HPV-mediated nuclear export of HP1γ drives cervical tumorigenesis by downregulation of p53

Abstract

E6 oncoprotein derived from high-risk human papillomavirus (HPV) drives the development of cervical cancer through p53 degradation. Because cervical cancer therapies to inactivate HPV or E6 protein are not available, alternative strategies are required. Here, we show that HPV-mediated nuclear export of human heterochromatin protein 1γ (HP1γ) reduces the stability of p53 through UBE2L3-mediated p53 polyubiquitination during cervical cancer progression. In general, HP1 plays a key role in heterochromatin formation and transcription in the nucleus. However, our immunostaining data showed that the majority of HP1γ is localized in the cytoplasm in HPV-mediated cervical cancer. We found that HPV E6 protein drives unusual nuclear export of HP1γ through the interaction between the NES sequence of HP1γ and exportin-1. The mutation of the NES sequence in HP1γ led to nuclear retention of HP1γ and reduced cervical cancer cell growth and tumor generation. We further discovered that HP1γ directly suppresses the expression of UBE2L3 which drives E6-mediated proteasomal degradation of p53 in cervical cancer. Downregulation of UBE2L3 by overexpression of HP1γ suppressed UBE2L3dependent p53 degradation-promoting apoptosis of cervical cancer cells. Our findings propose a useful strategy to overcome p53 degradation in cervical cancer through the blockage of nuclear export of HP1γ.

HP1γ Sensitizes Cervical Cancer Cells to Cisplatin through the Suppression of UBE2L3

“…Previously, we found a way to block the cervical cancer-specific p53 degradation by inducing HP1γ-mediated suppression of UBE2L3 [15]. Moreover, we revealed that E6 promoted cytoplasmic diffusion of HP1γ, disturbing the epigenetic silencing of UBE2L3 [15]. In the current study, we further demonstrate a pharmacological strategy targeting these mechanisms to resolve cisplatin resistance in cervical cancer cells.…”

“…The E6 protein derived from high-risk HPV stimulates poly-ubiquitination of p53 by E6AP (E3 ligase) and UBE2L3 (E2) [11,12]. Previously, we found a way to block the cervical cancer-specific p53 degradation by inducing HP1γ-mediated suppression of UBE2L3 [15]. Moreover, we revealed that E6 promoted cytoplasmic diffusion of HP1γ, disturbing the epigenetic silencing of UBE2L3 [15].…”

“…It has been reported that p53/p21 signal is more prominent in cisplatin-sensitive HeLa cells than cisplatin-resistant SiHa and CaSki cells [17]. As our previous study showed that overexpression of HP1γ induces apoptosis in cervical cancer cells by inhibiting the UBE2L3-mediated p53 degradation [15], we examined the involvement of HP1γ-UBE2L3-p53 axis in sensitizing cervical cancer cells to cisplatin. In HeLa cells, which were observed to be most sensitive to cisplatin among the three cell lines ( Figure 1A,B), cisplatin treatment increased the expression levels of p53 and Puma, a proapoptotic marker induced by p53 ( Figure 1C, left).…”

“…We previously revealed that the treatment of cervical cancer cells with LMB, an exportin-1 inhibitor, prevents the nuclear export of HP1γ, and thus, suppresses the expression of UBE2L3 while promoting p53 signaling [15]. Therefore, we examined whether co-treatment with LMB and cisplatin can sensitize cisplatin resistant (SiHa) cells.…”

“…Notably, our previous findings revealed that HP1γ, a reader protein of methylated histone H3 lysine 9 (H3K9me) [14], can recover the p53 protein expression levels through the epigenetic silencing of UBE2L3 [12]. We also found that HPV E6 expression leads to abnormal nuclear export of HP1γ, which contributes to excessive ubiquitination of p53 by UBE2L3 [15]. Thus, we strived to identify a way to overcome cisplatin resistance by utilizing HP1γ-mediated p53 stability.…”