Abstract
Insulin-induced gene 1 (INSIG1) regulates sterol synthesis by mediating the activation of sterol regulatory element-binding protein (SREBP) and the degradation of the HMG-CoA reductase (HMGCR). INSIG1 is up-regulated during HIV-1 infection, but its role in HIV-1 infection is unknown. In this report, using pseudovirus production, protein overexpression, and gene knockouts, we found that INSIG1 inhibits HIV-1 production by accelerating the degradation of the HIV-1 Gag protein. Unlike the degradation of HMGCR via the E3 ubiquitin ligase autocrine motility factor receptor (AMFR), a process that depends on the proteasome, INSIG1 coordinated with another ligase, translocation in renal carcinoma chromosome 8 (TRC8), and promoted Gag degradation through the lysosome pathway. We conclude that INSIG1 functions as a sentinel responsive to HIV-1 production and inhibits HIV-1 replication by degrading Gag, a process occurring at intracellular membrane sites such as the endoplasmic reticulum and endosomes where both INSIG1 and Gag may be located.
Highlights
Insulin-induced gene 1 (INSIG1) regulates sterol synthesis by mediating the activation of sterol regulatory element– binding protein (SREBP) and the degradation of the HMG-CoA reductase (HMGCR)
Unlike the degradation of HMGCR via the E3 ubiquitin ligase autocrine motility factor receptor (AMFR), a process that depends on the proteasome, INSIG1 coordinated with another ligase, translocation in renal carcinoma chromosome 8 (TRC8), and promoted Gag degradation through the lysosome pathway
No up-regulation of INSIG1 was observed in a single-cycle infection by the pseudovirus HIV-1 in either 293T or HIV-tropic Jurkat and THP-1 cells (Fig. 1E)
Summary
It is known that INSIG1 expression is up-regulated in HIV1–infected T cells [17], but in which step the INSIG1 is up-regulated during viral replication and the consequence of this up-regulation are not clear. The level of INSIG1 was not changed in a separate expression of HIV Gag-pol, VSV-G, and Env in 293T cells by the transfection of these plasmids individually (Fig. 1F). The overexpression of INSIG1 inhibited the production of HIV-1 significantly in both the Env and VSV-G enveloped pseudovirus (Fig. 2A). The protein level of Gag was increased in the insig knockout cells infected by the VSV-G enveloped pseudovirus (Fig. 4C). The inhibition of HIV-1 production by INSIG1 was largely hampered in the absence of TRC8 in Jurkat cells (Fig. 7, B and C). This observation confirmed that TRC8 is the Figure 7. This indicates that cholesterol might not be a critical factor in this process (Fig. 11)
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