High levels of serum C-peptide are associated with a decreased risk for incident renal progression in patients with type 2 diabetes: a retrospective cohort study

Correlation between serum C-peptide level and diabetic retinopathy according to estimated glomerular filtration rate in patients with type 2 diabetes

INTRODUCTION

Diabetic retinopathy is a common complication of diabetes and a leading cause of vision loss. The prevalence of DR increases with the duration of diabetes, poor glycemic control, and the presence of other systemic conditions such as hypertension and dyslipidemia. Serum C-peptide is a marker of endogenous insulin production and is commonly used to assess residual beta-cell function in patients with type 2 diabetes (T2D).

OBJECTIVES

To examine the relationship between serum C-peptide levels and diabetic retinopathy in patients with type 2 diabetes, categorized according to their estimated glomerular filtration rate.

METHODS

A cross-sectional study was conducted in VINAYAKA MISSION'S KIRUPANANDA VARIYAR MEDICAL COLLEGE AND HOSPITALS, VINAYAKA MISSION'S RESEARCH FOUNDATION, (DU), in the Department of General Medicine. The study was performed between September 2023 and July 2024. A total of 120 patients who had diabetes mellitus attending the OPD were included in the study. Blood samples were taken to measure serum C-peptide levels, fasting plasma glucose (FPG), and glycated hemoglobin (HbA1c). Serum C-peptide levels were measured using an electrochemiluminescence immunoassay. Also, eGFR was calculated using the CKD-EPI equation based on serum creatinine levels.

RESULTS

The majority of the participants had more than 10 years of diabetes (51.7%), while 48.3% had less than 10 years of diabetes. The mean HbA1c value was 8.2 ± 1.5, and the mean Serum C-peptide was 2.3 ± 1.1 ng/mL. The prevalence of diabetic retinopathy among the study participants was 45.0%.

CONCLUSION

Serum C-peptide levels are significantly correlated with both diabetic retinopathy and kidney function. Lower C-peptide levels (reflecting worse beta-cell function) are associated with a higher risk of retinopathy and worse kidney function.

C-peptide in diabetes: A player in a dual hormone disorder?

C-peptide, a byproduct of insulin synthesis believed to be biologically inert, is emerging as a multifunctional molecule. C-peptide serves an anti-inflammatory and anti-atherogenic role in type 1 diabetes mellitus (T1DM) and early T2DM. C-peptide protects endothelial cells by activating AMP-activated protein kinase α, thus suppressing the activity of NAD(P)H oxidase activity and reducing reactive oxygen species (ROS) generation. It also prevents apoptosis by regulating hyperglycemia-induced p53 upregulation and mitochondrial adaptor p66shc overactivation, as well as reducing caspase-3 activity and promoting expression of B-cell lymphoma-2. Additionally, C-peptide suppresses platelet-derived growth factor (PDGF)-beta receptor and p44/p42 mitogen-activated protein (MAP) kinase phosphorylation to inhibit vascular smooth muscle cells (VSMC) proliferation. It also diminishes leukocyte adhesion by virtue of its capacity to abolish nuclear factor kappa B (NF-kB) signaling, a major pro-inflammatory cascade. Consequently, it is envisaged that supplementation of C-peptide in T1DM might ameliorate or even prevent end-organ damage. In marked contrast, C-peptide increases monocyte recruitment and migration through phosphoinositide 3-kinase (PI-3 kinase)-mediated pathways, induces lipid accumulation via peroxisome proliferator-activated receptor γ upregulation, and stimulates VSMC proliferation and CD4+ lymphocyte migration through Src-kinase and PI-3K dependent pathways. Thus, it promotes atherosclerosis and microvascular damage in late T2DM. Indeed, C-peptide is now contemplated as a potential biomarker for insulin resistance in T2DM and linked to increased coronary artery disease risk. This shift in the understanding of the pathophysiology of diabetes from being a single hormone deficiency to a dual hormone disorder warrants a careful consideration of the role of C-peptide as a unique molecule with promising diagnostic, prognostic, and therapeutic applications.