Abstract

Background:

Hepatocellular carcinoma (HCC) is the most common primary malignancy of the liver and represents a major global health-care challenge. PC3 secreted microprotein (PSMP) is a novel chemotactic cytokine that can recruit peripheral blood monocytes and lymphocytes through its chemokine receptor CCR2. Our previous study found that PSMP is significantly highly expressed in human and mouse liver fibrosis/cirrhosis. PSMP promotes the progression of liver fibrosis by regulating the infiltration, activation and polarization of macrophages. However, the relationship between PSMP and the development and prognosis of HCC remains unclear.

Methods:

The expression of PSMP was detected in two independent HCC patients cohorts and its correlation with patients’ prognosis was analyzed. In vivo: In PSMP knockout (Psmp^-/-) and wild-type (WT) mice, subcutaneous tumorigenesis models and liver orthotopic tumorigenesis models were established using two mouse HCC cell lines (Hepa1-6, H22); In nude mice, subcutaneous tumorigenesis models were established using two human HCC cell lines (Huh7, HepG2). The direct effects of PSMP on the polarization of macrophages (human THP-1 cell line and mouse bone marrow-derived macrophages (BMDMs) were studied in vitro. In addition, we performed RNA sequencing of BMDMs from Psmp^-/- and WT mice.

Results:

Through the detection of clinical HCC patient samples, we found that PSMP is downregulated in human HCC tissues, and its expression level is positively correlated with the prognosis of HCC patients. In vivo, we found that genetic deletion of PSMP promotes subcutaneous and liver orthotopic tumor growth and metastasis in mice; Overexpression of PSMP inhibits the formation of subcutaneous tumors in nude mice. Mechanistically, the deletion of PSMP substantially suppresses the infiltration of CD8+ tumor-infiltrating lymphocytes (TILs) while promoting the infiltration and polarization of M2 tumor-associated macrophages (TAMs) within the liver. In vitro, we observed that PSMP possesses the capacity to induce M1-polarization and suppress M2-polarization of mouse BMDMs and human THP-1 cells. In addition, analysis of RNA sequencing results showed that PSMP may mediate the polarization of macrophages by regulating the PI3K/Akt signaling pathway. Then, we verified that PSMP mediates its inhibitory effect on M2-polarization of macrophages by suppressing the PI3K/Akt pathway through the inhibition of p85 and Akt phosphorylation.

Conclusion:

Collectively, PSMP may inhibit the M2 polarization of TAMs by downregulating PI3K/Akt pathway, and then promote the anti-tumor immune response of CD8+ T cells, and ultimately inhibit the progression of HCC. The results are expected to clarify the role and mechanism of PSMP in the liver tumor microenvironment for the first time, which has important theoretical significance, and may also provide new targets for the treatment of HCC, with potential application value.