One of the intriguing questions regarding corneal and limbal epithelial
cell proliferation is what mechanisms allow these cells to show
differential rates of proliferation during homeostasis and wound
repair. In initial experiments to examine this question, we surveyed
the localization of several cell-cycle–associated proteins in human
corneas.
11 In mammalian cells, the cell cycle is driven by
a family of protein kinases referred to as cyclin-dependent kinases
(CDKs), which are regulated by another family of proteins, termed
cyclins.
12 Although the CDKs are present throughout the
cell cycle, the synthesis of the cyclins is cell-cycle–dependent.
There are four main groups of cyclins, termed D, E, A, and B. Cyclin D
synthesis is stimulated by mitogenic signals, including growth factors.
It appears early in G1 and is degraded in the S and G2 phases of the
cell cycle. Three isoforms of cyclin D (termed 1, 2, and 3) are
known.
13 In normal, nontumorous cells, the synthesis of
cyclin E is initiated in mid-G1, its maximal expression occurs at the
time of entry into S-phase, and its degradation occurs as the cell
progresses through S-phase.
14 Accumulation of cyclins D
and E is required for cells to progress through a point late in G1
termed the “restriction point.” Once cells pass through the
restriction point, they are committed to progress through the remainder
of the cell cycle. Thus, most of the regulation of the cell cycle
occurs in G1, before the restriction point.
12 13 15 Cyclin
A appears in S-phase and is important in DNA
synthesis,
16 17 and B appears in G2 and is required for
M-phase.
16 17 Thus, the presence or absence of the cyclins
can be used as markers of the stages of the cell cycle. Before the
initiation of our project on human corneas, we predicted that limbal
basal cells were in a resting state, G0, and therefore would not
express any of the cyclins at an elevated level. However, we observed
high levels of cyclins E and D in the basal cell layer of the
limbus
11 and conjunctiva (unpublished observation). We
also found that basal cells in the central corneal epithelium expressed
low levels of these cyclins and therefore appeared to be primarily in
the G0-phase of the cell cycle. Suprabasal cells in both corneal and
conjunctival epithelium appeared to have exited the cell cycle.