracellular distribution of the Ca(2+)-binding protein calreticulin at the mRNA and protein levels have been studied during somatic and zygotic embryogenesis of Nicotiana plumbaginifolia Viv. A full-length cDNA sequence encoding calreticulin was isolated from a lembda Zap cDNA library from early developmental stages of somatic embryogenesis. The deduced amino acid sequence of the calreticulin from N. plumbaginifolia shows high homology to the corresponding proteins of tobacco (98.2% identity), maize (80%) and barley (76.5%), and more than 55% homology to animal calreticulins, and the sequence motifs with established functions found in calreticulins of other species were quite conserved. Northern experiments revealed a developmental regulation of the calreticulin transcript with a maximum during the early stages of somatic embryogenesis and an auxin dependence during in-vitro cell culture. alpha-Naphthaleneacetic acid stimulated calreticulin expression whereas 2,4-dichlorophenoxyacetic acid reduced it. Immunohistological analysis of calreticulin distribution in the ovaries during zygotic embryogenesis showed that calreticulin biosynthesis started tissue specifically, with a high abundance in the endothelium of the integument in the ovules, followed by calreticulin accumulation in the embryo proper and in the associated endosperm at the late globular stage of embryogenesis. Using immunogold labeling, calreticulin was intracellularly localized with a high abundance to the Golgi compartment and to patches on the surface of dividing protoplasts. Smaller amounts were found in the endoplasmic reticulum and plasma membranes. The functional role of calreticulin in posttranslational processing and translocation processes, apart from its postulated function in cellular Ca2+ homeostasis, is discussed.

No abstract

The constitutively active catalytic domain of protein kinase C (PKC)delta is an apoptotic effector generated by caspase-3 cleavage of full-length PKCdelta in response to a wide variety of apoptotic stimuli, including UV radiation. The PKCdelta catalytic domain induces apoptosis when ectopically expressed, however, the mechanism of apoptosis induction is unclear. We constructed a chimeric protein encoding the PKCdelta catalytic domain fused to a mutated estrogen receptor ligand-binding domain in order to selectively activate the PKCdelta catalytic domain. The enzymatic activity of the PKCdelta catalytic domain fusion protein was induced in human keratinocytes treated with 4-hydroxytamoxifen, and its activation triggered loss of mitochondrial membrane potential and apoptosis. The apoptosis was associated with release of cytochrome c from the mitochondria and caspase activation, and was blocked by caspase inhibitors and the anti-apoptotic proteins Bcl-2, and Bcl-x(L), suggesting a role for mitochondrial pore formation. Consistent with this, the activated PKCdelta catalytic domain triggered the redistribution and activation of Bax, a Bcl-2 family protein that can directly induce cytochrome c release. In summary, despite being an apoptotic effector activated late in the apoptotic cascade, PKCdelta also activates upstream components of the death effector pathway to insure the demise of cells committed to apoptosis.

Ultraviolet (UV) light exposure is a common cause of epithelial-derived skin cancers, and the epidermal response to UV-light has been extensively studied using both mouse models and cultured human keratinocytes (KCs). Elimination of cells with UV-induced DNA damage via apoptosis provides a powerful mechanism to minimize retention or expansion of genetically abnormal cells. This cell editing function has largely been ascribed to the biological role of the p53 tumor suppressor gene, as mutations or deletions involving p53 have been linked to skin cancer development. Rather than introducing mutations, or using cells with complete loss of wild-type p53, we used an siRNA-based approach to knockdown, but not eliminate, p53 levels in primary cultures of human KCs followed by UV-irradiation. Surprisingly, when p53 levels were reduced by 50-80% the apoptosis induced by exposure to UV-light was accelerated and markedly enhanced (two-to three-fold) compared to control siRNA treated KCs. The p53 siRNA treated KCs were characterized by elevated E2F-1 levels accompanied by accelerated elimination of the Mcl-1 and Bcl-x L antiapoptotic proteins, as well as enhanced Bax oligomerization. Forced overexpression of either Mcl-1 or Bcl-x L reduced the UV-light enhanced apoptotic response in p53 siRNA treated KCs. We conclude that p53 not only can provide proapoptotic signals but also regulates a survival pathway influencing Mcl-1 and Bcl-x L levels. This overlooked survival function of p53 may explain previous paradoxical responses noted by investigators using p53 heterozygous and knockout mouse models, and opens up the possibility that not all liaisons within the cell involving p53 necessarily represent fatal attractions.

SummaryProtein kinase C (PKC) is a heterogeneous family of serine ⁄ ⁄ threonine protein kinases that have different biological effects in normal and neoplastic melanocytes (MCs). To explore the mechanism behind their differential response to PKC activation, we analyzed the expression profile of all nine PKC isoforms in normal human MCs, HPV16 E6 ⁄ ⁄ E7 immortalized MCs, and a panel of melanoma cell lines. We found reduced PKCb and increased PKCf and PKCi expression at both the protein and mRNA levels in immortalized MCs and melanoma lines. We focused on PKCb as it has been functionally linked to melanin production and oxidative stress response. Re-expression of PKCb in melanoma cells inhibited colony formation in soft agar, indicating that PKCb loss in melanoma is important for melanoma growth. PKCbII, but not PKCbI, was localized to the mitochondria, and inhibition of PKCb significantly reduced UV-induced reactive oxygen species (ROS) in MCs with high PKCb expression. Thus alterations in PKCb expression in melanoma contribute to their neoplastic phenotype, possibly by reducing oxidative stress, and may constitute a selective therapeutic target.