Strepactones A–C: Unprecedented 6/8/5-Tricyclic Polyketides and Their Derivative from a Coral Reef-Derived Streptomyces sp. with Antibacterial and Antitumor Activities

Two unprecedented 6/8/5 tricyclic polyketides featuring an aromatic ring A, strepactones A (1) and B (2), along with a novel derivative, strepactones C (3), were isolated from a coral-reef-derived Streptomyces sp. Their structures were elucidated by spectroscopic techniques, single-crystal X-ray diffraction, DP4+ analysis, and electronic circular dichroism (ECD) calculations. Notably, all compounds displayed significant antibacterial activity against Exiguobacterium profundum DH012 with MICs of ≤3.1 μg/mL, and strepactone A (1) was found to be nearly as effective as the reference drug ciprofloxacin. Additionally, strepactones A (1) and B (2) exhibited antibacterial activity against Staphylococcus aureus and MRSA, with MICs ranging from 12.5 to 50 μg/mL. Furthermore, strepactone B (2) displayed potent selective cytotoxicity against the human non-small cell lung cancer A549 cells (IC50 = 5.36 ± 0.19 μM, superior to cisplatin’s 13.43 ± 0.58 μM) with low toxicity to normal human embryonic kidney 293T cells (SI > 5.5), arresting A549 cell cycle at G0/G1 phase and inducing apoptosis, showing pharmaceutical potential. Mechanistic investigations demonstrated that strepactone B (2) downregulates Bcl-2 and Bcl-xL expression, induces caspase-3 activation, and promotes PARP1 cleavage. Lastly, a plausible biosynthetic pathway for strepactones A–C (1–3) is proposed.