## 4 Discussion
Considering that TQ has no or minimal effects on normal human cells [24; 46] and that UHRF1 is overexpressed in several tumors, TQ could be a promising specific regulator of the HAUSP/UHRF1 axis that functions by inducing UHRF1 ubiquitination. Several studies in various cancer cells have shown that TQ induces a downregulation of the Bcl-2 oncogene [47; 48; 49; 50; 51; 52], one of the master activators of the anti-apoptotic effects in cancer. In the present study, we found that TQ induces a dose- and time-dependent downregulation of Bcl-2 in colon cancer cells. However, it apparently does not do so by inducing ubiquitination, based on the lack of high-molecular-weight bands of Bcl-2. Therefore, TQ can induce a fast ubiquitination of UHRF1 but apparently not of other oncogenes such as Bcl-2, and this ubiquitination is indispensable for UHRF1 degradation. Bcl-2 is degraded via the activation of the proteasome ubiquitination pathway in response to the natural product pseudolaric acid B [43], whereas TQ is suggested to induce Bcl-2 degradation in cancer cells through an ubiquitination-independent mechanism. In agreement with this hypothesis, TQ induced a time-dependent downregulation of Bcl-2 in human multiple myeloma cells through the inhibition of STAT3 [52], which is a main regulator of Bcl-2 [53; 54]. TQ, by virtue of its ability to induce the early ubiquitination of the oncogene UHRF1, followed by its degradation in both Jurkat cells as an experimental model of hematological malignancy as well as colon cancer as a model of solid tumors, the present study supports the chemopre and therapeutic potential of TQ for both human blood and solid tumors. However, due to its highly lipophilic nature, poor aqueous solubility and inadequate biological stability, TQ is characterized by poor bioavailability. However, many approaches to address TQ's pharmacokinetic limitations, such as formulations of TQ in lipid nanocarriers, are currently being developed [55; 56]. In conclusion, the present study shows that, unlike the anticancer drug doxorubicin, TQ can trigger the fast ubiquitination of UHRF1, but not of another oncogene, Bcl-2. The underlying mechanism could involve the downregulation of HAUSP, which normally would protect UHRF1 from ubiquitination (see the Graphical Abstract). Nevertheless, the profound mechanisms of TQ's effects on the ubiquitination status of UHRF1 still require further investigation. Objectively, due to the underlying complexity of the mechanisms of the interaction between UHRF1 and several proteins, including HAUSP [57], further studies are needed to explore the detailed mechanisms by which UHRF1 is rapidly polyubiquitinated in cancer cells in response to TQ and how HAUSP could be involved in the process. Conceptualization, M.A., O.A. and Z.O. ; methodology, M.A. and A.K. ; validation, M.A and Z.O. ; formal analysis, M.A. ; investigation, A.K. ; writing--original draft preparation, M.A. and A.K. ; writing--review and editing, M.A. and O.A. ; funding acquisition, O.A and Z.O. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by the deanship of Scientific Research at Umm Al-Qura University, grant number 19-MED-1-02-0008. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** All data generated or analyzed during this study are included in this published article or are available from the corresponding authors on reasonable request. **Acknowledgments:** The authors would like to thank the deanship of Scientific Research at Umm Al-Qura University for supporting this work by Grant Code: 19-MED-1-02-0008. **Conflicts of Interest:** The authors declare that they have no competing interests.