Phorbol Ester-induced Apoptosis in Prostate Cancer Cells via Autocrine Activation of the Extrinsic Apoptotic Cascade

Abstract

It is well established that activation of protein kinase C (PKC) by phorbol esters promotes apoptosis in androgen-dependent prostate cancer cells. However, there is limited information regarding the cellular mechanisms involved in this effect. In this report we identified a novel autocrine pro-apoptotic loop triggered by PKC␦ activation in prostate cancer cells that is mediated by death receptor ligands. The apoptotic effect of phorbol 12-myristate 13-acetate in LNCaP cells was impaired by inhibition or depletion of tumor necrosis factor alpha-converting enzyme, the enzyme responsible for tumor necrosis factor ␣ (TNF␣) shedding. Moreover, the apoptogenic effect of conditioned medium collected after phorbol 12-myristate 13-acetate treatment could be inhibited by blocking antibodies against TNF␣ and tumor necrosis factor-related apoptosisinducing ligand (TRAIL), but not FasL, as well as by RNA interference depletion of TNF␣ and TRAIL receptors. Moreover, depletion or inhibition of death receptor downstream effectors, including caspase-8, FADD, p38 MAPK, and JNK, significantly reduced the apoptogenic effect of the conditioned medium. PKC␦ played a major role in this autocrine loop, both in the secretion of autocrine factors as well as a downstream effector. Taken together, our results demonstrate that activation of PKC␦ in prostate cancer cells causes apoptosis via the release of death receptor ligands and the activation of the extrinsic apoptotic cascade. Protein kinase C (PKC)2 isozymes, a family of at least 10 related serine-threonine kinases, play important roles in the regulation of various cellular processes, including differentiation, proliferation, and malignant transformation, and have been widely implicated in the progression of cancer. This family of signaling kinases comprises the classical (␣, ␤I, ␤II, and ␥), novel (␦, ⑀, , and ), and atypical ( and /i) PKCs, which have differential patterns of cell and tissue distribution and unique modes of regulation (1). Phorbol esters, natural compounds that potently activate the classical and novel PKCs, trigger a plethora of cellular responses that vary depending on the cell type and the relative expression of individual PKC isozymes. Whereas phorbol esters are capable of promoting mitogenic or survival responses, many cell types undergo growth arrest or apoptosis in response to PKC activation. A major reason for such heterogeneity is the diversity of pathways activated by each PKC isozyme, their distinct relocalization, and their differential access to substrates upon activation (2). Such functional diversity is exemplified by the novel PKCs: whereas in most cases PKC⑀ acts as a mitogenic or anti-apoptotic kinase, PKC␦ generally inhibits proliferation, or in some cell types it triggers an apoptotic response and is required for drug-induced apoptosis (3). Dissecting the signaling events regulated by individual PKCs still represents a major challenge and will certainly help to understand the functional roles of PKC isozymes in normal and cancer cells.Androgen-de...

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“…Over cleavage prevented its antiapoptotic signal [51]. A down-regulation of PKC- ε by chronic exposure to a phorbol ester was associated with resistance to drug-induced apoptosis in human prostatic carcinoma cells [52]. However, consistently with our results, there is also evidence that suggest a pro-apoptotic role of PKC-ε.…”

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“…The phorbol esters themselves do not induce tumours but promote tumour growth following exposure to a subcarcinogenic dose of a carcinogen. They can thus be designated as co-carcinogens (Brodie and Blumberg, 2003;Gonzalez-Guerrico and Kazanietz, 2005).…”

ROCK Mediates Phorbol Ester-induced Apoptosis in Prostate Cancer Cells via p21Cip1 Up-regulation and JNK

“…Enzyme-linked Immunosorbent Assay-TNF␣ levels were determined by ELISA (Pepro Tech Inc.), essentially as described previously (11). Briefly, 100 l of conditioned medium (CM) were added into each well and incubated overnight at 4°C.…”

“…Our previous studies established that this effect is primarily mediated by PKC␦, and subsequent analysis revealed that this kinase promotes the activation of the extrinsic apoptotic cascade via an autocrine mechanism. PMA promotes the secretion of death factors from LNCaP cells via PKC␦, including TNF␣ and TRAIL, and the released factors promote cell death via activation of JNK and p38 MAPK cascades (11,12).…”