Porphyrins are used for cancer diagnostic and therapeutic applications, but the mechanism of how porphyrins accumulate in cancer cells remains elusive. Knowledge of how porphyrins enter cancer cells can aid the development of more accurate cancer diagnostics and therapeutics. To gain insight into porphyrin uptake mechanisms in cancer cells, we developed a flow cytometry assay to quantify cellular uptake of meso‐tetra (4‐carboxyphenyl) porphyrin (TCPP), a porphyrin that is currently being developed for cancer diagnostics. We found that TCPP enters cancer cells through clathrin‐mediated endocytosis. The LDL receptor, previously implicated in the cellular uptake of other porphyrins, only contributes modestly to uptake. We report that TCPP instead binds strongly ( $K_D=42\text{nM}$) to CD320, the cellular receptor for cobalamin/transcobalamin II (Cbl/TCN2). Additionally, TCPP competes with Cbl/TCN2 for CD320 binding, suggesting that CD320 is a novel receptor for TCPP. Knockdown of CD320 inhibits TCPP uptake by up to 40% in multiple cancer cell lines, including lung, breast, and prostate cell lines, which supports our hypothesis that CD320 both binds to and transports TCPP into cancer cells. Our findings provide some novel insights into why porphyrins concentrate in cancer cells. Additionally, our study describes a novel function for the CD320 receptor which has been reported to transport only Cbl/TCN2 complexes.

中文翻译：

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鉴定癌细胞中介四（4-羧基苯基）卟啉（TCPP）吸收的新机制

卟啉用于癌症的诊断和治疗应用，但是卟啉如何在癌细胞中蓄积的机制仍然不清楚。卟啉如何进入癌细胞的知识可以帮助开发更准确的癌症诊断和治疗方法。为了深入了解癌细胞中卟啉的摄取机制，我们开发了一种流式细胞术测定法来定量测定中四（4-羧基苯基）卟啉（TCPP）（目前正在开发用于癌症诊断的卟啉）的细胞摄取。我们发现TCPP通过网格蛋白介导的内吞作用进入癌细胞。先前与其他卟啉的细胞摄取有关的LDL受体仅适度地促进摄取。我们报告说TCPP相反具有较强的绑定力（${ķ}_d=42\text{纳米}$）CD320，即钴胺素/反钴胺素II（Cbl / TCN2）的细胞受体。此外，TCPP与Cbl / TCN2竞争CD320结合，表明CD320是TCPP的新型受体。抑制CD320在多种癌细胞系（包括肺癌，乳腺癌和前列腺癌细胞系）中可抑制TCPP吸收多达40％，这支持了我们的假设，即CD320既可与TCPP结合又可将TCPP转运到癌细胞中。我们的发现为卟啉为何集中在癌细胞中提供了一些新颖的见解。此外，我们的研究描述了CD320受体的一种新功能，据报道它只能转运Cbl / TCN2复合物。