Cellular Droplets Reveal How a Growth Factor Signal Drives Skin Pigmentation

In a study published in Frontiers in Molecular Biosciences, researchers report that a signaling protein long known for its role in metabolism also helps switch on melanin production in pigment cells—and that this new function depends on the protein’s ability to condense into microscopic droplets inside the cell. The work, carried out in mouse B16-F10 melanoma cells, identifies insulin receptor substrate 1 (IRS-1) as a molecular bridge linking the insulin-like growth factor 1 receptor (IGF-1R) to the well-established α-MSH/MITF pathway that governs melanogenesis, the process by which melanocytes synthesize the pigment melanin.

Melanogenesis has traditionally been understood through a single dominant route. α-melanocyte-stimulating hormone binds the melanocortin-1 receptor, raising intracellular cAMP and activating protein kinase A, which phosphorylates the transcription factor CREB. CREB then drives expression of microphthalmia-associated transcription factor (MITF), the master regulator of the pathway. MITF in turn upregulates the melanogenic enzymes tyrosinase (TYR), TYRP-1 and TYRP-2, the enzymes that build melanin. Disruptions in this balance matter clinically: excess melanin accumulation underlies hyperpigmentary disorders such as melasma and ephelides, while deficits cause albinism, conditions that the authors note can impose considerable psychological distress on patients.

The new study builds on earlier findings that the IGF-1/IGF-1R metabolic signaling axis can also induce melanin synthesis. IGF-1 is available to skin cells not only systemically but through local autocrine and paracrine production, particularly from dermal fibroblasts and keratinocytes of the stratum granulosum, and its pathway activity is reported to be enhanced under conditions such as aging and oxidative stress. Previous clinical work had observed that children with short stature treated with recombinant human growth hormone developed increased numbers of melanocytic naevi, hinting that growth factor signaling touches pigmentation biology in people. The precise mechanism, however, had remained unresolved.

To dissect that mechanism, the team first treated B16-F10 cells with α-MSH, IGF-1 or insulin. Both α-MSH and IGF-1 significantly increased melanin production, with IGF-1 less potent than α-MSH, while insulin had no promoting effect. IGF-1 and α-MSH also raised tyrosinase activity, an indicator of activation of the tyrosine metabolism pathway on which melanin synthesis depends, and upregulated both the messenger RNA and protein levels of MITF and its downstream melanogenic enzyme genes. The authors conclude from these experiments that IGF-1 is a genuine inducer of melanogenesis in this cell model.

The next question was architectural: does IGF-1/IGF-1R signaling run parallel to the α-MSH pathway or converge with it? The researchers silenced either IGF-1R or the insulin receptor (IR) with specific siRNA oligonucleotides before treating cells with α-MSH. The logic was straightforward. If the two pathways operated independently, removing IGF-1R should leave α-MSH-induced pigmentation untouched, since α-MSH could fully activate the downstream cascade on its own. Instead, knockdown of IGF-1R—but not IR—substantially reduced melanin synthesis, tyrosinase activity, the mRNA expression of Mitf, Tyr, Tyrp-1 and Tyrp-2, and the protein levels of MITF and TYR. Immunofluorescence staining likewise showed reduced nuclear MITF signaling when IGF-1R was depleted. The result supports a convergent model in which IGF-1R signaling co-activates the downstream MITF/TYR axis rather than working as a separate parallel route.

IRS-1 emerged as the connector. As the key downstream adaptor of both IGF-1R and the insulin receptor, IRS-1 was a natural candidate for the convergence point. When the team silenced IRS-1, α-MSH-induced melanin synthesis and tyrosinase activity fell significantly, accompanied by downregulation of the melanogenic genes Mitf, Tyr, Tyrp-1 and Tyrp-2, reduced MITF and TYR protein, and diminished endogenous MITF immunofluorescence. Conversely, overexpressing FLAG-tagged IRS-1 enhanced α-MSH-induced melanin production, increased tyrosinase activity, upregulated Mitf, Tyr and Tyrp-2 transcripts, and raised MITF and TYR protein levels. In cells expressing GFP-tagged IRS-1, MITF signal was significantly enhanced upon α-MSH stimulation compared with neighboring cells that did not take up the construct, while no obvious change appeared without α-MSH.

The most striking element of the study concerns phase separation, a biophysical process in which certain molecules demix from their surroundings to form a dense, droplet-like phase. Molecules capable of this behavior typically carry multiple interaction domains or intrinsically disordered regions that mediate multivalent contacts, and numerous membrane-less organelles and signalosomes assemble this way. The authors had previously shown that IRS-1 forms droplets through a self-association region (SAR) spanning residues 301–600 within its intrinsically disordered region, organizing signalosomes for insulin/IGF-1 signaling. In the present work, deleting this SAR abolished droplet formation: wild-type IRS-1 formed spherical cytoplasmic structures, while the SAR deletion mutant diffused throughout the cytoplasm.

Functionally, the difference was decisive. Overexpression of wild-type IRS-1 robustly increased melanin production upon α-MSH stimulation, but this effect was significantly reduced with the SAR deletion mutant. Expression of Tyr, Tyrp-1, Tyrp-2 and Mitf was markedly lower in mutant-expressing cells than in cells expressing wild-type IRS-1, and roughly 80 percent of IRS-1ΔSAR-overexpressing cells showed either undetectable or reduced MITF signaling compared with wild-type IRS-1-overexpressing cells after α-MSH treatment. The authors propose two roles for IRS-1 phase separation: concentrating key signaling components into droplets to integrate the signal, and rapidly assembling or disassembling in response to external stimuli, allowing melanocytes to respond quickly to IGF-1 and transmit the signal to melanogenic target genes. They report this as the first documented example of an endogenous signaling protein’s phase separation regulating melanogenesis.

Notably, insulin behaved very differently from IGF-1 despite the structural homology between the two ligands and partial cross-reactivity of their receptors. Neither insulin treatment nor IR knockdown affected α-MSH-induced melanogenesis, consistent with earlier reports that insulin alone inhibits tyrosinase activity and fails to stimulate melanin synthesis in vitro. The authors suggest this may reflect a tissue-specific function of the IGF-1/IGF-1R axis in melanocytes that is not cross-coupled by insulin/IR signaling, possibly involving ligand-specific signaling in which the same receptor triggers different downstream effects depending on which molecule activates it. They also flag an important caveat: their insulin experiments used 100 nM, a typical in vitro pharmacological concentration far above physiological or pathological levels in vivo, which has been reported to cross-activate IGF-1R in other tissues. How IRS-1 is differently modified after IGF-1 versus insulin stimulation remains an open question, as do the contributions of the PI3K/AKT and Ras-MAPK pathways downstream of IRS-1, whose roles in melanogenesis are reported to be mixed.

The findings come with clear scope limitations. All experiments were performed in a single mouse melanoma cell line, an established but artificial model for melanin synthesis, so whether IRS-1 phase separation operates the same way in human skin in vivo remains to be demonstrated. The authors also note that the in vivo association between high insulin and hyperpigmentation, seen in conditions such as acanthosis nigricans, is likely indirect and possibly mediated by signals from nearby keratinocytes rather than direct insulin signaling in melanocytes. Looking forward, they suggest that inhibiting IRS-1 phase separation could inform the development of skin-whitening agents, while promoting it might theoretically benefit patients with defects in melanin production such as albinism—ideas that remain speculative pending further study of the therapeutic potential of targeting this droplet-forming behavior in pigmentary disorders.

Subject of Research: The role of IRS-1 phase separation in connecting IGF-1R signaling to the MITF/TYR axis during melanogenesis

Article Title: IRS-1 phase separation bridges IGF-1R signaling and the MITF/TYR axis to control melanogenesis

Article References: Liang, H. D., Xu, S. B., Zhou, Y. T., Zheng, L. L., Koay, C. Y., Yap, H. Y., & Lua, B. L. (2026). IRS-1 phase separation bridges IGF-1R signaling and the MITF/TYR axis to control melanogenesis. Frontiers in Molecular Biosciences, 13, Article 1947504. https://doi.org/10.3389/fmolb.2026.1947504

Image Credits: AI Generated

DOI: 10.3389/fmolb.2026.1947504

Keywords: melanogenesis, IRS-1, IGF-1R, phase separation, MITF, tyrosinase, melanin, alpha-MSH, pigmentation, B16-F10 cells, signalosomes, intrinsically disordered region