The US Green Card is an immigration permit, meaning it allows you to move into the US permanently. If you decide to move somewhere else permanently, you are considered to have abandoned your US immigration plans, as you cannot be a permanent resident in two places.
Technically, if you are outside of the US for more than a year, you are considered to have abandoned your Green Card, unless you have special reasons and apply for an exception upfront. But even shorter times might be considered an abandonment: USCIS says (https://www.uscis.gov/green-card/after-green-card-granted/maintaining-permanent-residence): "You may also lose your permanent resident status by intentionally abandoning it. You may be found to have abandoned your status if you: Move to another country, intending to live there permanently."
Applying for an Italian permanent residence permit could well be considered as 'intending to live there permanently', and there's little to argue with that.
So, yes, you'll probably lose the Green Card. You can always apply again for it, if you change your mind again, but there is no guarantee that you get one again.
If you lived in the US five full years on the Green Card, you can apply for Citizenship. Once you are an American Citizen, you can move permanently to Italy without the risk of losing it.