Disappearance of p27(KIP1) and increase in proliferation of the lens cells after extraction of most of the fiber cells of the lens

Connexin Controls Cell-Cycle Exit and Cell Differentiation by Directly Promoting Cytosolic Localization and Degradation of E3 Ligase Skp2

Connexins and connexin channels play important roles in cell growth/differentiation and tumorigenesis. Here, we identified a relationship between a connexin molecule and a critical cell-cycle regulator. Our data show that connexin (Cx) 50 regulated lens cell-cycle progression and differentiation by modulating expression of cyclin-dependent kinase inhibitor p27/p57 and E3 ubiquitin ligase Skp2. Cx50 directly interacted with and retained Skp2 in the cytosol by masking the nuclear targeting domain of Skp2, and this effect was supported by an increased nuclear localization of Skp2, disruption of Skp2 interaction with importin-7, and decreased levels of p27/p57 in mouse lenses lacking Cx50. As a result, Cx50 increased auto-ubiquitination and subsequent degradation of Skp2. A mutation (V362E) on the C terminus of Cx50 disrupted the interaction between Cx50 and Skp2 and completely abolished such effects. Therefore, this study identifies a role for connexins in regulating cell-cycle modulators and, consequently, cell growth and differentiation.

Nuclear removal during terminal lens fiber cell differentiation requires CDK1 activity: appropriating mitosis-related nuclear disassembly

Lens epithelial cells and early lens fiber cells contain the typical complement of intracellular organelles. However, as lens fiber cells mature they must destroy their organelles, including nuclei, in a process that has remained enigmatic for over a century, but which is crucial for the formation of the organelle-free zone in the center of the lens that assures clarity and function to transmit light. Nuclear degradation in lens fiber cells requires the nuclease DNase IIβ (DLAD) but the mechanism by which DLAD gains access to nuclear DNA remains unknown. In eukaryotic cells, cyclin-dependent kinase 1 (CDK1), in combination with either activator cyclins A or B, stimulates mitotic entry, in part, by phosphorylating the nuclear lamin proteins leading to the disassembly of the nuclear lamina and subsequent nuclear envelope breakdown. Although most post-mitotic cells lack CDK1 and cyclins, lens fiber cells maintain these proteins. Here, we show that loss of CDK1 from the lens inhibited the phosphorylation of nuclear lamins A and C, prevented the entry of DLAD into the nucleus, and resulted in abnormal retention of nuclei. In the presence of CDK1, a single focus of the phosphonuclear mitotic apparatus is observed, but it is not focused in CDK1-deficient lenses. CDK1 deficiency inhibited mitosis, but did not prevent DNA replication, resulting in an overall reduction of lens epithelial cells, with the remaining cells possessing an abnormally large nucleus. These observations suggest that CDK1-dependent phosphorylations required for the initiation of nuclear membrane disassembly during mitosis are adapted for removal of nuclei during fiber cell differentiation.

Phosphorylation of p27(KIP1) in the Mitotic Cells of the Corneal Epithelium

PURPOSE

The mechanism in regulation of the cell cycle and proliferation of corneal epithelium in the homeostatic ocular surface remains unclear. The aim of this study is to examine the expression of p27(KIP1) and its phosphorylation in corneal epithelium.

METHODS

The eyes of C57BL/6 mice (7 weeks old) were enucleated. Formalin-fixed and paraffin-embedded tissue sections were examined using immunohistochemistry with anti-p27(KIP1), threonine 187 phosphorylated p27(KIP1) (T187-phospho-p27), and phosphorylated Histon H3 (pHiston H3) antibodies. Anti-T187-phospho-p27 and anti-pHiston H3 polyclonal antibodies were used for parallel immunofluorescent staining.

RESULTS

pHiston H3-immunopositive cells were noted in basal cells of the corneal epithelium. At high magnification of DAPI nuclear staining, mitotic and non-mitotic cells were observed in corneal basal layer. p27(KIP1)-positive nuclei were detected in corneal basal cells, where non-mitotic basal cells were located. In contrast, mitotic cells showed under detectable level on p27(KIP1) immunoreactivity. Immunoreactivity for T187-phospho-p27 was detected in basal cells of the corneal epithelium. At high magnification, it was confirmed that the immunopositive cells were mitotic cells. Immunoreactivity of T187-phospho-p27 as well as pHiston H3 was localized in the same corneal basal cells using double-staining immunohistochemistry.

CONCLUSIONS

These results suggested that degradation of p27(KIP1) regulates progression into mitosis in corneal basal cells.

Expression of p27(KIP1) and cyclin D1, and cell proliferation in human pterygium

BACKGROUND

The pterygium is a growth onto the cornea of fibrovascular tissue that is continuous with the conjunctiva, whereas the mechanisms of cell proliferation in pterygium epithelium are unknown.

AIM

To analyse the histopathology and the expression of cell cycle-related molecules in pterygium tissues.

METHODS

Seven pterygia were surgically removed using the bare-sclera procedure, and three normal bulbar conjunctivas were also obtained. Formalin-fixed, paraffin-wax-embedded tissues were analysed by immunohistochemistry with anti-p27(KIP1), cyclin D1 and Ki-67 antibodies.

RESULTS

Conjunctival epithelium consisted of several layers of round cells with a few goblet cells. Nuclear immunoreactivity for p27(KIP1) was noted in many normal epithelial cells, where cyclin D1 and Ki-67-positive nuclei were intermingled. A variety of goblet cells were located in the superficial layer of the pterygium head as well as those of the body epithelia. Several pterygium epithelial cells were p27(KIP1) positive, whereas nuclear immunoreactivity for cyclin D1 and Ki-67 was detected in many epithelial cells. By contrast, immunoreactivity for p27(KIP1), cyclin D1 and Ki-67 was hardly detected in the pterygium stroma.

CONCLUSION

It is suggested that pterygium growth and development are associated with the proliferation of epithelium, which is possibly involved in the expression of cell cycle-related molecules.