## 4 Discussion
However, in contrast to this finding, the PARP7 inhibitor used in this study, RBN-2397, has been reported to cause cancer regression in xenograft mouse models [21]. PARP7 is a key regulator of innate immunity by repressing TBK1, an important regulator of IFN-I signaling [20]. By inhibiting PARP7, the IFN-I signaling axis is restored, enabling immune cells to target cancer cells. RBN-2397, which exploits PARP7's role in regulating the IFN-I signaling, is currently in a Phase 1 clinical trial designed to assess its anti-tumor activity in patients with advanced-stage solid tumors (NCT04053673). Whether PARP7 inhibition-induced immune cell targeting of cancer cellsoverrides the increased activity of oncogenic transcription factors, such as ER\(\alpha\), remains to be determined. It would be interesting to compare the role of PARP7 in cancer cells versus host immune cells in syngeneic in vivo models. The ADP-ribose glycohydrolase, MACROD1 (LRP16), has been reported to interact with the AF-1 domain of ER\(\alpha\), and enhance ER\(\alpha\) transcriptional activity [39]. This report, and our findings, imply that PARP7 and MACROD1 might work in concert to regulate ER\(\alpha\) activity by catalyzing the transfer or removal of ADP-ribose on the receptor's AF-1 domain and may represent novel targets for ER positive breast cancer treatment. Recent studies have, however, shown that MACROD1 is almost exclusively expressed in the mitochondria [40]. Therefore, the possible role of MACROD1 as a co-regulator of transcription factor activity and its potential functions in the nucleus need to be fully clarified. It is possible that MACROD1 influences the cellular pool of NAD\({}^{+}\) or additional mitochondrial processes that indirectly impact ER\(\alpha\) function. We only examined PARP7 function in ER positive breast cancer cells. Whether or not PARP7 has a tumor suppressive effect in other subtypes of breast cancer is not known. Previous studies have shown that the androgen receptor (AR) regulates PARP7 expression [41; 42]. In addition, MACROD1 has been reported to act as a co-activator in AR signaling [43], inferring that AR could be a target for PARP7-mediated mono-ADP-ribosylation. Since AR has been proposed as an important regulator of carcinogenesis in a subset of triple negative breast cancers (TNBC), the interplay between PARP7 and AR could be studied to further understand the role of PARP7 in breast cancer. In summary, we show that PARP7 negatively regulates ER\(\alpha\), establishing a link between PARP7-mediated mono-ADP-ribosylation and ER\(\alpha\) signaling. Further studies are needed to determine how mono-ADP-ribosylation affects ER\(\alpha\) protein levels and stability. In addition to promoting the degradation of AHR, HIF1\(\alpha\) and ER\(\alpha\), PARP7 also promotes the degradation of c-Myc [19], suggesting that PARP7 may represent a key regulatory factor controlling and suppressing the expression of several oncogenic transcription factors. In contrast, PARP7 is a negative regulator of IFN-I signaling, which allows tumor cells to \"hide\" from immunosurveillance. Thus, inhibition or loss of PARP7 expression would be expected to prevent tumors from evading the immune system, leading to increased anti-tumorigenic responses. It will be important to determine whether PARP7's immunomodulatory role, which may induce immune cell mediated tumor cytotoxicity, supersedes the increased activity of oncogenic transcription factors. The following are available online at [[https://www.mdpi.com/2073-440](https://www.mdpi.com/2073-440)]([https://www.mdpi.com/2073-440](https://www.mdpi.com/2073-440)) 9/10/3/623/s1, Figure S1: Characterization of lab generated anti-PARP7 antibody in CO51 cells and E0771 cells. Figure S2: Moment of precursor ion selection for MS2 fragmentation leading to ADP-R specific reporter ions in the generated fragment spectra. Figure S3: Liquid chromatography and mass spectrometry (LC/MS) of protein samples used for the identification of peptides in ER\(\alpha\). Conceptualization, M.R., S.T. and J.M.