Department of Biochemistry, The Jikei University School of Medicine, Tokyo, Japan.
Division of Gastroenterology and Hepatology, Department of Internal Medicine, The Jikei University School of Medicine, Tokyo, Japan.
Cancer Sci. 2022 Jul;113(7):2378-2385. doi: 10.1111/cas.15386. Epub 2022 May 12.
Protein kinase C delta (PKCδ) is a multifunctional PKC family member and has been implicated in many types of cancers, including liver cancer. Recently, we have reported that PKCδ is secreted from liver cancer cells, and involved in cell proliferation and tumor growth. However, it remains unclear whether the extracellular PKCδ directly regulates cell surface growth factor receptors. Here, we identify epidermal growth factor receptor (EGFR) as a novel interacting protein of the cell surface PKCδ in liver cancer cells. Imaging studies showed that secreted PKCδ interacted with EGFR-expressing cells in both autocrine and paracrine manners. Biochemical analysis revealed that PKCδ bound to the extracellular domain of EGFR. We further found that a part of the amino acid sequence on the C-terminal region of PKCδ was similar to the putative EGFR binding site of EGF. In this regard, the point mutant of PKCδ in the binding site lacked the ability to bind to the extracellular domain of EGFR. Upon an extracellular PKCδ-EGFR association, ERK1/2 activation, downstream of EGFR signaling, was apparently induced in liver cancer cells. This study indicates that extracellular PKCδ behaves as a growth factor and provides a molecular basis for extracellular PKCδ-targeting therapy for liver cancer.
蛋白激酶 C 亚型 δ（PKCδ）是多功能蛋白激酶 C 家族成员，与多种类型的癌症有关，包括肝癌。最近，我们报道 PKCδ 从肝癌细胞分泌，并参与细胞增殖和肿瘤生长。然而，细胞外 PKCδ 是否直接调节细胞表面生长因子受体仍不清楚。在这里，我们确定表皮生长因子受体（EGFR）是肝癌细胞表面 PKCδ 的一种新型相互作用蛋白。成像研究表明，分泌的 PKCδ 以自分泌和旁分泌的方式与表达 EGFR 的细胞相互作用。生化分析显示 PKCδ 与 EGFR 的细胞外结构域结合。我们进一步发现 PKCδ C 端区域的一部分氨基酸序列与 EGF 的 EGFR 结合位点相似。在这方面，结合位点处 PKCδ 的点突变体缺乏与 EGFR 细胞外结构域结合的能力。细胞外 PKCδ-EGFR 结合后，肝癌细胞中 EGFR 信号下游的 ERK1/2 激活明显增加。这项研究表明，细胞外 PKCδ 具有生长因子的作用，并为肝癌的细胞外 PKCδ 靶向治疗提供了分子基础。
