What is the significance of Yosef's opening words "Shim'u Na"?
Seforno and Ramban (in Pasuk 8): He didn't just relate his dream; he told them to pay attention to it, so as to understand its interpretation. 1
Ohr ha'Chayim #1: They did not want to hear it, so he said Na - please [listen]! 2
Ohr ha'Chayim #2: [Listen] now - the primary interpretation of a dream is the same day (Brachos 56a). This is why one may fast due to a bad dream on Shabbos.
Ohr ha'Chayim #3: Do not think that I already told it to 1overs and received a good interpretation (and therefore I do not fear lest you interpret it unfavorably. 3 ) No - I tell it to you now for the first time; I trust that you love me, and will interpret it favorably.
Malbim: The dream affects you. It is Hashem's prophetic decree about you. A Midrash says that in the future, Nevi'im will rebuke Yisrael "Shim'u Na..."
Ha'Emek Davar: He appeased them, and they consented to listen. They never imagined that it is relevant to them, so they listened.
Which merely had the effect of increasing their hatred - since its implications were obvious.
It does not explicitly say so there. Ohr ha'Chayim understands that this is the reason why one should fast the same day, even on Shabbos.
Ohr ha'Chayim: Even though R. Bena'ah once received 24 interpretations of a dream, and all were fulfilled (Brachos 55b), this is when they are not contradictory.