The metabolism of Vitamin A and its derivative retinoid happen in the retina. As a product of Vitamin A metabolism, RA is a critical factor of maintaining the normal functions of the eye. It has been reported that the structure and functions of RPE cells is regulated by RA [
22]. RA causes growth arrest of RPE cells and induces hexagonal changes in their shapes, which plays a crucial role in maintaining the RPE’s monolayer structure [
23]. In addition, RA was reported to up-regulate TSP-1 and enhance the concentration of TGF-β1 in RPE cells [
24]. In our research, we attempted to investigate the effect of RA on TGF-β2, which is closely related to myopia. We found that ATRA could up-regulate the expression of TGF-β2 in the RPE cells and increase the secretion of TGF-β2 time-dependently. In the mice, guinea pig and mouse, TGF-β2 was proved to be expressed in the retina, RPE-choroid complex and sclera [
25,
26]. In a recent study, the TGF-β2 concentration of aqueous in highly myopic patients was significantly up-regulated, and was positively related to axial length of myopic patients.. According to these results, we inferred that increased RA in the retina of patients with myopia might induce the up-regulation of TGF-β2 in the RPE, which leads to the chondrogenic change in the sclera during the development of myopia. Nonetheless, the signaling pathway by which RA regulates TGF-β2 in RPE was not clear until now.
In the RPE, RA is bound to the retinoic X receptor (RXR) and retinoic acid receptor (RAR) to regulate gene transcription. There are two signaling pathways in RPE: (i) phospholipase C (PLC), calcium(Ca
2+) channels, and protein kinase C (PKC), and (ii) adenylyl cyclase, calcium (Ca
2+) channels, and cyclic AMP (cAMP)-dependent protein kinase A (PKA) [
27]. In our study, we used two inhibitors (U73122 and SQ22536) to inhibit the signaling pathways. As the inhibitor of phospholipase C and A2, U73122 can reduce the free cytosolic Ca2
+ via inhibiting the hydrolysis of PPI (phosphatidylinositol) to IP3 (inositol triphosphate). It also inhibits the activation of G-protein phospholipase C coupling while remaining unaffected by the production of cAMP [
28]. SQ22536 is a cell-permeable adenylyl cyclase inhibitor. SQ22536 reduces the activity of adenylate cyclase, when the concentrations of GTP increases with Mg
++ and Mn
++, SQ22536 can inhibit that the activating effect of Mg
++ and Mn
++ on adenylate cyclase in the presence of catecholamines, [
29]. We found that after treatment with U73122 + ATRA, the concentration of secreted TGF-β2 was significantly lower than that of the ATRA-treated group. When the concentration of U73122 reached to 40 μM, the concentration of secreted TGF-β2 was not significantly different from that of the control group. These results indicated that ATRA stimulated the secretion of TGF-β
2 via the PKC signaling pathway in RPE. On the other hand, we found that in the SQ22536-treated groups, there was no significant difference between the secretion of TGF-β
2 in the SQ22536 + ATRA-treated group and the ATRA-treated group, which indicated that the adenylyl cyclase signaling pathway was not involved in the effect of the stimulation of ATRA on the secretion of TGF-β
2. We concluded that retinal RA might bind to the RA receptors of RPE and activate the PKC signaling pathway, stimulating the secretion of TGF-β
2 in the RPE and inducing scleral remodeling in the myopia.
In further studies, our group will inject ATRA and U73122 into the sub-retina of mice, observe the effects of the suppression of U73122 on the changes in RPE induced by ATRA, and determine whether the growth of eyeball induced by ATRA can be suppressed by U73122. We will attempt to clarify the mechanism behind ATRA’s control of myopia.