Tumour metastasis is the major cause of death from cancer. Metastasis initiates when cancer cells leave the primary tumour and enter the adjacent tissues. Breast cancer and squamous cell carcinoma of head and neck (SCCHN) are common types of human cancers that metastasize to distant organs. Both types of cancers lead to incurable diseases by commonly metastasizing to lungs or bones. Proliferating cancer cells prefer glycolysis over oxidative phosphorylation even in the presence of oxygen relative to normal cells which produce ATP by oxidative phosphorylation. The cause of this metabolic switch and its contribution to tumour growth and metastasis is unknown. 

Previously, upstream phosphorylation of Lactate dehydrogenase A (LDHA) is identified in cancer cell invasion and tumour metastasis [1]. LDHA is a metabolic enzyme that converts pyruvate and NADH to lactate and NAD⁺ during glycolysis. Cancer cells are known to have an upregulation of LDHA which promotes aerobic glycolysis in cancer cells. Downregulation of LDHA suppress tumour growth and metastasis in breast cancer, and hepatocarcinoma cells [2,3]. Previous findings show that LDHA plays an important role in tumour growth and metastasis, however, the molecular mechanism by which LDHA is regulated is unknown.  In the study entitled, “Phosphorylation-mediated activation of LDHA promotes cancer cell invasion and tumour metastasis”, Jin et al., (2017) reports the phosphorylation of LDHA by upstream kinases HER2 and Src, and their contribution in breast cancer and SCCHN cells invasion and tumour metastasis.

To confirm the role of LDHA in cancer cell invasion and metastasis, Jin et al. first knocked down LDHA by LDHA shRNA in various invasive cell lines of breast cancer and SCCHN and reported significantly decreased cell invasion and increased anoikis induction. Anoikis is the apoptotic cell death induced by detachment from the surrounding extracellular matrix. As detachment changes occur during metastasis, it is necessary for metastatic tumour cells to overcome anoikis to spread. To better characterize the role of LDHA in cancer cell invasion and metastasis, they further tested the expression and phosphorylation of LDHA in various breast cancer and SCCHN with diverse invasive ability. Y10 phosphorylation of LDHA was found to be positively correlated with anoikis resistance and cancer cell invasion ability.

To uncover the molecular mechanism by which LDHA phosphorylation is regulated, Jin et al., performed in vitro kinase assays to examine the upstream kinases including HER2, Src, and epidermal growth receptor factor (EGFR), which are usually dysregulated in breast cancer and SCCHN. They found HER2 and Src, but not EGFR, phosphorylate LDHA at tyrosine 10, and contribute to cell invasion and metastasis in breast cancer and SCHNN. Additionally, they found HER2 partially signals through Src to phosphorylate LDHA at Y10, which suggests that Src is not only an important regulator of cell invasion and metastasis in Src positive cancer cells but in HER2 positive cancer cells as well.

Furthermore, Jin et al. performed knockdown of LDHA activity by shRNA LDHA in MDA-MB231 cells, then expressed either catalytically less active LDHA Y10F or WT LDHA. They found WT LDHA rescued the decreased cell invasion and increased anoikis and not the Y10F mutant. This shows that Y10F LDHA, which is not phosphorylated at tyrosine 10, decreases cancer cell invasion and metastasis relative to Y10 LDHA, which contributed to cell invasion and anoikis resistance by rescuing LDHA hyperactivity in MDA-MB231. Additionally, they observed high ROS levels in Y10 LDHA, which also lead to increased cell invasion and resistance to anoikis.

To demonstrate clinical importance of Y10 phosphorylation, Jin et al. first showed higher expression of WT LDHA relative to Y10F mutant using the xenograft mouse model of tumour metastasis. Then, they advanced their study to primary human tissues from breast cancer patients, and they found a functional relationship between LDHA activity and tumour metastasis using IHC staining.

In conclusion, this is the first study that demonstrates that HER2 and Src signaling is responsible for tyrosine 10 phosphorylation of metabolic enzyme LDHA, which in turn activates LDHA and controls internal ROS levels and results in increased cell invasion, increased resistance to anoikis, and tumour metastasis in breast cancer and SCCHN (Table 1). This study providesevidence of LDHA necessity and lactate presence for tumour cell invasion and metastasis. Overall, this study provides three potential therapeutic targets—Src, HER2 and LDHA—for breast cancer and SCCHN.

Table 1. phosphorylation of LDHA at tyrosine 10 is positive for HER2 and Src, which results in increased breast cancer and SCCHN cells invasion, increased resistance to anoikis, increased metastasis, and increased ROS levels.

References.

1. Fan, J., Hitosugi, T., Chung, T. W., Xie, J., Ge, Q., Gu, T. L., … Chen, J. (2011). Tyrosine phosphorylation of lactate dehydrogenase A is important for NADH/NAD(+) redox homeostasis in cancer cells. Molecular and cellular biology, 31(24), 4938–4950. doi:10.1128/MCB.06120-11.
2. Rizwan, A., Serganova, I., Khanin, R., Karabeber, H., Ni, X., Thakur, S., … Koutcher, J. A. (2013). Relationships between LDH-A, lactate, and metastases in 4T1 breast tumors. Clinical cancer research: an official journal of the American Association for Cancer Research, 19(18), 5158–5169.
3. Sheng SL, Liu JJ, Dai YH, Sun XG, Xiong XP, Huang G. Knockdown of lactate dehydrogenase A suppresses tumor growth and metastasis of human hepatocellular carcinoma. FEBS J. 2012;279:3898–3910