Evidence that the Axis Comprised of TSP1-TGFβ-CTGF Plays a Major Role In the Pathophysiology of Diabetic Retinopathy:Regulatory Role Of IL-4 and IL-13

Diabetic retinopathy is the most severe of several ocular complications of diabetes. Diabetes afflicts approximately 6% of the US population, thus retinopathy remains an important problem (PMID: 14702427). Our laboratory has provided evidence of the significant role of a pro-inflammatory axis during chronic inflammation (PMID: 19485899). The axis is comprised of TSP1-TGFβ-CTGF. We now provide evidence of an additional player within the axis, namely IL-4. A series of biochemical measurements were used to assess the role of the above mentioned axis in the progression of disease, from non-proliferative diabetic retinopathy (NPDR) to proliferative diabetic retinopathy (PDR).

Institutionally approved IRB and informed consent allowed this prospective study to recruit a total of 40 individuals from a vulnerable population disproportionately afflicted by this disease and representative of the population that we served in North Philadelphia with type-2 diabetes. Western-blotting technique was utilized to document the presence of fragments from CTGF in plasma, which have been shown in vitreous fluid to be angiogenic (PMID: 14988298). Multiplexed protein profiling on microarrays by rolling-circle amplification was employed to determine IL-4, IL-13, and MIP-1β levels. Commercially available ELISA was the method to determine the plasma concentration of TSP1, TGFβ and CTGF.

TSP1 (p<0.002), TGFβ (p<0.001), CTGF (p<0.05), IL4 (p<0.0002), MIP-1β (p<0.05) but not IL-13 were found significantly elevated in NPDR patients when compared to PDR patients or control group. Fragments of CTGF were more abundant in patients with PDR when compared to NPDR or the control group. Interestingly, the systemic plasma circulation of CTGF during the NPDR phase (elevated) may explain the subsequent vitreous accumulation of CTGF fragments described in the literature during the PDR phase apparently with angiogenic and fibrotic properties. Furthermore, IL-4 regulates CTGF (PMID:11967989) by interfering with TGFβ-induced transcriptional activation of the CTGF gene (blocking 50% of the SMAD pathway). Thus, the elevated levels during the NPDR phase may be a protective response to prevent CTGF synthesis and secretion. The steady-state levels of IL-13 (normal control group) where significantly higher when compared to the NPDR or PDR group with a trend for higher levels of IL-13 in the NPDR group. IL-13 is anti-angiogenic and thus its progressive decrease as a function of length of disease is of interest. Mean H1ac levels were not significantly different in the NPDR patients (8.0 ± 3.5) when compared with PDR patients (8.3 ± 2.2). In summary, our data suggests that the pro-inflammatory axis described above, now including IL-4 and IL-13, overall, plays a major role in the pathophysiology of diabetic retinopathy. The mechanistic pathway is currently under investigation in our laboratory.

No relevant conflicts of interest to declare.