Yixue Cao

NF-κB in cancer: from innocent bystander to major culprit

Nuclear factor of κB (NF-κB) is a sequence-specific transcription factor that is known to be involved in the inflammatory and innate immune responses. Although the importance of NF-κB in immunity is undisputed, recent evidence indicates that NF-κB and the signalling pathways that are involved in its activation are also important for tumour development. NF-κB should therefore receive as much attention from cancer researchers as it has already from immunologists.

The Lymphotoxin-β Receptor Induces Different Patterns of Gene Expression via Two NF-κB Pathways

The lymphotoxin-beta receptor (LTbetaR) plays critical roles in inflammation and lymphoid organogenesis through activation of NF-kappaB. In addition to activation of the classical NF-kappaB, ligation of this receptor induces the processing of the cytosolic NF-kappaB2/p100 precursor to yield the mature p52 subunit, followed by translocation of p52 to the nucleus. This activation of NF-kappaB2 requires NIK and IKKalpha, while NEMO/IKKgamma is dispensable for p100 processing. IKKbeta-dependent activation of canonical NF-kappaB is required for the expression but not processing of p100 and for the expression of proinflammatory molecules including VCAM-1, MIP-1beta, and MIP-2 in response to LTbetaR ligation. In contrast, IKKalpha controls the induction by LTbetaR ligation of chemokines and cytokines involved in lymphoid organogenesis, including SLC, BLC, ELC, SDF1, and BAFF.

IKKα Provides an Essential Link between RANK Signaling and Cyclin D1 Expression during Mammary Gland Development

Abstract To identify functions of the IKKα subunit of IκB kinase that require catalytic activity, we generated an Ikkα AA knockin allele containing alanines instead of serines in the activation loop. Ikkα AA/AA mice are healthy and fertile, but females display a severe lactation defect due to impaired proliferation of mammary epithelial cells. IKKα activity is required for NF-κB activation in mammary epithelial cells during pregnancy and in response to RANK ligand but not TNFα. IKKα and NF-κB activation are also required for optimal cyclin D1 induction. Defective RANK signaling or cyclin D1 expression results in the same phenotypic effect as the Ikkα AA mutation, which is completely suppressed by a mammary specific cyclin D1 transgene. Thus, IKKα is a critical intermediate in a pathway that controls mammary epithelial proliferation in response to RANK signaling via cyclin D1 .

IκB kinase (IKK)β, but not IKKα, is a critical mediator of osteoclast survival and is required for inflammation-induced bone loss

Transcription factor, nuclear factor κB (NF-κB), is required for osteoclast formation in vivo and mice lacking both of the NF-κB p50 and p52 proteins are osteopetrotic. Here we address the relative roles of the two catalytic subunits of the IκB kinase (IKK) complex that mediate NF-κB activation, IKKα and IKKβ, in osteoclast formation and inflammation-induced bone loss. Our findings point out the importance of the IKKβ subunit as a transducer of signals from receptor activator of NF-κB (RANK) to NF-κB. Although IKKα is required for RANK ligand-induced osteoclast formation in vitro, it is not needed in vivo. However, IKKβ is required for osteoclastogenesis in vitro and in vivo. IKKβ also protects osteoclasts and their progenitors from tumor necrosis factor α–induced apoptosis, and its loss in hematopoietic cells prevents inflammation-induced bone loss.

NF-kappaB in mammary gland development and breast cancer.

Nuclear factor of κB (NF-κB) is a group of sequence-specific transcription factors that is best known as a key regulator of the inflammatory and innate immune responses. Recent studies of genetically engineered mice have clearly indicated that NF-κB is also required for proper organogenesis of several epithelial tissues, including the mammary gland. Mice have shown severe lactation deficiency when NF-κB activation is specifically blocked in the mammary gland. In addition, there are strong suggestions that NF-κB may play an important role in the etiology of breast cancer. Elevated NF-κB DNA-binding activity is detected in both mammary carcinoma cell lines and primary human breast cancer tissues.

IκB kinase α kinase activity is required for self-renewal of ErbB2/Her2-transformed mammary tumor-initiating cells

NF-κB is constitutively active in many solid tumors, including breast cancer. However, the role of NF-κB in breast carcinogenesis is unknown. IkkαAA/AA “knockin” mice in which activation of IκB kinase α (IKKα) is prevented by replacement of activation loop serines with alanines exhibit delayed mammary gland growth during pregnancy, because IKKα activity is required for cyclin D1 induction and proliferation of lobuloalveolar epithelial cells. Given the role of cyclin D1 in breast and mammary cancer, we examined involvement of IKKα in mammary carcinogenesis induced by oncogenes or a chemical carcinogen, 7,12-dimethylbenz[a]anthracene (DMBA). The IkkαAA mutation retarded tumor development in response to either 7,12-dimethylbenzaanthracene or the MMTV-c-neu (ErbB2/Her2) transgene but had no effect on MMTV-v-Ha-ras-induced cancer, although both oncogenes rely on cyclin D1. Strikingly, primary IkkαAA/AA/MMTV-c-neu carcinoma cells exhibited diminished self-renewal capacity, resulting in the inability to establish secondary tumors. IkkαAA/AA/MMTV-c-neu carcinoma cells underwent premature senescence when cultured under conditions used for propagation of mammary gland stem cells. Thus, IKKα is not only a regulator of mammary epithelial proliferation, but is also an important contributor to ErbB2-induced oncogenesis, providing signals that maintain mammary tumor-initiating cells. IKKα may represent a novel and specific target for treatment of ErbB2-positive breast cancer.

Proteotyping of mammary tissue from transgenic and gene knockout mice with immunohistochemical markers: a tool to define developmental lesions.

Through the use of transgenic and gene knockout mice, several studies have identified specific genes required for the functional development of mammary epithelium. Although histological and milk protein gene analyses can provide useful information regarding functional differentiation, they are limited in their ability to precisely define the molecular lesions. For example, mice that carry a mutation in one of the subunits of the IκB kinase, IKKα, cannot lactate despite the presence of histologically normal alveolar compartment and the expression of milk protein genes. To further define and understand such lesions on a molecular level, we sought evidence for proteins that are differentially expressed during mammary gland development with a view to generating a tissue proteotype. Using database screens and immunohistochemical analyses, we have identified three proteins that exhibit distinct profiles. Here, using mouse models as test biological systems, we demonstrate the development and application of mammary tissue proteotyping and its use in the elucidation of specific developmental lesions. We propose that the technique of proteotyping will have wide applications in the analyses of defects in other mouse models.