Oxysterol-binding Protein-related Protein 4L（ORP4L） Promotes Cell Proliferation By Sustaining Intracellular Ca²⁺ Homeostasis

One of the characteristics of tumor cells is infinite proliferation. Oxysterol binding protein（OSBP）-related proteins（ORPs） comprise a mammalian gene family with 12 members implicated in a spectrum of different cellular processes including sterol and phosphoinositide sensors coordinating transport, signaling and metabolism. ORP4 is a member of the ORPs family. Studies have reported that ORP4 knockout mice exhibit a teratozoospermia, also ORP4 is essential for cell proliferation and survival, but the underlying mechanism is unclear.ORP4 is expressed as three variants, ORP4 L, ORP4 M and ORP4 S. Here, we reported that silencing of ORP4 L with specific small interfering RNA（siRNA） inhibited the proliferation of human cervical cancer cell lines C33 A, HeLa and CaSki, the reverse effect being observed in ORP4 L overexpressing cells. For molecular insight, we found that ORP4 L maintained intracellular Ca²⁺ homeostasis. We found that ORP4 L could interact with inositol-1,4,5-trisphosphate receptor 1（IP3R1） with yeast two-hybrid analysis. IP3R1 is predominant in maintenance of cellular Ca²⁺ signals as Ca²⁺ channel. We also confirmed that ORP4 L interacted with IP3R1 with different methods.Ca²⁺ is a versatile second messenger with a wide range of central physiological roles in processes such as cell growth or proliferation. We found that ORP4 L could regulate nuclear factor of activated T cells（NFAT） activity, NFAT are usually activated by increased intracellular Ca²⁺ levels. ORP4 L promoted expression of a gene cluster which supported cell proliferation through regulating NFAT activity. Of note, ORP4 L sustained IP3R1 expression at both mRNA and protein levels via Ca²⁺-dependent NFAT3 activation, This offered a mechanic explanation for the role of ORP4 L in cellular Ca²⁺ homeostasis, ORP4 L maintained cellular Ca²⁺ homeostasis through keeping IP3R1 stable expression. Furthermore, ORP4 L knockdown markedly inhibited tumor growth in a C33 A cell xenograft mouse model.To conclude, our results revealed that ORP4 L promoted cell proliferation through maintaining intracellular Ca²⁺ homeostasis and IP3R1 stable expression by ORP4 L and IP3R1 protein interaction. This indicate that ORP4 L may be a putative new candidate target for the development of cancer treatment.