Although gastrointestinal cancers are frequently associated with chronic inflammation, the underlying molecular links have not been comprehensively deciphered. Using loss- and gain-of-function mice in a colitis-associated cancer model, we establish here a link comprising the gp130/Stat3 transcription factor signaling axis. Mutagen-induced tumor growth and multiplicity are reduced following intestinal epithelial cell (IEC)-specific Stat3 ablation, while its hyperactivation promotes tumor incidence and growth. Conversely, IEC-specific Stat3 deficiency enhances susceptibility to chemically induced epithelial damage and subsequent mucosal inflammation, while excessive Stat3 activation confers resistance to colitis. Stat3 has the capacity to mediate IL-6- and IL-11-dependent IEC survival and to promote proliferation through G1 and G2/M cell-cycle progression as the common tumor cell-autonomous mechanism that bridges chronic inflammation to tumor promotion.

Protein kinase C (PKC) isozymes are key signal transducers involved in normal physiology and disease and have been widely implicated in cancer progression. Despite our extensive knowledge of the signaling pathways regulated by PKC isozymes and their effectors, there is essentially no information on how individual members of the PKC family regulate gene transcription. Here, we report the first PKC isozyme-specific analysis of global gene expression by microarray using RNAi depletion of diacylglycerol/phorbol ester-regulated PKCs. A thorough analysis of this microarray data revealed unique patterns of gene expression controlled by PKC␣, PKC␦, and PKC⑀, which are remarkably different in cells growing in serum or in response to phorbol ester stimulation. PKC␦ is the most relevant isoform in controlling the induction of genes by phorbol ester stimulation, whereas PKC⑀ predominantly regulates gene expression in serum. We also established that two PKC␦-regulated genes, FOSL1 and BCL2A1, mediate the apoptotic effect of phorbol esters or the chemotherapeutic agent etoposide in prostate cancer cells. Our studies offer a unique opportunity for establishing novel transcriptional effectors for PKC isozymes and may have significant functional and therapeutic implications.

Bryostatin 1, a macrocyclic lactone that has been widely characterized as an ultrapotent protein kinase C (PKC) activator, displays marked pharmacological differences with the typical phorbol ester tumor promoters. Bryostatin 1 impairs phorbol 12-myristate 13-acetate (PMA)-induced tumor promotion in mice and is in clinical trials as an anticancer agent for a number of hematopoietic malignancies and solid tumors. In this study, we characterized the effect of bryostatin 1 on LNCaP prostate cancer cells, a cellular model in which PKC isozymes play important roles in the control of growth and survival. Although phorbol esters promote a strong apoptotic response in LNCaP cells via PKC␦-mediated release of TNF␣, bryostatin 1 failed to trigger a death effect even at high concentrations, and it prevented PMA-induced apoptosis in these cells. Mechanistic analysis revealed that bryostatin 1 is unable to induce TNF␣ release, and it impairs the secretion of this cytokine from LNCaP cells in response to PMA. Unlike PMA, bryostatin 1 failed to promote the translocation of PKC␦ to the plasma membrane. Moreover, bryostatin 1 prevented PMA-induced PKC␦ peripheral translocation. Studies using a membrane-targeted PKC␦ construct revealed that the peripheral localization of the kinase is a requisite for triggering apoptosis in LNCaP cells, arguing that mislocalization of PKC␦ may explain the actions of bryostatin 1. The identification of an antiapoptotic effect of bryostatin 1 may have significant relevance in the context of its therapeutic efficacy.

Protein kinase C⑀ (PKC⑀), a diacyglycerol-and phorbol esterresponsive serine-threonine kinase, has been implicated in mitogenic and survival control, and it is markedly overexpressed in human tumors, including in prostate cancer. Although prostate cancer cells undergo apoptosis in response to phorbol ester stimulation via PKC␦-mediated release of death factors, the involvement of PKC⑀ in this response is not known. PKC⑀ depletion by RNAi Protein kinase C (PKC), 3 a family of serine-threonine kinases that comprises the classical (cPKCs ␣, ␤, and ␥), novel (nPKCs ␦, ⑀, , and ), and atypical (aPKCs and ) PKCs, is a key signaling component of growth factor and cytokine pathways. Despite their structural similarities, PKC isozymes have unique modes of regulation as well as differential patterns of cell and tissue expression (1, 2). Only cPKCs and nPKCs are regulated by phorbol esters and diacylglycerol, a lipid second messenger generated upon activation of G protein-coupled receptors and tyrosine kinases (1-3). Phorbol esters are capable of promoting opposite responses in different cell types, such as mitogenesis/ survival versus growth arrest/apoptosis. This paradigm of functional diversity is exemplified by the nPKCs: whereas in most cases PKC⑀ acts as a mitogenic or antiapoptotic kinase, activation of PKC␦ inhibits proliferation or triggers an apoptotic response (4 -8). PKC⑀ can signal to mitogenesis via Raf/MEK/ ERK and cyclin D1 induction (9, 10) or can even transform cells (4, 6, 11). In addition, PKC⑀ has been linked to cell survival through the activation of Akt and Bax (12,13).Tumor cells display in many cases an altered balance in PKC isozyme expression, potentially reflecting the involvement of PKCs in the etiology and progression of cancer. Most notably, many cancer cells show marked up-regulation of PKC⑀. PKC⑀ is elevated in prostate cancer, particularly in high grade tumors, and is implicated in prostate tumor progression and the transition to androgen-independence (14 -17). The functional complexity of PKC in prostate cancer cells is nonetheless highlighted by the fact that phorbol esters promote an apoptotic response in androgen-dependent prostate cancer cells (18,19). Previous work from our laboratory and others established that PKC␦ is the major mediator of the death effect of PMA in LNCaP cells (19 -21). In this context, the role of PKC⑀ remains controversial, as unlike in other cell types, PKC⑀ was found to be either dispensable for prostate cancer cell survival or even contribute to the proapoptotic effects of PKC activators (22-24). Given our limited understanding of the mechanistic insights of PKC-driven responses in prostate cancer cells, additional studies would be required to ascertain the specific contribution of PKC isozymes to this paradox.We have recently demonstrated that PKC␦-mediated prostate cancer cell death involves the activation of an apoptotic autocrine loop through the release of death factors (primarily TNF␣), and the subsequent activation of the extrinsic apoptotic cascade ...

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