Actinolactomycin from Streptomyces sp. CB00271: Isolation, Identification, and Potent Anti-Triple-Negative Breast Cancer

TL;DR

A new compound from a soil bacterium shows strong anti-cancer effects against a difficult-to-treat breast cancer subtype.

Contribution

Actinolactomycin is identified as a potent anti-triple-negative breast cancer agent with significant potency compared to cisplatin.

Findings

Actinolactomycin showed 9-fold and 31-fold higher potency than cisplatin against MDA-MB-231 and 4T1 TNBC models.

Streptomyces sp. CB00271 is a rare microbial source of actinolactomycin, offering a new supply avenue for future development.

Abstract

Triple-negative breast cancer (TNBC) is a highly aggressive subtype with limited targeted therapies, underscoring an urgent need for novel agents. The soil-derived Streptomyces sp. CB00271, isolated from a biodiversity hotspot, was investigated for its bioactive metabolites. Bioassay-guided isolation led to the identification of actinolactomycin (1), alongside daidzein (2) and genistein (3). Remarkably, actinolactomycin (1) exhibited potent cytotoxicity against TNBC models, with IC50 values of 0.72 ± 0.12 μM (MDA-MB-231) and 0.15 ± 0.02 μM (4T1), demonstrating approximately 9-fold and 31-fold greater potency than cisplatin, respectively, and suggesting action through targeted pathway inhibition. This study constitutes the first report to systematically highlight the exceptional anti-TNBC potential of this rare natural product, establishing it as a promising lead compound against this challenging subtype. Furthermore, Streptomyces sp. CB00271 is identified as a valuable and scarce microbial resource for actinolactomycin, providing a new avenue to address its supply limitation and facilitate future development.

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1. Introduction

Breast cancer remains a major global health challenge, with triple-negative breast cancer (TNBC) representing one of its most aggressive and problematic subtypes. Defined by the lack of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) expression, TNBC accounts for approximately 15–20% of all breast cancer cases and is characterized by a high risk of early recurrence, metastasis, and poor overall survival [1,2]. The absence of established molecular targets renders TNBC patients insensitive to endocrine therapy and HER2-targeted agents, leaving conventional chemotherapy as the primary systemic treatment option [2]. However, the clinical efficacy of chemotherapy is often compromised by dose-limiting toxicities and the rapid development of drug resistance [3]. Consequently, the discovery of novel, potent, and more selective lead compounds that can address the unique vulnerabilities of TNBC is an urgent and critically important pursuit in oncology drug discovery [4].

The search for therapeutic agents from nature has a long history, deeply rooted in traditional medicine. In the modern era, this paradigm has evolved into systematic drug discovery from natural sources, with microbial metabolites emerging as a preeminent reservoir of bioactive molecules. Among these, actinomycetes of the genus Streptomyces are exceptionally prolific, responsible for producing a vast array of clinically used therapeutics, including antitumor antibiotics such as bleomycin, actinomycin D, and mitomycin [5,6,7]. The advent of genome mining has revolutionized natural product discovery, enabling the systematic exploration of ‘silent’ or poorly expressed biosynthetic gene clusters (BGCs) in Streptomyces strains, thereby unveiling their hidden chemical potential [8,9]. In this context, the unique ecosystems of Yunnan Province in China, a recognized biodiversity hotspot, offer a largely untapped reservoir of microbial diversity. The distinctive low-latitude and high-altitude environments of this region are conducive to the evolution of unique actinomycete lineages, presenting significant opportunities for the discovery of novel microorganisms and their bioactive metabolites [10,11].

Streptomyces sp. CB00271, isolated from a pristine soil sample in Yunnan, has emerged as a particularly promising strain. Previous research has demonstrated its ability to produce the [5,6]-spiroketal-containing antitumor compound β-rubromycin and a new derivative, β-rubromycin acid (Figure 1), with the latter exhibiting cytotoxicity across a panel of tumor cell lines [12]. Subsequent optimization of fermentation conditions led to the discovery of chalkophomycin (Figure 1), a structurally unprecedented copper-chelating metallophore that shows potent cytotoxicity against several cancer cell lines, significantly surpassing the efficacy of cisplatin [13,14]. Whole-genome sequencing of CB00271 further revealed an impressive biosynthetic capacity, harboring 45 BGCs, including 18 non-ribosomal peptide synthetase (NRPS) clusters, 6 NRPS–type I polyketide synthase (T1PKS) hybrid clusters, and 10 BGCs of unknown types, underscoring its potential as a prolific source of novel natural products [15]. However, despite its rich biosynthetic potential, the full spectrum of its secondary metabolites, particularly against specific and recalcitrant cancers like TNBC, remains underexplored.

Actinolactomycin (ALM, Figure 1) is a structurally distinctive cyclic 2-oxonanonide natural product first reported from Streptomyces flavoretus 18522 [16]. It has demonstrated inhibitory activity against several human cancer cell lines (e.g., A2780, K562, HCT-15, A549, and HeLa) and has been shown to induce G_0_/G_1_ cell cycle arrest and apoptosis [16,17]. Mechanistic studies suggest that ALM exerts its effects by inhibiting the mammalian target of rapamycin (mTOR) signaling pathway and suppressing the translation of hypoxia-inducible factor 1α (HIF-1α) [18]. Intriguingly, the mTOR–HIF-1α axis is critically implicated in the metabolic reprogramming, invasive phenotype maintenance, and therapy resistance of TNBC [19,20,21], theoretically positioning ALM as a potential targeted therapeutic candidate for this aggressive subtype. Nonetheless, its actual efficacy against TNBC models has never been experimentally evaluated. Despite its promising bioactivity, ALM remains an exceedingly scarce natural product. To date, only four actinomycete strains capable of producing ALM have been reported worldwide, namely Streptomyces flavoretus 18522 (from Yunnan soil, 2005) [16], strain L39-3 (from Bohai Sea mud, 2010) [17], Streptomyces sp. K4 (from soil, 2024) [22], and Streptomyces pratensis TRM74023 (from rhizosphere soil of the Taklamakan Desert, 2026) [23]. Its complete biosynthetic pathway also remains unelucidated, severely hindering further research and development. In this study, we report for the first time the isolation of actinolactomycin from Streptomyces sp. CB00271 and provide a systematic evaluation of its potent cytotoxicity against TNBC models, thereby introducing a new microbial source for this rare compound and directly investigating its previously unreported anti-TNBC potential.

Given the pressing clinical challenge of TNBC and the scarcity of ALM, this study focused on the soil-derived Streptomyces sp. CB00271. We systematically investigated the secondary metabolites of this strain, leading to the isolation and structural elucidation of several compounds. The primary objective was to evaluate the antitumor activity of the rare component actinolactomycin against breast cancer models, with a particular emphasis on the refractory TNBC subtype. Our findings not only identify actinolactomycin as a highly potent anti-TNBC agent but also establish Streptomyces sp. CB00271 as a valuable microbial resource for this scarce compound, thereby providing a promising lead candidate and a solid foundation for the development of novel therapeutics against triple-negative breast cancer.

2. Results

2.1. Phylogenetic Analysis and Strain Identification of Streptomyces sp. CB00271

To confirm the taxonomic status of the soil-derived strain CB00271, we performed 16S rRNA gene sequencing and phylogenetic analysis. The nearly full-length 16S rRNA gene sequence (1451 bp) was obtained and deposited in the GenBank database under accession number MT672586.1. BLAST analysis revealed that the sequence shared >99% identity with those of several known Streptomyces species. A phylogenetic tree constructed using the neighbor-joining method demonstrated that strain CB00271 forms a stable clade with Streptomyces sp. QLS85, Streptomyces microfavus HBUM174246, and Streptomyces microfavus HBUM174141 (Figure 2), unambiguously assigning it to the genus Streptomyces. Notably, this study identifies CB00271 as a novel microbial source of actinolactomycin-type natural products.

2.2. Isolation and Structural Elucidation of Secondary Metabolites

To investigate the chemical constituents of Streptomyces sp. CB00271, large-scale fermentation (20 L) in M2 medium was performed, yielding 6.3 g of crude extract. The metabolic profile of the initial silica gel column fraction (Fr.1, 4.3 g) is shown in Figure 3A. Subsequent purification of Fr.1 through repeated silica gel chromatography, recrystallization, and semi-preparative HPLC led to the isolation of three compounds: 1 (80 mg), 2 (1 mg), and 3 (1 mg). Their structures were unequivocally determined by comprehensive spectroscopic analysis, including HR-ESI-MS and NMR, and by comparison with literature data. The chemical structures of actinolactomycin (1), daidzein (2), and genistein (3) are illustrated in Figure 3B–D.

Compound 1 was obtained as a white crystalline powder, soluble in chloroform (CHCl_3_). Its structure was determined to be actinolactomycin (ALM) through comprehensive spectroscopic analysis and comparison with literature data. NMR spectra were recorded in deuterated chloroform (CDCl_3_). The ^1^H-NMR (500 MHz, CDCl_3_) data were as follows: (3H, 2 methyl protons of C9–CH_3_ and C3–CH_3_) of 1.19 d (J = 6.2 Hz) and 1.04 d (J = 6.9 Hz), respectively; (2H, 4 methylene protons of H-5a/b, H-6a/b, H-8a/b) of 1.88 m, 1.58 m, 1.44 m, 1.72 m; (1H, 4 methine protons of H-3, H-4, H-7, H-9) of 2.46 m, 3.97 dd (J = 10.3, 4.7 Hz), 3.82 m, 4.94 m. The ^13^C-NMR (125 MHz, CDCl_3_) data were: δC 174.21 (C-2), 80.01 (C-4), 76.32 (C-7), 69.02 (C-9), 45.21 (C-3), 42.24 (C-8), 31.36 (C-6), 28.12 (C-5), 20.48 (C-9–CH_3_), 12.83 (C-3–CH_3_). All the above spectroscopic data are in full agreement with those reported for actinolactomycin in the literature [16], confirming its identity as ALM. Due to the lack of a conjugated chromophore, this compound showed no significant UV absorption at 254 nm, explaining the absence of a corresponding peak in the chromatogram shown in Figure 3A. A summary of the ^1^H and ^13^C NMR correlation data for compound 1 and the reference compound is provided in Table 1.

Compound 2 was obtained as a light yellow amorphous powder, soluble in dimethyl sulfoxide (DMSO). HR-ESI-MS showed a pseudo-molecular ion peak at m/z 255.0651 [M + H]^+^ (calcd. for C_15_H_10_O_4_, 255.0652), confirming the molecular formula. The ^1^H-NMR (400 MHz, DMSO-d6) spectrum exhibited characteristic signals of an isoflavone skeleton: (1H, aromatic proton H-2) of 8.23 s; (1H, aromatic proton H-5) of 7.90 d (J = 8.8 Hz); (2H, aromatic protons H-2′/H-6′) of 7.37 d (J = 8.7 Hz); (1H, aromatic proton H-6) of 6.85 dd (J = 9.1, 1.9 Hz); (1H, aromatic proton H-8) of 6.81 d (J = 2.1 Hz); (2H, aromatic protons H-3′/H-5′) of 6.78 d (J = 8.7 Hz). All spectroscopic data were identical to those reported in the literature for daidzein (7,4′-dihydroxyisoflavone) [24], unambiguously confirming its identity.

Compound 3 was obtained as a light yellow crystalline powder, soluble in DMSO. ESI-MS showed ions at m/z 271.01 [M + H]^+^ (positive mode) and m/z 269.06 [M – H]^−^ (negative mode), consistent with the molecular formula C_15_H_10_O_5_. The 1H-NMR (400 MHz, DMSO-d6) data were as follows: (1H, aromatic proton H-2) of 8.19 s; (2H, aromatic protons H-2′/H-6′) of 7.35 d (J = 8.7 Hz); (2H, aromatic protons H-3′/H-5′) of 6.80 d (J = 8.7 Hz); (1H, aromatic proton H-8) of 6.18 s; (1H, aromatic proton H-6) of 6.04 s. These data matched perfectly with literature values for genistein (4′,5,7-trihydroxyisoflavone) [25], leading to its definitive identification.

2.3. Cytotoxic Activity Evaluation of Compound 1

The cytotoxic potential of actinolactomycin (ALM, 1) was assessed against the human triple-negative breast cancer cell line MDA-MB-231 and the murine breast cancer cell line 4T1 using the CCK-8 assay, with cisplatin serving as a positive control. As summarized in Table 2, after a 72 h exposure, compound 1 demonstrated potent cytotoxic effects, with IC_50_ values of 0.72 ± 0.12 μM in MDA-MB-231 cells and 0.15 ± 0.02 μM in 4T1 cells. Notably, the activity of ALM was approximately 9-fold and 31-fold higher than that of cisplatin (IC_50_ = 6.37 ± 0.26 μM and 4.61 ± 1.30 μM, respectively), demonstrating its superior potency in these in vitro models.

The inhibitory kinetics of compound 1 exhibited a distinct profile compared to the classical chemotherapeutic agent cisplatin. As illustrated in Figure 4A–C, the inhibitory effect of ALM was clearly time-dependent, with cell viability decreasing progressively over 72 h. However, unlike cisplatin, which displayed a typical concentration-dependent response (Figure 4D,F), compound 1 did not achieve complete inhibition (100%) within the tested concentration range (up to 200 μM), nor did it exhibit a classic concentration-effect relationship (Figure 4C,E). This non-classical kinetic profile, where the effect saturates at sub-maximal inhibition, is consistent with a mechanism involving the modulation of specific cellular signaling pathways rather than direct, nonspecific cytotoxicity. It aligns with prior reports that ALM can induce cell cycle arrest and apoptosis [18]. The plateau effect may reflect the pharmacodynamic saturation of its target pathway, a phenomenon occasionally observed with targeted therapeutics.

3. Discussion

This study reports two primary findings with significant implications. First, we identify the soil-derived Streptomyces sp. CB00271 as a novel and valuable microbial source for the exceedingly rare natural product actinolactomycin (ALM). To our knowledge, this strain represents only the fifth reported natural producer of ALM worldwide. Second, and more importantly, we provide the first systematic evaluation demonstrating ALM’s exceptional cytotoxicity against triple-negative breast cancer (TNBC) models. ALM exhibited potent antiproliferative activity against both human MDA-MB-231 and murine 4T1 cells, with IC_50_ values in the sub-micromolar range (0.72 and 0.15 μM, respectively). Its potency surpassed that of the conventional chemotherapeutic cisplatin by approximately 9-fold and 31-fold, respectively. Notably, this anti-TNBC activity is substantially higher than previously reported efficacies against other cancer types (e.g., IC_50_ of 1.4–15.4 μM in K562 and A549 cells) [16,17] suggesting a particularly pronounced vulnerability of TNBC cells to this compound.

To contextualize the potency of ALM, its activity against TNBC cells places it among the most potent natural product leads identified for this subtype. The IC_50_ values (0.15–0.72 μM) are notably superior to those of many well-studied natural products with reported anti-TNBC effects. For instance, epigallocatechin-3-gallate (EGCG) and curcumin, two widely investigated dietary polyphenols, typically exhibit IC_50_ values in the range of 10–80 μM against MDA-MB-231 cells [26,27]. Even shikonin, a potent naphthoquinone, shows IC_50_ values around 1–5 μM in similar models [28]. This comparison underscores the exceptional, sub-micromolar potency of ALM, highlighting its significant potential as a lead compound for TNBC drug development.

The inhibitory kinetics of ALM revealed a distinct, non-classical profile. While its effect was strongly time-dependent, the dose–response curve reached a plateau without achieving complete (100%) inhibition, even at high concentrations. This pattern fundamentally diverges from the classic sigmoidal curve of direct DNA-damaging agents like cisplatin and is characteristic of agents that act by modulating specific signaling pathways rather than causing generalized cytotoxicity. This observation aligns with prior reports that ALM can induce G_0_/G_1_ cell cycle arrest and, importantly, apoptosis in cancer cells [17,18]. The plateau effect may reflect the pharmacodynamic saturation of its target pathway, a common feature observed with some targeted therapeutics, rather than merely indicating a resistant subpopulation. This interpretation is strongly supported by the established molecular mechanism of ALM, which involves the inhibition of the mTOR-HIF-1α signaling axis [18].

The targeting of the mTOR-HIF-1α axis provides a compelling and specific mechanistic rationale for ALM’s observed potency against TNBC. This pathway is a master regulator of cellular metabolism, proliferation, and adaptation to stress, and its hyperactivation is a well-documented driver of the aggressive phenotype, metabolic reprogramming, and therapy resistance in TNBC [19,20,21]. This mechanism positions ALM distinctly among natural products explored against TNBC. While many phytochemicals exert effects through modulating other key pathways, such as PI3K/Akt and STAT3 signaling, inducing reactive oxygen species (ROS), or regulating autophagy [29], the mTOR-HIF-1α axis functions as a central signaling hub that integrates multiple oncogenic inputs. ALM’s upstream inhibition of this nexus may offer a more fundamental intervention against TNBC’s core vulnerability compared to agents targeting individual downstream effectors. The established ability of ALM to inhibit this pathway and induce apoptosis provides a coherent explanation for its cytotoxic effects [18]. Collectively, these findings position ALM not as a broad-spectrum genotoxic agent, but as a promising, mechanism-driven lead compound that precisely targets a key adaptive and survival pathway critically implicated in TNBC pathogenesis.

Beyond the biological activity of ALM, the genomic context of the producing strain, Streptomyces sp. CB00271, significantly amplifies the impact of this discovery. Genomic analysis has revealed that this strain harbors a rich biosynthetic potential, with 45 putative biosynthetic gene clusters [15]. This makes CB00271 not merely an alternative source, but a genetically tractable platform to address the critical supply bottleneck that has hindered ALM research. Future genome-mining and heterologous expression studies based on this strain are essential to elucidate the complete biosynthetic pathway, enable sustainable production, and facilitate structural diversification for optimized drug candidates.

This study has certain limitations that chart clear courses for future research. While we link ALM’s activity to the mTOR-HIF-1α axis based on prior literature, its direct molecular target(s) within TNBC cells remain to be experimentally validated. Furthermore, to fully define its therapeutic potential, future work should assess its cytotoxicity against non-cancerous cell lines to evaluate its therapeutic window. The compelling in vitro efficacy reported here necessitates validation in in vivo TNBC models to assess its pharmacokinetic profile and antitumor effectiveness in a physiological context. Finally, given its targeted mechanism, exploring the synergistic potential of ALM with other targeted agents or standard chemotherapies represents a promising strategic direction. Addressing these points will be crucial for fully realizing the translational potential of ALM as a novel therapeutic candidate against TNBC.

4. Materials and Methods

4.1. Strain Source

The strain used in this study, Streptomyces sp. CB00271 (hereafter referred to as CB00271), was isolated from pristine soil samples collected in Yunnan Province, China. After isolation and purification in the laboratory, the strain was deposited in the China Center for Type Culture Collection (CCTCC) under the accession number CCTCC M 2020176.

4.2. Culture Media

The compositions of the media used were as follows:

G1 Solid Medium (g/L): Soluble starch 20, NaCl 0.5, KNO_3_ 1, MgSO_4_·7H_2_O 0.5, FeSO_4_·7H_2_O 0.01, K_2_HPO_4_ 0.5, agar 20; pH 7.0.

TSB Seed Medium (g/L): Tryptic Soy Broth 30; pH 7.0.

M2 Liquid Fermentation Medium (g/L): Soluble starch 15, cottonseed meal 5, CaCO_3_ 2, CuSO_4_·5H_2_O 0.1, NaI 0.005; pH 7.1–7.2.

4.3. Reagents and Instruments

All analytical grade organic solvents, including petroleum ether, dichloromethane, ethyl acetate, and methanol, were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). HPLC-grade acetonitrile was obtained from ACS (Houston, TX, USA). Cisplatin was acquired from Beijing Solarbio Science & Technology Co., Ltd. (Beijing, China). Diaion HP20 macroporous resin was from Mitsubishi Chemical Corporation (Tokyo, Japan). Silica gel (200–300 mesh) was from Qingdao Bangkai Hi-Tech Materials Co., Ltd. (Qingdao, China). The analytical column (AQ-C18, 5 µm, 4.6 mm × 250 mm) was from Waters Technologies (Shanghai) Ltd. (Shanghai, China). The semi-preparative column (5 µm, 250 mm × 10 mm) was from Welch Materials, Inc. (Shanghai, China). The bacterial genomic DNA extraction kit was from Sangon Biotech (Shanghai) Co., Ltd. (Shanghai, China). The CCK-8 cell proliferation assay kit was from Dojindo Molecular Technologies, Inc. (Kumamoto, Japan).

Instrumentation included: NMR spectrometers (400/500 MHz, Bruker Corporation, Billerica, MA, USA); a high-resolution mass spectrometer (Agilent Technologies, Inc., Santa Clara, CA, USA); HPLC, semi-preparative HPLC, and LC-MS systems (Waters Technologies (Shanghai) Ltd., Shanghai, China); a rotary evaporator (Hangzhou Yijie Technology Co., Ltd., Hangzhou, China); a biochemical incubator (Shanghai Boxun Industry & Commerce Co., Ltd. Medical Equipment Factory, Shanghai, China); a shaking incubator (Shanghai Zhicheng Analytics Instrument Manufacture Co., Ltd., Shanghai, China); and a PCR instrument (Eppendorf AG, Hamburg, Germany).

4.4. Strain Identification

Total genomic DNA of the strain was extracted using the SteadyPure Bacterial Genomic DNA Extraction Kit [30]. The 16S rRNA gene fragment was amplified via PCR using total DNA as the template and universal primers 27F (5′-AGAGTTTGATCMTGGCTCAG-3′) and 1492R (5′-TACGGYTACCTTGTTACGACTT-3′). The PCR product was purified by gel electrophoresis, and the target band was recovered using the Trelief^®^ DNA Gel Extraction Kit. The purified product was sent to Tsingke Biotechnology Co., Ltd. (Beijing, China) for bidirectional sequencing. The resulting sequence was compared using the BLAST (version 2.17.0) (Basic Local Alignment Search Tool) algorithm in the NCBI database. A phylogenetic tree was constructed using the neighbor-joining method in MEGA 11 software [31].

4.5. Fermentation

Streptomyces sp. CB00271 was inoculated from a glycerol stock onto G1 solid medium and cultivated at 28 °C for 5–7 days. Colonies were then transferred to TSB seed medium and incubated at 28 °C with shaking at 200 rpm for 24–48 h. Subsequently, a 10% (v/v) inoculum was transferred into M2 liquid fermentation medium containing 3% (w/v) HP20 resin and cultivated at 30 °C with shaking at 220 rpm for 9 days.

4.6. Isolation and Purification of Secondary Metabolites

A total of 20 L of fermentation broth was collected. The HP20 resin was eluted with methanol, and the eluate was concentrated under reduced pressure. The resulting concentrate was sequentially extracted with petroleum ether and ethyl acetate. The ethyl acetate phase was concentrated to obtain a crude extract (6.3 g). This extract was subjected to silica gel (200–300 mesh) column chromatography and eluted with a stepwise gradient of ethyl acetate/methanol (10:1 to 0:1, v/v) to yield three fractions (Fr.1–Fr.3). Fr.1 was further separated by silica gel column chromatography using a gradient of dichloromethane/methanol (200:1 to 0:1, v/v), yielding six subfractions (Fr.1.1–Fr.1.6). Fr.1.4 was re-chromatographed on a silica gel column with a gradient of dichloromethane/methanol (200:1 to 0:1, v/v), resulting in four subfractions. Fr.1.4.3 afforded compound 1 (80 mg) after recrystallization. Fr.1.5 was subjected to silica gel column chromatography eluted with a gradient of dichloromethane/methanol (100:1 to 0:1, v/v), yielding three subfractions. Fr.1.5.2 was purified by semi-preparative HPLC under the following conditions: column, Welch Xtimate^®^ C18 (5 µm, 250 mm × 10 mm i.d.); mobile phase A, 0.1% (v/v) formic acid in H_2_O; mobile phase B, 100% acetonitrile; gradient: 0–1 min, 95% A; 1–18 min, 95% A to 5% A; 18–20 min, 5% A; 20–20.5 min, 5% A to 95% A; 20.5–25 min, 95% A; flow rate, 2.0 mL/min; detection wavelength, 254 nm. This process yielded compound 2 (tR = 11.9 min, ~1 mg) and compound 3 (tR = 13.3 min, ~1 mg).

4.7. Structural Elucidation

The structures of the isolated compounds were determined using High-Resolution Electrospray Ionization Mass Spectrometry (HR-ESI-MS) and Nuclear Magnetic Resonance (NMR) spectroscopy.

4.8. Cytotoxicity Assay

The cytotoxicity of compound 1 against MDA-MB-231 and 4T1 cell lines was evaluated using the Cell Counting Kit-8 (CCK-8) assay, with cisplatin as the positive control. Cells were seeded into 96-well plates at a density of 3000–5000 cells per well and allowed to adhere for 24 h. Subsequently, the cells were treated with various concentrations of compound 1 for 24, 48, and 72 h. Then, 10 µL of CCK-8 reagent was added to each well, followed by incubation for 0.5–2 h. The absorbance was measured at 450 nm. Cell viability was calculated using the formula: (Aexperiment − Ablank)/(Acontrol − Ablank) × 100%. The IC_50_ values were determined by nonlinear regression analysis using GraphPad Prism 6.0 software.

5. Conclusions

In conclusion, this study makes two significant contributions to the field. First, we have identified the soil-derived Streptomyces sp. CB00271 as a novel and genetically rich microbial source for the scarce natural product actinolactomycin (ALM). Second, and more critically, we provide the inaugural systematic evidence that ALM exhibits exceptional potent cytotoxicity against triple-negative breast cancer (TNBC) models, with potency surpassing the clinical standard cisplatin by an order of magnitude. This represents a significant advancement beyond previous reports of its activity against other cancer types, directly addressing the urgent need for novel agents against this refractory subtype. Its unique, non-classical inhibitory kinetics align with a mechanism involving the targeted disruption of key oncogenic signaling pathways, specifically through the inhibition of the mTOR-HIF-1α axis as supported by prior mechanistic studies, rather than through direct DNA damage, positioning ALM as a promising precision-oriented lead compound against this aggressive subtype.

Critically, the discovery of ALM production in Streptomyces sp. CB00271 transcends the identification of a mere alternative source. The strain’s sequenced genome and extensive biosynthetic potential establish it as a tangible platform to address the long-standing supply bottleneck through future genome-guided biomanufacturing. To fully realize the translational potential of ALM, subsequent research must prioritize the experimental validation of its direct molecular target(s) in TNBC, the assessment of its in vivo efficacy and pharmacokinetics, the evaluation of its therapeutic window against normal cells, and the exploration of its synergy with existing therapeutic regimens.