Connexin proteins are the principle structural components of the gap junctions. Colocalization and tissue-specific expression of diverse connexin molecules are reported to occur in a variety of organs. Impairment of gap junctional intercellular communication, caused by mutations, gain of function, or loss of function of connexins, is involved in a number of diseases including the development of cancer. Here we show that human breast cancer cells, MCF-7, and breast tumor tissues express a novel gap junction protein, connexin 46 (Cx46) and it plays a critical role in hypoxia. Previous studies have shown that connexin46 is predominantly expressed in lens and our studies find that Cx46 protects human lens epithelial cells (HLEC) from hypoxia induced death. Interestingly, we find that Cx46 is upregulated in MCF-7 breast cancer cells and human breast cancer tumors. Downregulation of Cx46 by siRNA promotes 40% MCF-7 cell death at 24 hour under hypoxic conditions. Furthermore, direct injection of anti-Cx46 siRNA into xenograft tumors prevents tumor growth in nude mice. This finding will provide an exciting new direction for drug development for breast cancer treatment and suggests that both normal hypoxic tissue (lens) and adaptive hypoxic tissue (breast tumor) utilize the same protein, Cx46, as a protective strategy from hypoxia.

While there are many reviews which examine the group of proteins known as protein kinase C (PKC), the focus of this article is to examine the cellular roles of two PKCs that are important for stress responses in neurological tissues (PKCγ and ε) and in cardiac tissues (PKCε). These two kinases, in particular, seem to have overlapping functions and interact with an identical target, connexin 43 (Cx43), a gap junction protein which is central to proper control of signals in both tissues. While PKCγ and PKCε both help protect neural tissue from ischemia, PKCε is the primary PKC isoform responsible for responding to decreased oxygen, or ischemia, in the heart. Both do this through Cx43.It is clear that both PKCγ and PKCε are necessary for protection from ischemia. However, the importance of these kinases has been inferred from preconditioning experiments which demonstrate that brief periods of hypoxia protect neurological and cardiac tissues from future insults, and that this depends on the activation, translocation, or ability for PKCγ and/or PKCε to interact with distinct cellular targets, especially Cx43.This review summarizes the recent findings which define the roles of PKCγ and PKCε in cardiac and neurological functions and their relationships to ischemia/reperfusion injury. In addition, a biochemical comparison of PKC γ and PKC ε and a proposed argument for why both forms are present in neurological tissue while only PKC ε is present in heart, are discussed. Finally, the biochemistry of PKCs and future directions for the field are discussed, in light of this new information.

Gap junctions are intercellular aqueous channels composed of transmembrane proteins called connexins (1). The gap junction-dependent or independent functions of connexins are important in the regulation of several cellular processes, including growth, proliferation, differentiation, protection, and cell death (2, 3). Distinct expression patterns and highly dynamic turnover rates are the key components that regulate tissue-specific activity of different connexin molecules. The expression and turnover of connexins are fine-tuned balances of several processes such as gene expression, mRNA stability, protein synthesis and transport, and degradation (4, 5). Connexin turnover and function is also modulated by several intrinsic and extrinsic factors, including intra-and extracellular pH, various phosphorylation events, cellular status, and chemical reagents such as the tumor-promoting phorbol ester, TPA 3 (6 -10). One tissue that relies on the gap junction-mediated communication for normal function and growth is the vertebrate lens. The lens is naturally avascular and, therefore, gap junctionmediated functions play a major role in maintaining proper homeostasis and transparency of the lens. The vertebrate lens endogenously expresses three connexin proteins required for proper lens development and function, connexin43 (Cx43), connexin46 (Cx46), and connexin50 (Cx50) (11)(12)(13)(14)(15)(16)(17). These connexins show differential spatial distributions that are related to their specific functions at different regions of the lens.

SUMMARY Cataracts, or lens opacities, are the leading cause of blindness worldwide. Cataracts increase with age and environmental insults, e.g. oxidative stress. Lens homeostasis depends on functional gap junctions. Knockout or missense mutations of lens gap junction proteins, Cx46 or Cx50, result in cataractogenesis in mice. We have previously demonstrated that protein kinase Cγ (PKCγ) regulates gap junctions in the lens epithelium and cortex. In the current study, we further determined whether PKCγ control of gap junctions protects the lens from cataractogenesis induced by oxidative stress in vitro, using PKCγ knockout and control mice as our models. The results demonstrate that PKCγ knockout lenses are normal at 2 days post-natal when compared to control. However, cell damage, but not obvious cataract, was observed in the lenses of 6-week-old PKCγ knockout mice,suggesting that the deletion of PKCγ causes lenses to be more susceptible to damage. Furthermore, in vitro incubation or lens oxidative stress treatment by H2O2 significantly induced lens opacification (cataract) in the PKCγ knockout mice when compared to controls. Biochemical and structural results also demonstrated that H2O2 activation of endogenous PKCγ resulted in phosphorylation of Cx50 and subsequent inhibition of gap junctions in the lenses of control mice, but not in the knockout. Deletion of PKCγaltered the arrangement of gap junctions on the cortical fiber cell surface,and completely abolished the inhibitory effect of H2O2on lens gap junctions. Data suggest that activation of PKCγ is an important mechanism regulating the closure of the communicating pathway mediated by gap junction channels in lens fiber cells. The absence of this regulatory mechanism in the PKCγ knockout mice may cause those lenses to have increased susceptibility to oxidative damage.