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P.C. Kothary et al.

48.4 Discussion

The molecular mechanism for retinal fibro-genesis is unknown and thinking in this area is in a state of evolution. CTGF has been implicated in the pathogenesis of retinal fibro-genesis (Hinton et al. 2004). However, its signaling pathway in this system has not been elucidated. In the present study, we show that hyperglycemia and IGF-1 both stimulate CTGF synthesis, a proposed modulator of pathological retinal fibrosis, in cultured hRPE cells and this stimulation is inhibited the ERK pathway inhibitor, PD98059, in hRPE from control eyes but not in hRPE cells obtained from a patient with AMD.

Elevated glucose stimulates CTGF synthesis in a dose dependent manner in hRPE cells from AMD patients and to a lesser extent Non-AMD patients (Fig. 48.1). Hyperglycemia (20 mM glucose) stimulated CTGF synthesis was not completely blocked by the ERK inhibitor, PD98059 in hRPE cells from an AMD patient, while PD98059 completely blocked CTGF synthesis in hRPE cells Non-AMD patients (Fig. 48.3). Thus our data show that exposure of hRPE cells from Non-AMD patients to high glucose leads to an increase of CTGF expression, and the induction of CTGF by high glucose is mediated largely via ERK kinase pathway. However, exposure of hRPE cells from an AMD patient to high glucose leads to an increase of CTGF expression, and the induction of CTGF by high glucose is not completely blocked by ERK kinase pathway inhibitor.

To study the role of the ERK kinase signaling pathway in IGF-1 action, we exposed hRPE cells from an AMD patient and Non-AMD patients to increasing concentrations of IGF-1 and demonstrated that IGF-1 stimulates CTGF synthesis in dose dependent manner in hRPE cells from AMD as well as Non-AMD patients (Fig. 48.2). However, IGF-1 (25 nM) stimulated CTGF synthesis was not blocked by ERK inhibitor, PD98059, in hRPE cells from an AMD patient while PD98059 completely blocked CTGF synthesis in hRPE cells from Non-AMD patients (Fig. 48.4). Thus our data show that exposure of hRPE cells from Non-AMD patients to IGF- 1 leads to an increase of CTGF expression, and the induction of CTGF by IGF-1 is mediated primarily via ERK kinase pathway. However, exposure of hRPE cells from an AMD patient to IGF-1 leads to an increase of CTGF expression, and the induction of CTGF by IGF-1 is not completely blocked by ERK kinase pathway inhibitor.

Our study demonstrates that normal hRPE cells produce CTGF via the ERK kinase signaling pathway. This result is consistent with the finding that ERK pathway mediates CTGF induction in fibroblasts (Blalock et al. 2003). In addition, our study also demonstrates that ERK pathways are impaired and there may be abnormal CTGF synthesis regulation in response to glucose and IGF-1 in hRPE cells a patient with AMD.

This differential response may play a role in the fibrotic peri-retinal membrane formation seen in patients with AMD-related proliferative vitreo-retinopathy. Our data supports the Shi-wen et al. (2000) postulation that the constitutive over expression of connective tissue growth factor may contribute directly to chronic, persistent fibrosis. In addition, these studies suggest that the source of elevated CTGF in AMD

48 Up-Regulation of CTGF-Synthesis

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patients may be hRPE cells. In our studies of cultured hRPE cells from an AMD patient, we demonstrated that glucose and IGF-1 can induce abnormally elevated CTGF synthesis and anomalous regulatory pathways may be responsible. However, our in vitro studies cannot exclude that in vivo leakage of CTGF from the circulation through the defective blood retinal barrier (Kuiper et al. 2006) may also play a role.

In summary, our study has demonstrated that glucose and IGF-1 stimulation of CTGF synthesis in hRPE cells is mediated via ERK signaling. Impaired ERK signaling in hRPE cells in an AMD patient may result in abnormal synthesis of CTGF, which may play a role in the fibrotic peri-retinal membrane formation seen in AMD-related proliferative vitreo-retinopathies

Acknowledgments The authors thank Angela Joy Verkade for her assistance in preparing this chapter. This research was funded by the Skillman Foundation.

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