New Insights into Traditional Chinese Medicine in Treatment of Diabetic Nephropathy

High-calorie diet and sedentary lifestyles are strongly associated with the development and onset of type 2 diabetes (T2D). According to the International Diabetes Federation, the global incidence of diabetes among individuals aged 20–79 years was 9.3% in 2019. The current estimated total number of patients with diabetes is 463 million, with an expected increase to over 578 million by 2030.^[1] Systemic insulin resistance and insufficient insulin secretion in T2D result in several complications that severely impair a patients quality of life, especially the development of T2D nephropathy (T2DN).^[2]

Increasing evidence has shown that the dysregulation of TGF-β signaling contributes to the development of T2DN. For example, the TGF-β/Smad3 signaling pathway is dysregulated in the kidneys of T2D patients, where activation of Smad3 and loss of its inhibitor Smad7 lead to severe glomerular and tubulointerstitial fibrosis.^[3] T2D associated with high levels of blood glucose and advanced glycation end-products can induce Smad3 phosphorylation in mesangial cells, tubular epithelial cells, and vascular smooth muscle cells in vitro.^[3–4] Smad3 also reportedly downregulated the expression of the transcription factors Pdx-1, Nkx6.1, and NeuroD1 in the islet beta cells of db/db mice,^[6,7] which transcriptionally suppresses the proliferation and function of islet beta cells and therefore further promotes the progression of T2D. In contrast, Smad7, a natural negative feedback regulator of Smad3, can reduce renal inflammation in diabetic nephropathy by promoting islet beta cell proliferation by targeting an NF-κB-dependent mechanism.^[8] Thus, rebalancing the equilibrium between Smad3 and Smad7 in TGF-β downstream signaling may represent a therapeutic strategy for T2DN.^[9]

Traditional Chinese medicine (TCM) has been reported to have therapeutic effects in T2D. For example, a clinical study showed that Tianqi can significantly slow down diabetes progression and minimize diabetes risk by enhancing glucose tolerance.^[10] Berberine, the main compound of Coptis, reduced insulin resistance in rats with streptozotocin-induced diabetes by suppressing oxidative stress and aldose reductase.^[11] Combination therapy is a well-documented therapeutic protocol for TCM, which involves the simultaneous or sequential use of more than one herbal component to maximize the therapeutic outcome by parallel regulation of numerous signaling pathways and transcriptional events. In fact, our group has invented a novel TCM-based combination therapy named AANG, containing a natural Smad7 agonist, asiatic acid (AA) from Centella asiatica,^[12] and the Smad3 inhibitor naringenin (NG) from Citrus or Millettiareticulata Benth,^[13] which synergistically inhibit multidrug resistance in hepatocellular carcinoma by targeting Smad3-dependent p-glycoprotein expression at the transcriptional level.^[14] These findings revealed the therapeutic potential of AANG in other human diseases associated with the aberration of TGF-β/Smad3 signaling.

Encouragingly, our recently published article in the International Journal of Biological Sciences^[15] provides novel insights into Smad3-dependent T2DN. In this study, we used AANG to rebalance the Smad3/Smad7 equilibrium in the diabetic injured kidneys of db/db mice, which not only reduced NF-κB-driven renal inflammation but also prevented Smad3-mediated renal fibrosis. Moreover, the collective results of the study found that AANG combination had a better effect than monotherapy with either AA or NG on the regulation of Smad3/Smad7 signaling in diabetic kidneys. Our data suggested that AANG effectively protected islet beta cells against diabetes-induced regression in a Smad3-dependent manner. Furthermore, we observed the synergistic effect of AANG on the induction of Smad7 for protecting the diabetic kidney against NF-κB-mediated renal inflammation.^[16,17] In addition, we detected that AANG can inhibit the expression of the Smad3-dependent fibrogenic lncRNA Erbb4-IR^[18] and inflammatory lncRNA LRNA9884^[19] in db/db mice, which may also represent a protective mechanism against T2DN.

Importantly, in the same study,^[15] we unexpectedly discovered the preventive effect of AANG therapy on the primary disease T2D. We found that AANG synergistically prevented the onset of T2D in db/db mice, resulting in improved insulin sensitivity and glucose tolerance. The preventive effect of AANG on T2D was achieved by starting treatment in 4-week-old db/db mice at the pre-diabetic stage but not at the post-diabetic stage (12-week-old db/db mice). We further identified that AANG exerts a preventive effect on pancreatic islets, the niche of T2D onset. These interesting findings suggested that AANG can prevent T2D if applied to individuals at the pre-diabetic stage.

In summary, our new study^[15] demonstrated the efficiency of AANG in suppressing the development of T2DN in db/db mice by systematically rebalancing Smad3/Smad7 signaling in the pancreas and kidneys, which consequently suppressed Smad3-mediated renal fibrosis and NF-κB-driven renal inflammation. In addition, we unexpectedly discovered a novel therapeutic function of AANG in preventing T2D onset by reserving beta cell proliferation and insulin production capacity in islets under diabetic conditions. This research provides new insights and important clinical rationale for translating this AANG combined therapy as a unique and an effective treatment for T2DN, as well as a TCM-based formula for preventing T2D onset in patients with pre-diabetic conditions.