The transcriptional regulation of peroxisome proliferator-activated receptor (PPAR) α by post-translational modification, such as ubiquitin, has not been described. We report here for the first time an ubiquitin ligase (muscle ring finger-1/MuRF1) that inhibits fatty acid oxidation by inhibiting PPARα, but not PPARβ/δ or PPARγ in cardiomyocytes in vitro. Similarly, MuRF1 Tg+ hearts showed significant decreases in nuclear PPARα activity and acyl-carnitine intermediates, while MuRF1−/− hearts exhibited increased PPARα activity and acyl-carnitine intermediates. MuRF1 directly interacts with PPARα, mono-ubiquitinates it, and targets it for nuclear export to inhibit fatty acid oxidation in a proteasome independent manner. We then identified a previously undescribed nuclear export sequence in PPARα, along with three specific lysines (292, 310, 388) required for MuRF1s targeting of nuclear export. These studies identify the role of ubiquitination in regulating cardiac PPARα, including the ubiquitin ligase that may be responsible for this critical regulation of cardiac metabolism in heart failure.

Oridonin belongs to ent-kaurane tetracyclic diterpenoid and was first isolated from Isodon species. It exhibits inhibitory activities against a variety of tumor cells, and pharmacological study shows that oridonin could inhibit cell proliferation, DNA, RNA and protein synthesis of cancer cells, induce apoptosis and exhibit an antimutagenic effect. In addition, the large amount of the commercially-available supply is also very important for the natural lead oridonin. Moreover, the good stability, suitable molecular weight and drug-like property guarantee its further generation of a natural-like compound library. Oridonin has become the hot molecule in recent years, and from the year 2010, more than 200 publications can be found. In this review, we summarize the synthetic medicinal chemistry work of oridonin from the first publication 40 years ago and share our research experience of oridonin for about 10 years, which may provide useful information to those who are interested in this research field.

A series of nine enmein-type ent-kaurane diterpenoid and furoxan-based nitric oxide (NO) donor hybrids (10a-i) were designed and synthesized from commercially available oridonin (1). These hybrids were evaluated for their antiproliferative activity against Bel-7402, K562, MGC-803, and CaEs-17 human cancer cell lines and L-02 normal liver cells. The antiproliferative activity against tumor cells was stronger than the lead compound 1 and parent molecule 9 in most cases. Especially, compound 10f showed the strongest activity against human hepatocarcinoma Bel-7402 cell line with an IC 50 of 0.81 µM and could also release 33.7 µmol/L NO at the time point of 60 min. Compounds 10a-i also showed cytotoxic selectivity between tumor and normal liver cells with IC 50 ranging from 22.1 to 33.9 µM. Furthermore, the apoptotic properties on Bel-7402 cells revealed that 10f could induce S phase cell cycle arrest and apoptosis at low micromolar concentrations. The effects of 10f on apoptosis-related proteins were also investigated. The potent antiproliferative activities and mechanistic studies warrant further preclinical investigations.

MuRF1 inhibits fatty acid oxidation while increasing glucose metabolism by its specific inhibition of PPARα activity. Surprisingly, MuRF1 does not degrade PPARα through poly‐ubiquitination. Alternatively, we identified that MuRF1 mono‐ubiquitinates PPARα. To identify the underlying mechanism of MuRF1's inhibition of PPARα, we increased MuRF1 expression and determined PPARα's nuclear localization by confocal microscopy. Increasing MuRF1 cleared nuclear PPARα in 80% of the cells (vs. <0.1% of controls), leading to the hypothesis that MuRF1 enhances PPARα nuclear export. Using the nuclear export inhibitor leptomycin B, we prevented MuRF1‐dependent PPARα nuclear export, suggesting a role of the nuclear export machinery. We next identified two potential non‐canonical nuclear export signals in PPARα and mutated their leucines to confirm their role in nuclear export. Only the PPARα NES1 mutant was enriched in its nuclear localization. We next mutated the three adjacent lysines to PPARα's NES1 to determine their role in MuRF1's ubiquitination and activity. Mutating any one of the three lysines around NES1 prevented MuRF1‐dependent nuclear export of PPARα. These data suggest that the increased MuRF1 that occurs in cardiac stress may drive the mono‐ubiquitination of PPARα, resulting in enhanced nuclear export and inhibition of PPARα activity.