Electrical inhibition of lens epithelial cell proliferation: an additional factor in secondary cataract?

Abstract

Cataract is the most common cause of blindness but is at least curable by surgery. Unfortunately, many patients gradually develop the complication of posterior capsule opacification (PCO) or secondary cataract. This arises from stimulated cell growth within the lens capsule and can greatly impair vision. It is not fully understood why residual lens epithelial cell growth occurs after surgery. We propose and show that cataract surgery might remove an important inhibitory factor for lens cell growth, namely electric fields. The lens generates a unique pattern of electric currents constantly flowing out from the equator and entering the anterior and posterior poles. We show here that cutting and removing part of the anterior capsule as in cataract surgery significantly decreases the equatorial outward electric currents. Application of electric fields in culture inhibits proliferation of human lens epithelial cells. This inhibitory effect is likely to be mediated through a cell cycle control mechanism that decreases entry of cells into S phase from G1 phase by decreasing the G1-specific cell cycle protein cyclin E and increasing the cyclin-Cdk complex inhibitor p27kip1. Capsulorrhexis in vivo, which reduced endogenous lens electric fields, significantly increased LEC growth. This, together with our previous findings that electric fields have significant effects on the direction of lens cell migration, points to a controlling mechanism for the aberrant cell growth in posterior capsule opacification. A novel approach to control growth of lens epithelial cells using electric fields combined with other controlling mechanisms may be more effective in the prevention and treatment of this common complication of cataract surgery.

Figure 1. Removal of part of anterior capsule significantly changes profiles of lens electric current.

A) Profile of electric currents of ocular lens. Outward currents (flow from positive to negative) at the equators and inward currents at anterior and posterior poles. B, C) Vibrating probe measurements of lens electric currents before and after mimicking cataract surgery. Steady inward currents (filled bars in C) at the anterior pole of rat lens in situ at positions indicated in (B) were completely reversed in polarity (outward currents; open bars) and were significantly higher than the original currents (P<0.004 for all positions before and after anterior capsule removal). This would reduce the outward currents at the bow regions where cells proliferate. Making a hole on the anterior capsule (E, F) significantly reduced (paired t-test; P<0.01) normal equatorial outward currents. D, G) are typical original records for (C) and (F), respectively.

Figure 2. An applied EF inhibits the proliferation of lens epithelial cells.

A) Human LEC line was cultured in DMEM with 12.5% fetal bovine serum and exposed to a dc EF in an incubator (37°C, 5% CO₂). Control cells were not exposed to an EF. Cell density (B) and growth rate (C) were reduced after EF exposure of 200 mV/mm. *P<0.01 compared with control. Data are expressed as mean ± SEM and were collected from 52 fields from at least 2 independent experiments.

Figure 3. Electric fields inhibit human lens epithelial cell proliferation through G1/S phase cell cycle checkpoint control.

A, B) Flow cytometry showed that EFs inhibited transition of human LECs from G₁ to S phase at 24 h after onset of EF. C) Western blot analyses showed significantly decreased G1-specific cell cycle protein cyclin E and increased cyclin-Cdk complex inhibitor p27^kip1 in LECs 12 and 24 h in EF. The expression of Cdk2 remained largely unchanged. Results were confirmed from at least two independent experiments. EF: 200 mV/mm.

Figure 4

Proliferation of lens epithelial cells at the bow region (A, B, D) and in the center of the capsular bag (C) after extracapsular lens extraction in vivo. A) Immediately after extracapsular lens extraction, the capsular bag appeared clean. LECs were present only under the anterior lens capsule and at the lens bow (arrowheads), but not on the posterior lens capsule (arrow). B) Six hours after extracapsular lens extraction, some proliferation of LECs at the lens bow was evident (arrow). C) Three days after extracapsular lens extraction, multiple layers of lens epithelial cells were observed at the center of the capsular bag (*), and wrinkling in the posterior capsule was evident (arrows). D) Seven days after extracapsular lens extraction, LEC differentiated into lens fibers at the lens bow (*). Magnification: (A, C, D) ×40; (B) ×20. (H&E staining).