MMP1 and proliferative diabetic retinopathy: Notably, upregulation of thrombin/MMP1/protease-activated receptor-1 (PAR1) pathway in retinas of diabetic rats and in vitreous samples from patients with proliferative diabetic retinopathy compared to non-diabetic patients has been reported to promote angiogenesis and progression of proliferative diabetic retinopathy [220].