Volume regulation in lens epithelial cells and differentiating lens fiber cells.

Previous studies from our laboratory have led us to conclude that lens cell elongation is caused by an increase in cell volume. This volume increase results from an increase in the potassium content of the cells due to decreased potassium efflux. In contrast, an increase in the volume of most cells triggers a regulatory volume decrease (RVD) that is usually mediated by increased potassium efflux. For this reason, chicken embryo lens epithelial cells were tested to see whether they were capable of typical cell volume regulation. Changes in cell volume during lens fiber differentiation were first estimated by 3H2O water uptake. Cell water increased in proportion to cell length in elongating lens cells. Treatment of epithelial cells cultured in basal medium with dilute or concentrated medium, or with medium containing 50 mM sucrose, resulted in typical volume regulatory responses. Cells lost or gained volume in response to osmotic stress, then returned to their previous volume. In addition, the elongation and increase in cell volume that accompanies lens fiber cell differentiation occurred normally in either hypo- or hypertonic media. This observation showed that the activation of mechanisms to compensate for osmotic stress did not interfere with the increase in volume that accompanies elongation. The ability of elongating cells to volume regulate was also tested. Lens epithelial cells were stimulated to elongate by exposure to embryonic vitreous humor, then challenged with hypotonic medium. These elongating cells regulated their volume as effectively as unstimulated cells. Therefore, cells that were increasing their volume due to reduced potassium efflux could adjust their volume in response to osmotic stress, presumably by increasing potassium efflux. This suggests that the changes in potassium efflux that occur during differentiation and RVD are regulated by distinct mechanisms.

Lentropin, a protein that controls lens fiber formation, is related functionally and immunologically to the insulin-like growth factors.

Lentropin, a factor present in the vitreous humor of the eye, stimulates lens fiber differentiation from chicken embryo lens epithelial cells in vitro. Lentropin has been partially purified but has not been isolated in sufficient quantity or purity for direct comparison with other growth and differentiation factors. Previous studies have shown that insulin and fetal bovine serum share with lentropin the ability to stimulate lens fiber formation from cultured epithelial cells. In the present study, a number of hormones and growth factors were assayed for lentropin activity. Of those tested, the only substances that had this activity were the insulin-like growth factors (IGFs) somatomedin C (Sm-C/IGF-I) and multiplication-stimulating activity (MSA/IGF-II). Sm-C/IGF-I was approximately 30 times more potent than insulin or MSA/IGF-II in promoting fiber cell formation. A monoclonal antibody to human Sm-C/IGF-I inhibited purified Sm-C/IGF-I, fetal bovine serum, and chicken vitreous humor from stimulating fiber cell differentiation in vitro. This antibody has been shown not to crossreact with insulin and did not block insulin-stimulated lens fiber formation. These findings indicate that lentropin is related to the IGFs and that these factors may play important roles in controlling cell differentiation, in addition to their better-known ability to stimulate cell division.