Precise regulation of porcupine activity is required for physiological Wnt signaling.

Gradients of diverse Wnt proteins regulate development, renewal, and differentiation. Porcupine (PORCN) is a membrane-bound O-acyltransferase that is required for post-translational modification of all Wnts to enable their transport, secretion, and activity. Mutations in PORCN are associated with focal dermal hypoplasia (FDH), whereas gene deletion causes embryonic lethality in mice. To study the protein in more detail, zinc finger nucleases were used to edit the PORCN genomic locus, establishing two HT1080 fibrosarcoma clones null for PORCN activity that facilitate the study of PORCN structure and function. We establish that PORCN is a key non-redundant node for the regulation of global Wnt signaling because PORCN null cells are completely incapable of autocrine Wnt signaling. The strength of Wnt signaling is exquisitely sensitive to PORCN expression, with a dynamic range of at least 3 orders of magnitude, suggesting that PORCN activity is a key modulator of all Wnt ligand activity. Consistent with this, we find that multiple FDH-associated mutants have only subtle alterations in enzyme activity yet are associated with a severe FDH phenotype. These studies support an essential regulatory role of PORCN in shaping Wnt signaling gradients.

PORCN moonlights in a Wnt-independent pathway that regulates cancer cell proliferation.

Porcupine (PORCN) is a membrane-bound O-acyl transferase that is required for the palmitoylation of Wnt proteins, and that is essential in diverse Wnt pathways for Wnt-Wntless (WLS) binding, Wnt secretion, and Wnt signaling activity. We tested if PORCN was required for the proliferation of transformed cells. Knockdown of PORCN by multiple independent siRNAs results in a cell growth defect in a subset of epithelial cancer cell lines. The growth defect is transformation-dependent in human mammary epithelial (HMEC) cells. Additionally, inducible PORCN knockdown by two independent shRNAs markedly reduces the growth of established MDA-MB-231 cancers in orthotopic xenografts in immunodeficient mice. Unexpectedly, the proliferation defect resulting from loss of PORCN occurs in a Wnt-independent manner, as it is rescued by re-expression of catalytically inactive PORCN, and is not seen after RNAi-mediated knockdown of the Wnt carrier protein WLS, nor after treatment with the PORCN inhibitor IWP. Consistent with a role in a Wnt-independent pathway, knockdown of PORCN regulates a distinct set of genes that are not altered by other inhibitors of Wnt signaling. PORCN protein thus appears to moonlight in a novel signaling pathway that is rate-limiting for cancer cell growth and tumorigenesis independent of its enzymatic function in Wnt biosynthesis and secretion.

Chemical genetics analysis of an aniline mustard anticancer agent reveals complex I of the electron transport chain as a target.

The antitumor agent 11ß (CAS 865070-37-7), consisting of a DNA-damaging aniline mustard linked to an androgen receptor (AR) ligand, is known to form covalent DNA adducts and to induce apoptosis potently in AR-positive prostate cancer cells in vitro; it also strongly prevents growth of LNCaP xenografts in mice. The present study describes the unexpectedly strong activity of 11ß against the AR-negative HeLa cells, both in cell culture and tumor xenografts, and uncovers a new mechanism of action that likely explains this activity. Cellular fractionation experiments indicated that mitochondria are the major intracellular sink for 11ß; flow cytometry studies showed that 11ß exposure rapidly induced oxidative stress, mitochondria being an important source of reactive oxygen species (ROS). Additionally, 11ß inhibited oxygen consumption both in intact HeLa cells and in isolated mitochondria. Specifically, 11ß blocked uncoupled oxygen consumption when mitochondria were incubated with complex I substrates, but it had no effect on oxygen consumption driven by substrates acting downstream of complex I in the mitochondrial electron transport chain. Moreover, 11ß enhanced ROS generation in isolated mitochondria, suggesting that complex I inhibition is responsible for ROS production. At the cellular level, the presence of antioxidants (N-acetylcysteine or vitamin E) significantly reduced the toxicity of 11ß, implicating ROS production as an important contributor to cytotoxicity. Collectively, our findings establish complex I inhibition and ROS generation as a new mechanism of action for 11ß, which supplements conventional DNA adduct formation to promote cancer cell death.