Crucial involvement of transforming growth factor beta (TGF-β)/Smad signaling regulation by miR-204-5p in cataract and posterior capsular opacification

Abstract

Worldwide, cataracts remain the leading cause of irreversible blindness, primarily driven by age-related changes and free radical-mediated lens damage. Posterior capsular opacification (PCO) is a common complication following cataract surgery, arising from aberrant proliferation and epithelial-mesenchymal transition (EMT) of residual lens epithelial cells (LECs). The transforming growth factor-β (TGF-β)/Smad signaling cascade is among the most critical pathways underlying both cataractogenesis and PCO, as it drives EMT and subsequent fibrotic alterations. MicroRNAs have emerged as key regulators of ocular homeostasis, with miR-204-5p being particularly relevant. Highly expressed in the lens, miR-204-5p has been shown to target TGFBR2, a core component of the TGF-β/Smad pathway, and thereby suppress downstream signaling. Downregulation of miR-204-5p appears to enhance TGF-β signaling, which may contribute to lens fibrosis and opacification during cataract development or after surgery. This article reviews current understanding of the interplay between miR-204-5p and the TGF-β/Smad signaling pathway in LECs, focusing on their roles in EMT and fibrotic progression. We also highlight experimental evidence supporting miR-204-5p as a promising candidate for PCO prevention. Emerging evidence also suggests that miR-204-5p regulates oxidative stress and mitochondrial homeostasis through additional targets in lens epithelial cells, highlighting its broader therapeutic potential. Elucidating crosstalk between signaling and epigenetic regulation in LEC biology opens new avenues for non-surgical management of cataracts and PCO. This review underscores the need for further research into miRNA-mediated modulation of fibrotic signaling, establishing this as an innovative and promising direction in molecular ophthalmic therapy.

Clinical trial number

not applicable.

Highlights

• •miR-204-5p is a key regulator of TGF-β/Smad signaling in lens epithelial cells.
• •Downregulation of miR-204-5p promotes EMT and fibrosis in PCO.
• •miR-204-5p targets TGFBR2 and Smad4 to suppress fibrotic signaling.
• •miRNA-based strategies may provide non-surgical prevention of PCO.

1. Introduction

Cataracts, characterized by the opacification of the eye's natural lens, represent the leading cause of blindness globally, accounting for approximately 47.8% of all blindness cases [[1], [2], [3]]. Projections suggest that by 2025, over 40 million individuals worldwide will be affected by cataract-induced blindness [1,[4], [5], [6]]. Various factors, including aging, ocular trauma, diabetes mellitus, ultraviolet radiation exposure, substance abuse, and other eye diseases, increase the likelihood of cataract development [[7], [8], [9]]. With the growing global aging population, age-related cataracts have become the primary cause of reversible blindness [[10], [11], [12]]. Non-surgical management of cataract primarily focuses on delaying disease progression and mitigating risk factors. Key strategies include protecting the eyes from ultraviolet radiation using UV-blocking eyewear, cessation of smoking, and optimal control of diabetes and systemic oxidative stress. Nutritional approaches, such as diets rich in antioxidants (vitamins C and E, lutein, zeaxanthin), and experimental agents like N-acetylcarnosine or lanosterol eye drops, are under investigation for their potential to preserve lens transparency {Forouzanfar, 2025 #209}. Although no pharmacological cure exists yet, these preventive and supportive measures may reduce the incidence or delay the need for surgical intervention (Fig. 1). Despite advancements in phacoemulsification and intraocular lens implantation, postoperative complications, particularly posterior capsular opacification (PCO), remain significant clinical challenges [[15], [16], [17]]. PCO occurs when residual lens epithelial cells (LECs) proliferate, undergo epithelial-to-mesenchymal transition (EMT), and migrate. Researchers are currently investigating the molecular mechanisms that regulate these cellular processes [[18], [19], [20]]. Transforming Growth Factor Beta (TGF-β) is a crucial cytokine involved in fibrotic responses and plays a significant role in the development of cataracts and PCO. TGF-β primarily transmits signals via the Smad-dependent pathway, influencing the transcription of genes associated with EMT, extracellular matrix deposition, and the apoptosis of LECs [21,22] (Table 1).

Fig. 1.

Illustration showing non-surgical strategies to delay cataract progression, including UV protection, antioxidant intake, and experimental eye drops. [Created with BioRender.com] [13,14].

Table 1.

Key signaling pathways involved in Cataract and posterior capsular opacification (PCO).

Signaling Pathway	Role in Lens Biology	Contribution to Cataract/PCO	Key References
TGF-β/Smad	Induces epithelial-to-mesenchymal transition (EMT) in LECs	Promotes fibrosis, opacification, and PCO development	[23]
PI3K/Akt	Regulates survival and proliferation of lens epithelial cells (LECs)	Enhances resistance to apoptosis and contributes to abnormal LEC persistence	[24]
MAPK/ERK	Mediates cellular growth and stress responses	Facilitates inflammatory and proliferative responses in the lens	[24]
Wnt/β-catenin	Controls differentiation and maintenance of epithelial phenotype	Drives EMT and LEC migration during fibrotic PCO	[23]
Notch	Balances LEC proliferation and differentiation	Disruption may lead to aberrant growth and lens opacity	[25]

The aberrant activation of the TGF-β/Smad axis is significantly linked to lens fibrosis and opacification. Recent studies have emphasized the function of microRNAs (miRNAs)—small noncoding RNAs that regulate gene expression post-transcriptionally—in modulating TGF-β/Smad signaling across different ocular pathologies {Arabi, 2025 #207}. MiR-204-5p has been identified as a significant regulator of lens homeostasis and epithelial cell behavior. Downregulation of miR-204-5p has been noted in cataractous and PCO tissues, which correlates with increased TGF-β signaling and expression of EMT markers [26].

This review examines the regulatory relationship between miR-204-5p and the TGF-β/Smad signaling pathway in relation to cataract and PCO. This study analyzes the molecular mechanisms and subsequent effects of the interaction, proposing a new therapeutic approach that targets miR-204-5p to prevent or reduce lens opacification.

2. TGF-β/smad signaling in lens epithelial cell biology

The TGF-β signaling pathway is a critical mechanism by which the lens epithelium maintains cellular homeostasis. TGF-β maintains LECs in a precise equilibrium of growth, differentiation, and apoptosis under standard conditions [27]. This maintains the clarity and structural integrity of the lens. The binding of TGF-β ligands (TGF-β1, TGF-β2, and TGF-β3) to a heterotetrameric receptor complex composed of type I and type II serine/threonine kinase receptors, specifically TGFBR1 and TGFBR2, predominantly activates the pathway [28]. Activation of this receptor complex results in the addition of phosphate groups to Smad proteins regulated by intracellular receptors (R-Smads: Smad2 and Smad3) [29]. These proteins subsequently associate with Smad4 to create heteromeric complexes. These complexes translocate to the nucleus, where they regulate the transcription of target genes implicated in various cellular processes [30].

TGF-β/Smad signaling is rigorously regulated in lens epithelial biology under normal physiological conditions. It contributes to the maintenance of lens epithelial quiescence and the mediation of responses to mild oxidative stress or micro-injuries [28]. Nonetheless, dysregulation or sustained activation of this pathway, as seen in aging, oxidative stress, or post-surgical wound healing, results in abnormal phenotypic changes in LECs. This encompasses the induction of EMT, a process in which LECs lose their epithelial characteristics, such as polarity and tight junctions, while acquiring mesenchymal traits, including increased motility, fibrotic activity, and resistance to apoptosis [31].

Elevated TGF-β2 levels in aqueous humor have been consistently observed in age-related cataracts, indicating a chronic inflammatory and fibrotic microenvironment in the anterior segment. After cataract extraction, residual LECs are subjected to elevated levels of TGF-β resulting from surgical trauma, which initiates pathological EMT and fibrotic remodeling, ultimately leading to PCO [32]. Experimental studies confirm that exogenous TGF-β treatment of cultured LECs induces the expression of α-smooth muscle actin (α-SMA), fibronectin, and collagen type I, which are all characteristics of a mesenchymal phenotype [33].

Sustained TGF-β/Smad activation enhances oxidative stress by disrupting antioxidant defenses, particularly the glutathione (GSH) system and catalase activity, thereby establishing a feed-forward loop that further undermines lens cell function [34]. The downstream effectors of this pathway comprise EMT-related transcription factors, including Snail, ZEB1, and Twist, which regulate chromatin remodeling and suppress epithelial markers such as E-cadherin. Smad-independent, or non-canonical TGF-β signaling—through MAPK (ERK, p38), Rho GTPases, and PI3K/Akt pathways—contributes to lens fibrosis by influencing cytoskeletal rearrangement, cellular migration, and survival. The Smad-dependent axis is the primary mechanism directly responsible for transcriptional reprogramming in lens fibrosis and cataract progression [35].

Recent studies have focused on the epigenetic regulation of TGF-β/Smad signaling, especially via noncoding RNAs like microRNAs (miRNAs). Specific miRNAs, such as miR-204-5p, miR-26a, and members of the miR-200 family, have been recognized as modulators of critical nodes within this pathway [36]. The findings underscore the intricate feedback mechanisms influencing lens epithelial cell fate and indicate possible intervention points to prevent or reverse fibrotic alterations in cataract and PCO. The importance of TGF-β/Smad signaling in various physiological and pathological contexts necessitates careful modulation of this pathway [37]. Specifically, the role of post-transcriptional regulators such as miR-204-5p presents a viable strategy for preserving lens transparency, inhibiting EMT, and reducing fibrotic complications following cataract surgery.

3. Functional roles of miR-204-5p in lens biology and pathology

MiR-204-5p is a highly conserved microRNA found in substantial amounts in multiple ocular tissues, such as the retina, cornea, and lens epithelium. The expression of this factor in the lens is essential for maintaining epithelial homeostasis, as it regulates multiple cellular processes, including proliferation, apoptosis, and responses to oxidative stress [38]. Research has consistently shown that miR-204-5p levels are diminished in cataractous lenses, especially in advanced nuclear and subcapsular types [39]. This suggests that miR-204-5p may play a role in lens opacity.

The decrease in miR-204-5p expression is associated with heightened oxidative damage, mitochondrial dysfunction, and the expedited aging of LECs. Experimental evidence demonstrates that miR-204-5p directly or indirectly inhibits pro-apoptotic targets, including BCL2L11 (BIM), as well as genes related to EMT, such as ZEB2 and TGFBR2 [39]. Additionally, miR-204-5p plays a role in cellular protection from oxidative damage by targeting regulators of the NRF2 pathway, which increases cellular resistance to this type of damage, while additional targets such as Smad4, IGFBP5, and TXNIP may contribute to its broader regulatory effects in lens epithelial cells [40]. Research demonstrates that the downregulation of miR-204-5p correlates with abnormal fibrotic remodeling and sustained activation of EMT in residual lens epithelial cells after cataract surgery, which may lead to posterior capsular opacification (PCO), a common postoperative complication [41]. Dysregulation promotes the conversion of dormant epithelial cells into migratory, contractile myofibroblasts, leading to posterior capsule opacification and compromised vision.

MiR-204-5p directly regulates the TGF-β/Smad signaling pathway, which is primarily implicated in EMT within lens pathology (33). Bioinformatics and luciferase reporter assays demonstrate that miR-204-5p interacts with the 3′ untranslated regions (UTRs) of Smad4 and TGFBR2. Both points hold significance within the canonical TGF-β cascade. Research demonstrates that the enforced expression of miR-204-5p in human lens epithelial cells in vitro significantly reduces Smad-mediated transcriptional activity, lowers the levels of fibrosis-related markers including α-SMA and fibronectin, and hinders cellular motility [42]. The results demonstrate that miR-204-5p acts as a protective molecular switch, preserving epithelial cell health and inhibiting their conversion into fibrotic cells.

Although substantial progress has been made in elucidating the molecular functions of miR-204-5p in lens epithelial biology, evidence from direct clinical studies remains limited. To date, no interventional clinical trials have specifically evaluated miR-204-5p–based therapies for cataract or posterior capsular opacification [38]. Nevertheless, several mechanistic investigations using human lens epithelial cells, ex vivo human lens tissues, and in vivo animal models have provided compelling evidence for the regulatory role of miR-204-5p in lens fibrosis. Comparative expression analyses of cataractous versus normal human lenses consistently demonstrate a significant downregulation of miR-204-5p, which correlates with increased activation of TGF-β/Smad signaling and elevated expression of EMT markers [43]. Furthermore, functional studies employing miR-204-5p mimics or inhibitors in cultured human LECs and animal models have revealed its capacity to suppress Smad-dependent transcriptional activity, reduce fibrotic marker expression, and inhibit pathological cell migration. Despite these promising mechanistic insights, several challenges hinder the translation of miR-204-5p–based strategies into clinical practice. These include the lack of long-term safety data, interspecies differences in miRNA expression profiles, and the need for efficient and lens-specific delivery systems. Addressing these limitations through advanced experimental models and well-designed translational studies will be essential to bridge the gap between preclinical findings and future clinical applications.

4. Mechanistic model: miR-204-5p as a modulator of TGF-β/smad signaling

Recent evidence indicates that miR-204-5p functions as a key regulator of the TGF-β/Smad signaling pathway in lens epithelial cells. This pathway is recognized for its involvement in fibrogenesis and tissue remodeling, and it becomes pathologically activated during cataractogenesis and the development of PCO [44]. MiR-204-5p seems to inhibit this activation via a multi-faceted suppression mechanism involving both upstream and downstream effectors.

MiR-204-5p targets the 3′ UTR of TGF-β receptor II (TGFBR2), reducing the receptor's surface expression and restricting ligand-induced activation of the signaling cascade. Secondly, miR-204-5p disrupts the core transcriptional machinery of the TGF-β pathway by downregulating Smad4, the co-mediator Smad essential for nuclear translocation and transcriptional complex formation [45]. Thirdly, miR-204-5p modulates the expression of downstream EMT-promoting transcription factors, including Snail, ZEB1, and ZEB2, thereby diminishing the fibrotic phenotype {Ashrafizadeh, 2020 #103; Peng, 2021 #210}. Although targeting TGFBR2 is a key mechanism of miR-204-5p in LECs, many evidence suggests that its role extends beyond this pathway {Peng, 2021 #210{Xie, 2024 #211}}. Studies have shown that miR-204-5p also regulates IGFBP5, TXNIP and Smad4, through mitochondrial function, oxidative stress and inflammation {Abbasi, 2024 #212{Cao, 2025 #213}}. However, the relative contribution of these mechanisms in cataract and PCO requires further clarification.

This layered inhibition results in a net decrease in the expression of fibrotic markers, cytoskeletal reorganization, and extracellular matrix deposition. As a result, lens epithelial cells preserve their epithelial characteristics, rate of proliferation, and integrity of the monolayer—attributes crucial for lens transparency and visual function (34). The antagonistic relationship between miR-204-5p and TGF-β signaling offers mechanistic insights into the development of cataracts and posterior capsule opacification, while also indicating a wider role for miR-204-5p in lens wound healing and response to surgical trauma.

5. Future directions and therapeutic potential

Recent progress in the prevention and treatment of primary cataracts and secondary PCO has been strongly supported by an improved understanding of the inhibitory role of miR-204-5p in TGF-β-mediated fibrotic signaling. Therapeutic strategies aiming to restore or enhance miR-204-5p expression offer a targeted approach capable of effectively suppressing EMT in lens epithelial cells while preserving cell viability (Fig. 2). Among the promising delivery strategies are synthetic miR-204-5p mimics encapsulated in biocompatible nanoparticles, which could allow for sustained intraocular release through minimally invasive, topical administration. Additionally, gene-editing platforms such as CRISPR/dCas9 activators hold potential for upregulating endogenous miR-204-5p expression, presenting a more durable therapeutic option, particularly for high-risk individuals. Given the multifactorial nature of lens pathologies, combination therapies involving antioxidant agents, TGF-β inhibitors, and miRNA mimics may yield more comprehensive and effective disease modulation. However, several important challenges remain to be addressed before clinical translation becomes viable. Achieving tissue-specific delivery is critical to minimize off-target effects on non-lenticular ocular structures, such as the cornea or retina, and to reduce the risk of local inflammation or immune responses associated with viral vectors or nanoparticle formulations {Sadeghi, 2025 #206}. Furthermore, current evidence is largely derived from in vitro and animal models, underscoring the need for validation in human lens tissues, particularly due to interspecies variation in miRNA expression and the unique microenvironments of the human lens. The complexity of cataractogenesis, involving multiple overlapping pathways and substantial individual variability, further complicates the attribution of specific regulatory roles to miR-204-5p in isolation. Consequently, future research should prioritize the use of advanced techniques such as single-cell transcriptomics, spatial gene expression profiling, and in vivo functional studies utilizing lens-specific models. These efforts will be essential to refine delivery platforms, optimize dosing strategies, and ultimately develop miRNA-based therapies—such as miR-204-5p modulators—that could provide innovative, non-surgical interventions for preventing cataract progression and PCO formation.

Fig. 2.

Cataract Progression by age and non-coding RNA effect. [Created with BioRender.com].

6. Possible role of other miRNAs and other signaling pathways

To determine how miRNA regulation influences cataract formation, the researchers compared the data from recent studies (see Table 2). Numerous microRNAs, including miR-204-5p, miR-15a, and miR-34a, are crucial for regulating vital biological pathways, including TGF-β/Smad, Wnt/β-catenin, and PI3K/Akt (Table 1). The growth, death, EMT, and fibrotic responses of the lens epithelium are all regulated by these miRNAs. MiR-204-5p effectively inhibits Smad signaling, which in turn inhibits the EMT caused by TGF-β. This implies that it guards against cataract development and PCO. In addition to miR-204-5p, accumulating evidence indicates that multiple microRNAs are involved in cataractogenesis and PCO through the regulation of oxidative stress–responsive and fibrotic signaling pathways. Oxidative stress is a major driving force in lens epithelial cell (LEC) dysfunction, acting both as an initiator and an amplifier of TGF-β–mediated EMT. Several miRNAs have been identified as critical modulators of redox balance, apoptosis, and EMT in the lens [50]. For instance, miR-34a is consistently upregulated in age-related and diabetic cataracts and has been shown to promote oxidative stress–induced apoptosis in LECs by targeting anti-apoptotic and mitochondrial protective genes. Similarly, the miR-15a/16-1 cluster contributes to cataract formation by enhancing oxidative damage and apoptosis through repression of BCL-2 and related survival pathways [50,51]. In contrast, miR-26a has been reported to exert protective effects by modulating PI3K/Akt signaling, thereby limiting aberrant LEC proliferation and fibrotic remodeling (see Table 3).

Table 2.

Key miRNAs Involved in Cataractogenesis and PCO.

miRNA	Targeted Pathway or Gene	Primary Function	Role in Cataract/PCO	Expression Status	Ref.
miR-204-5p	TGF-β/Smad	Inhibits EMT and maintains lens transparency	Suppresses fibrosis and PCO formation	Downregulated	[41,44]
miR-15a/16–1	Anti-apoptotic genes (e.g., Bcl-2)	Induces apoptosis in LECs	Contributes to cataract onset	Upregulated	[46]
miR-34a	p53/ROS-related pathways	Regulates oxidative stress and cell cycle	Promotes lens opacification	Upregulated	[47]
miR-23b	Smad3	Inhibits TGF-β-induced EMT	Suppresses fibrotic PCO progression	Downregulated	[48]
miR-26b	PI3K/Akt	Regulates LEC survival and proliferation	Reduces aberrant LEC proliferation	Variable	[49]

Table 3.

Functional alterations of lens epithelial cells (LECs) in Cataract and PCO.

Biological Process	Normal Lens Function	Pathological Changes in Cataract/PCO
Epithelial Differentiation	Maintains transparency and lens integrity	EMT leads to fibrosis and capsular opacification
Apoptosis	Controlled cell turnover and homeostasis	Excessive or insufficient apoptosis contributes to pathology
Oxidative Stress Response	Balanced ROS detoxification by antioxidants	Impaired response causes protein aggregation and opacity
Proliferation & Migration	Limited, spatially regulated	Aberrant proliferation and migration in PCO

Importantly, these miRNAs do not function independently but rather operate within an interconnected regulatory network that converges on major fibrogenic pathways, including TGF-β/Smad, Wnt/β-catenin, and MAPK signaling. Within this network, miR-204-5p appears to act as a central regulatory node by simultaneously suppressing oxidative stress–associated signaling and directly inhibiting key components of the canonical TGF-β pathway. This broader perspective underscores that miR-204-5p functions as part of a coordinated miRNA-mediated regulatory system rather than as an isolated molecular factor in PCO pathogenesis [38].

7. Conclusion

In summary, miR-204-5p is an essential regulatory molecule that alters TGF-β/Smad signaling at multiple regulatory junctures to maintain the homeostasis of lens epithelial cells. The downregulation in tissues affected by PCO and cataracts underscores its significance for post-operative vision and lens clarity. By directly inhibiting TGFBR2 and Smad4, and indirectly obstructing transcription factors that induce EMT, miR-204-5p serves as a physiological gatekeeper that averts fibrotic remodeling and preserves the lens's transparency (Fig. 3). Due to its dual functions—impeding the transition of epithelial cells into fibrotic cells and inhibiting fibrotic transformation—miR-204-5p occupies a pivotal role at the convergence of aging biology, wound healing, and molecular treatments in ophthalmology. The therapeutic activation, by gene modification or miRNA mimics, demonstrates significant potential as an innovative strategy to avert PCO and cataracts, potentially decreasing the global demand for procedures and improving vision for everybody (Fig. 4).

Fig. 3.

Possible role of other miRNAs and other signaling pathways.

Fig. 4.

Schematic illustration of the regulatory role of miR-204-5p in modulating TGF-β/Smad signaling in lens epithelial cells. MiR-204-5p suppresses TGF-β signaling by targeting TGFBR2 and Smad4, leading to reduced Smad-dependent transcription, inhibition of epithelial–mesenchymal transition, and attenuation of fibrotic remodeling associated with posterior capsular opacification.

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Funding

This research did not receive any specific grant.

CRediT authorship contribution statement

Javad Sadeghi: Conceptualization, Data curation, Formal analysis, Validation, Visualization, Writing – original draft, Writing – review & editing. Somayeh Ghavidel Yazdi: Conceptualization, Data curation, Formal analysis, Funding acquisition, Visualization, Writing – original draft, Writing – review & editing. Kimia Arabi: Conceptualization, Investigation, Methodology, Writing – original draft, Writing – review & editing. Meysam Moghbeli: Conceptualization, Validation, Visualization, Writing – original draft, Writing – review & editing. Vahid Hajali: Conceptualization, Data curation, Writing – original draft, Writing – review & editing. Fatemeh Forouzanfar: Conceptualization, Data curation, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing. Ehsan Saburi: Conceptualization, Data curation, Formal analysis, Visualization, Writing – original draft, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

No data was used for the research described in the article.