Phosphatidylserine synthesis controls oncogenic B cell receptor signaling in B cell lymphoma.

Cancer cells harness lipid metabolism to promote their own survival. We screened 47 cancer cell lines for survival dependency on phosphatidylserine (PS) synthesis using a PS synthase 1 (PTDSS1) inhibitor and found that B cell lymphoma is highly dependent on PS. Inhibition of PTDSS1 in B cell lymphoma cells caused a reduction of PS and phosphatidylethanolamine levels and an increase of phosphoinositide levels. The resulting imbalance of the membrane phospholipidome lowered the activation threshold for B cell receptor (BCR), a B cell-specific survival mechanism. BCR hyperactivation led to aberrant elevation of downstream Ca2+ signaling and subsequent apoptotic cell death. In a mouse xenograft model, PTDSS1 inhibition efficiently suppressed tumor growth and prolonged survival. Our findings suggest that PS synthesis may be a critical vulnerability of malignant B cell lymphomas that can be targeted pharmacologically.

Histone demethylase KDM5D upregulation drives sex differences in colon cancer.

Sex exerts a profound impact on cancer incidence, spectrum and outcomes, yet the molecular and genetic bases of such sex differences are ill-defined and presumptively ascribed to X-chromosome genes and sex hormones1. Such sex differences are particularly prominent in colorectal cancer (CRC) in which men experience higher metastases and mortality. A murine CRC model, engineered with an inducible transgene encoding oncogenic mutant KRASG12D and conditional null alleles of Apc and Trp53 tumour suppressors (designated iKAP)2, revealed higher metastases and worse outcomes specifically in males with oncogenic mutant KRAS (KRAS*) CRC. Integrated cross-species molecular and transcriptomic analyses identified Y-chromosome gene histone demethylase KDM5D as a transcriptionally upregulated gene driven by KRAS*-mediated activation of the STAT4 transcription factor. KDM5D-dependent chromatin mark and transcriptome changes showed repression of regulators of the epithelial cell tight junction and major histocompatibility complex class I complex components. Deletion of Kdm5d in iKAP cancer cells increased tight junction integrity, decreased cell invasiveness and enhanced cancer cell killing by CD8+ T cells. Conversely, iAP mice engineered with a Kdm5d transgene to provide constitutive Kdm5d expression specifically in iAP cancer cells showed an increased propensity for more invasive tumours in vivo. Thus, KRAS*-STAT4-mediated upregulation of Y chromosome KDM5D contributes substantially to the sex differences in KRAS* CRC by means of its disruption of cancer cell adhesion properties and tumour immunity, providing an actionable therapeutic strategy for metastasis risk reduction for men afflicted with KRAS* CRC.

Potent and Selective PTDSS1 Inhibitors Induce Collateral Lethality in Cancers with PTDSS2 Deletion.

SIGNIFICANCE: This study identifies a specific dependency on PTDSS1 for phosphatidylserine synthesis following PTDSS2 deletion and introduces novel PTDSS1 inhibitors as a therapeutic option to induce collateral lethality in cancer with PTDSS2 loss.

AR-negative prostate cancer is vulnerable to loss of JMJD1C demethylase.

Prostate cancer is a leading cause of cancer-related mortality in men. The widespread use of androgen receptor (AR) inhibitors has generated an increased incidence of AR-negative prostate cancer, triggering the need for effective therapies for such patients. Here, analysis of public genome-wide CRISPR screens in human prostate cancer cell lines identified histone demethylase JMJD1C (KDM3C) as an AR-negative context-specific vulnerability. Secondary validation studies in multiple cell lines and organoids, including isogenic models, confirmed that small hairpin RNA (shRNA)-mediated depletion of JMJD1C potently inhibited growth specifically in AR-negative prostate cancer cells. To explore the cooperative interactions of AR and JMJD1C, we performed comparative transcriptomics of 1) isogenic AR-positive versus AR-negative prostate cancer cells, 2) AR-positive versus AR-negative prostate cancer tumors, and 3) isogenic JMJD1C-expressing versus JMJD1C-depleted AR-negative prostate cancer cells. Loss of AR or JMJD1C generates a modest tumor necrosis factor alpha (TNFα) signature, whereas combined loss of AR and JMJD1C strongly up-regulates the TNFα signature in human prostate cancer, suggesting TNFα signaling as a point of convergence for the combined actions of AR and JMJD1C. Correspondingly, AR-negative prostate cancer cells showed exquisite sensitivity to TNFα treatment and, conversely, TNFα pathway inhibition via inhibition of its downstream effector MAP4K4 partially reversed the growth defect of JMJD1C-depleted AR-negative prostate cancer cells. Given the deleterious systemic side effects of TNFα therapy in humans and the viability of JMJD1C-knockout mice, the identification of JMJD1C inhibition as a specific vulnerability in AR-negative prostate cancer may provide an alternative drug target for prostate cancer patients progressing on AR inhibitor therapy.

Shank2 Binds to aPKC and Controls Tight Junction Formation with Rap1 Signaling during Establishment of Epithelial Cell Polarity.

Epithelial cells establish apicobasal polarity by forming tight junctions (TJs) at the apical-lateral boundary, which play fundamental roles in physiological functions. An evolutionarily conserved atypical protein kinase C (aPKC)-partitioning defective (PAR) complex functions as a platform for TJ assembly during cell polarity establishment. However, how this complex converts the spatial cues into a subsequent active unit is unclear. Here, we identify an epithelial isoform of Shank2 as a mediator of the aPKC-PAR complex. Shank2 binds to and colocalizes with aPKC at apical junctional regions of polarized epithelial cells. Shank2 knockdown results in defects in TJ formation. Mechanistically, we find that the N-terminal SPN domain is required for the junctional localization of Shank2 and binds to the active form of Rap1 small GTPase, which is involved in TJ formation. Our findings suggest that a close physical and functional relationship between aPKC and Shank2-active Rap1 signaling serves as the platform for TJ assembly to regulate epithelial cell polarity.

High expression of KIBRA in low atypical protein kinase C-expressing gastric cancer correlates with lymphatic invasion and poor prognosis.

Overexpression of atypical protein kinase Cλ/ι (aPKCλ/ι), a regulator of cell polarity, is frequently associated with the poor prognoses of several cancers, including gastric cancer. Recent studies revealed a molecular link between aPKC and KIBRA, an upstream regulator of tumor suppressor Hippo pathway that regulates cell proliferation and apoptosis. Further, KIBRA directly inhibits the kinase activity of aPKC to regulate epithelial cell polarity. These observations suggest that the KIBRA-aPKC connection plays a role in cancer progression; however, clinical significance of the correlation between these factors remains unclear. Here we examined the correlation between KIBRA/aPKCλ/ι expression, as detected by immunohistochemistry, and clinicopathological outcomes in 164 gastric cancer patients using Fisher's exact test and Kaplan-Meier log-rank test. We found an intimate correlation between the expression level of KIBRA and aPKCλ/ι (P = 0.012). Furthermore, high expression of KIBRA is correlated with lymphatic (P = 0.046) and venous invasion (P = 0.039). The expression level of KIBRA by itself did not correlate with the prognosis; however, high expression of KIBRA in low aPKCλ/ι-expressing gastric cancer correlated with disease-specific (P = 0.037) and relapse-free survival (P = 0.041) by Kaplan-Meier with log-rank test and higher lymphatic invasion cases by Fisher's exact test (P = 0.042). Furthermore, overexpression of the aPKC-binding region of KIBRA disrupted tight junctions in epithelial cells. These results suggest that high expression of KIBRA in low aPKC-expressing cells causes massive loss of aPKC activity, leading to loss of polarity and invasiveness of gastric cancer cells.

The KIBRA-aPKC connection: A potential regulator of membrane trafficking and cell polarity.

The kidney and brain protein (KIBRA) is a scaffold or an adaptor-like protein with WW, C2-like and atypical protein kinase C (aPKC)-binding domains. Genetic studies in Drosophila revealed that KIBRA is an upstream regulator of the conserved Hippo pathway, which is implicated in organ size determination. In addition, genome-wide studies revealed an association between the single nucleotide polymorphism in the KIBRA gene locus and human episodic memory performance. However, the mechanism of action through which KIBRA is linked to these functions remains poorly understood. Recent studies on the biochemical and cellular properties of KIBRA reveal the role of KIBRA as a regulator of membrane trafficking. Further, KIBRA directly inhibits the activity of the cell polarity regulator, aPKC, which is required for apical protein exocytosis. Here, we discuss how this KIBRA-aPKC connection, a potential regulator of membrane trafficking and cell polarity, can contribute to the recently discovered functions of KIBRA.

KIBRA suppresses apical exocytosis through inhibition of aPKC kinase activity in epithelial cells.

Epithelial cells possess apical-basolateral polarity and form tight junctions (TJs) at the apical-lateral border, separating apical and basolateral membrane domains. The PAR3-aPKC-PAR6 complex plays a central role in TJ formation and apical domain development during tissue morphogenesis. Inactivation and overactivation of aPKC kinase activity disrupts membrane polarity. The mechanism that suppresses active aPKC is unknown. KIBRA, an upstream regulator of the Hippo pathway, regulates tissue size in Drosophila and can bind to aPKC. However, the relationship between KIBRA and the PAR3-aPKC-PAR6 complex remains unknown. We report that KIBRA binds to the PAR3-aPKC-PAR6 complex and localizes at TJs and apical domains in epithelial tissues and cells. The knockdown of KIBRA causes expansion of the apical domain in MDCK three-dimensional cysts and suppresses the formation of apical-containing vacuoles through enhanced de novo apical exocytosis. These phenotypes are restored by inhibition of aPKC. In addition, KIBRA directly inhibits the kinase activity of aPKC in vitro. These results strongly support the notion that KIBRA regulates epithelial cell polarity by suppressing apical exocytosis through direct inhibition of aPKC kinase activity in the PAR3-aPKC-PAR6 complex.

KIBRA Co-localizes with protein kinase Mzeta (PKMzeta) in the mouse hippocampus.

KIBRA is a WW domain-containing protein that can bind to protein kinase Czeta (PKCzeta). The SNP of the ninth intron of the KIBRA gene is associated with human episodic memory performance. Protein kinase Mzeta (PKMzeta), a brain-specific variant of PKCzeta, plays important roles in memory formation. Here we examined the interaction of KIBRA and PKMzeta in the adult mouse brain. Immunoprecipitation using newly-raised anti-KIBRA antibody revealed the interaction between KIBRA and PKMzeta in the brain. KIBRA was co-localized with PKMzeta in a single cultured neuron. Distribution analysis by immunohistochemistry and in situ hybridization indicated that KIBRA was highly localized with PKMzeta in the hippocampal CA1, CA3, and dentate gyrus. These results suggest that KIBRA functions in memory performance via interaction with PKMzeta.